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University Microfilms International

BOO N /hi B HOAD. ANN ARBOR, Ml 48100 IB HI 1.11 OHO HOW. I ONDON WC I R 41 .1. I NO I. AND YOnEpoj

POTTER, DANIEL ANDREW GUARDING, AGGEEPOIVE BEHAVIOR AND MATING PHCOEP; IN MALE GPIDER MITEG (ACARI : TETRAHYGH1DAE).

THE OHIO GTATE IJNIVEEGITY. PH.D., 1 O'fC

University M icrofilm s International .won/> I II ihiaii. ANN n m m u mi -iiniH, GUARDING, AGGRESSIVE BEHAVIOR AND MATING SUCCESS IN MALE SPIDER MITES (ACARI : TETRANYCHIDAE)

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By Daniel Andrew Potter, B.S., M.Sc.

* * * * *

The Ohio State University 1978

Reading Committee Approved By J .F. Downhower D.J. Horn D.E. Johnston

S.S.Y. Young v - ''^ U Q I* ^ ^ Adviser Department pf Entomology "The magic of the microscope is not that it makes little creatures larger, but that it makes a large one smaller. We are too big for our world. The microscope takes us down from our proud and lonely immensity and makes us, for a time, fellow citizens with the great majority of living things. It lets us share with them the strange and beautiful world where a meter amounts to a mile and yesterday was years ago."

- Asher Treat (1975) ACKNOWLEDGEMENTS

It is a great pleasure to acknowledge my friends and teachers, Professors Donald E. Johnston and Dana L. Wrensch, for their patient guidance, constant inspiration and encour­ agement throughout these studies. To have had the privilege and good fortune to work with two such thoughtful and dedi­ cated scientists has, above all else, made my tenure in the Acarology Laboratory a rewarding one. The members of my advisory committee, Drs. J.F. Downhow- er, D.J. Horn, W.C. Rothenbuhler and S.S.Y. Young deserve special thanks for the time, ideas and advice they so will­ ingly gave me. I am also indebted to Dr. Rodger Mitchell for valuable and enlightening discussions, and to Dr. G.W. Whar­ ton for sharing with me his wisdom and good humor. My re­ search has benefitted immeasurably from my association with each of the above-mentioned individuals, and to each I offer my sincerest thanks. Dr. W. Helle of the Laboratory of Applied Entomology, Amsterdam, provided colonies of wild-type and albino Tetrany- chus urticae. For aid in maintaining the OSU greenhouse spi­ der mite cultures, I thank Mr. John McCabe. I also thank the Instructional and Research Computer Center, OSU, for providing computer time on the IBM 370/165. An expression of gratitude is extended to those faculty members and graduate students at the Ohio State University who have helped to provide the creative and stimulating en­ vironment necessary for research and study. For their friend­ ship and encouragement, I am particularly grayeful to B.M. Drees, B.J. Lenoble, M.P. Murtaugh and M.J. Walsh. Finally, I wish to thank my parents, Professor Norman N. Potter and Mrs. Adele B. Potter, for their years of unwaver­ ing love, guidance, encouragement and sacrifice that have allowed me the opportunity to pursue my interests in biology. It gives me great pleasure to dedicate this dissertation to them.

iv VITA

July 16, 1952 ...... Born - Ames, Iowa 1971-1974 ...... Research Assistant, Dept, of Entomology, Cornell University, Ithaca, N.Y. 1974 ...... B.S., Cornell University, Ithaca, N.Y. 1975-1976 ...... NIH Predoctoral Trainee, The Acarology Laboratory, The Ohio State University, Columbus, Ohio 1976-1978 ...... Teaching Associate, Department of Ento­ mology, The Ohio State University, Columbus, Ohio 1977 ...... M.Sc., The Ohio State University, Columbus, Ohio 1978 ...... Ph.D., The Ohio State University, Columbus, Ohio

PUBLICATIONS Potter, D .A. and D.E. Johnston. 1976. Canestriniphis megalo- dacne n.g., n.sp. (Acari : Eviphididae) from a pleasing fungus beetle, Megalodacne heros. Ann. Entomol. Soc. Am. 69: 494-96. Potter, D.A., D.L. Wrensch and D.E. Johnston. 1976. Guarding, aggressive behavior, and mating success in male two- spotted spider mites. Ann. Entomol. Soc. Am. 69: 707-11. Potter, D.A., D.L. Wrensch and D.E. Johnston. 1976. Aggres­ sion and mating success in male spider mites. Science 193: 160-61. Potter, D .A. and D.E. Johnston. 1978. Raillietia whartoni sp. n. (Acari : Mesostigmata) from the Uganda kob. J. Para­ sitology 64: 139-42.

v Potter, D.A. 1978. Functional sex ratio in the carmine spider mite. Ann. Entomol. Soc. Am. 71: 218-22. Potter, D.A. 1978. Reproductive behavior and sexual selection in tetranychine mites. In Recent Advances in Acarology. Academic Press, New York. In Press. Potter, D.A. and D.L. Wrensch. 1978. Interrupted matings and the effectiveness of second inseminations in the two- spotted spider mite. Ann. Entomol. Soc. Am. In Press.

FIELDS OF STUDY Major Field: Entomology Area of Specialization: Acarology. Behavior and ecology of plant-feeding mites. Professors Donald E. Johnston and Dana L. Wrensch.

vi TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ...... iii VITA ...... v LIST OF TABLES ...... ix LIST OF FIGURES ...... x INTRODUCTION ...... 1 Chapter 1. REPRODUCTIVE BEHAVIOR AND SEXUAL SELECTION IN TETRANYCHINE MITES Introduction ...... 8 Methods . . 8 Results ...... 9 Discussion ...... 25 11. FUNCTIONAL SEX RATIO IN SPIDER MITES Introduction ...... 29 Methods ...... 30 Results and Discussion ...... 31 111. INTERRUPTED MATINGS AND THE EFFECTIVENESS OF SECOND INSEMINATIONS IN THE TWOSPOTTED SPIDER MITE Introduction ...... 50 Methods ...... 51 Results ...... 52 Discussion ...... 59 IV. FACTORS AFFECTING FREQUENCY AND INTENSITY OF FIGHTING Introduction ...... 64 Methods ...... 65

vii TABLE OF CONTENTS (CONTINUED) Page Results ...... 67 Discussion ...... 83 GENERAL DISCUSSION ...... 87 LIST OF REFERENCES ...... 94

viii LIST OF TABLES

Table Page 1. Tests of assortative vs. non-assortative mating in wild-type (+) and albino (a) T. urticae ...... 11 2. Proportion of female T. cinnabarinus guarded by males as a function of time before ecdysis .... 16 3. Size and success of male Tetranychus cinna­ barinus in aggressive encounters ...... 24 4. Mating success of old, experienced vs. young virgin male Tetranychus urticae ...... 26 5. Multiple regression of the proportion of multiply guarded females on male density and functional sex ratio ...... 45 6. Comparison of effectiveness of first (albino) and second (wild-type) matings early vs. late in oviposition period of doubly inseminated females ...... 57 7. Effects of sguare root (/x+1) transformation on Bartlett's x statistic for homogeneity of sample variances ...... 68 8. Analysis of variance: summary table for 60 one- hour observation periods ...... 6 9 9. Total number of aggressive behaviors ...... 70 10. Total aggressive behaviors per male ...... 71 11. Total weighted aggressive behaviors ...... 7 2 12. Weighted aggressive behaviors per male ...... 73 13. Success of guarders in aggressive encounters over old vs. young females ...... 79 14. Overall fighting success of guarding males ...... 80

ix LIST OF FIGURES

Figure Page 1. Male T. urticae in guarding position alongside quiescent female deutonymph ...... 13 2. Male T. urticae in typical guarding position on top of quiescent female deutonymph ...... 14 3. Aggressive encounter between 2 T. urticae males for possession of a quiescent female deutonymph ...... 18 4. Changes in functional sex ratio (= no. sex­ ually capable males : no. available females) of Tetranychus cinnabarinus through a hypothe­ tical 48-day colonizing episode ...... 34 5. Changes in functional sex ratio of T. cin­ nabarinus during a colonizing episode on bean plants ...... 37 6. Numbers of adult males and quiescent female deutonymphs during a colonizing episode on bean plants ...... 39 7. Numbers of singly guarding, multiply guarding and wandering males and of quiescent female deutonymphs during a colonizing episode on bean plants ...... 42 8. Relationship between time of interruption of the first copulation and the effectiveness of the second mating ...... 55 9. Male and teneral female T. cinnabarinus in copula ...... 63 10. Mean intensity of aggressive encounters in controls, and in presence of exuviae, young females, and old females ...... 77

x Figure Page 11. Frequency distributions of encounters in­ volving males guarding young vs. old females ... 82

xi INTRODUCTION

Changes in agricultural practices since World War II have seriously increased the potential for spider mite out­ breaks. The heavy and sometimes needless reliance on organo­ phosphorous and chlorinated hydrocarbon insecticides has resulted in the destruction of populations of predatory ar­ thropods and permitted spider mites to become pests of major importance (Chant 1966, Huffaker et al. 1970, Mcmurtry et al. 1970). Associated with the intensive application of pes­ ticides and fertilizers have been changes in host plant phys­ iology, and much evidence exists that mite fecundity has been enhanced by improved nutritional conditions (Garman and Kene­ dy 1949, Huffaker and Spitzer 1950, Rodriguez 1951, 1958, Hueck et al. 1952). These problems have been further com­ pounded by the facility with which spider mites develop resistance to chemical acaricides (Helle 1962, 1965). These difficulties have stimulated a renewed interest in tetranychid biology, and the resultant search for outbreak- causing factors and for better means of control has generated a vast literature. Nonetheless, basic studies of spider mite behavior, nutrition, physiology and diapause have lagged be­ hind, and in many areas are lacking or inadequate (Huffaker

et al. 1970). 2

In this dissertation I present the results of a compre­ hensive analysis of male precopulatory behavior in the two- spotted spider mite Tetranychus urticae Koch and the carmine spider mite T. cinnabarinus (Boisduval). In addition to pro­ viding new information on spider mite reproductive biology, my experiments have shown these mites to present an ideal system for the study of intrasexual competition and strate­ gies for maximizing fitness. Biology The general biology of tetranychoid mites was reviewed by Boudreaux (1963) and van de Vrie et al. (1972). The pre­ sent review will deal specifically with the twospotted spider mite complex (including T. urticae and T. cinnabarinus) and will be restricted to those aspects pertinent to the under­ standing of spider mite reproductive biology. Spider mites are minute, phytophagous, colonizing organ­ isms with a short life cycle (minimum ca. 10 days) (Helle and Overmeer 1973) and a high intrinsic rate of increase (Laing 1969, Wrensch and Young 1975). Reproduction is by arrhenoto- kous parthenogenesis, with virgin females producing only haploid male progeny and inseminated females producing both haploid male and diploid female offspring. While some degree of variation in sex ratio has been reported for different populations, females generally outnumber males by about 3:1 (Laing 1969, Overmeer 1972, Wrensch 1979). 3 After eclosion both sexes pass through three immature instars- the hexapod larva, the octapod protonymph and the penultimate instar, or deutonymph. Each of these active, feeding instars is concluded with a quiescent period during which the mite fastens itself to the leaf surface with silk and undergoes the molt to the next life stage. In the female, the final quiescent period lasts about 24-28 hours. Within a cohort males develop more quickly and emerge while females are still undergoing the final molt. Guided by a sex pheromone (Cone et al. 1971a,b), wandering males lo­ cate the pharate females, where they normally remain in at­ tendance until the adult female emerges and mating is accom­ plished . As she nears ecdysis, the smooth integument of the molting female becomes wrinkled and silvery, due apparently to air trapped between the new and old cuticles (Laing 1969) . Often the guarding male will help to pull the old cuticle from the ecdysing female with his front tarsi. Mating nearly always occurs as soon as the female is free from the exuviae. This prolonged precopulatory guarding enables a male to retain access to a potential mate until she becomes recep­ tive. In this respect, it is similar to the sequestering of females by certain male insects (Parker 1970) , amphipods (Hynes 1955), ticks (Wilkinson 1953) and aquatic crabs (Nou- vel and Nouvel 1937, Hartnell 1969). Spider mites differ from most insects, however, in that generally only the first mating is effective (Boudreaux 1963, Helle 1967, Potter and Wrensch 1978). Hence, to mate successfully, a male must gain control of virgin females prior to ecdysis. The production of the sex pheromone may be linked to the formation of oocytes within the quiescent female (Cone et al. 1971b). Regev and Cone (1975, 1976) analyzed crude ether ex­ tracts of quiescent female deutonymphs of T. urticae and dis­ covered the presence of the sesquiterpene alcohols farnesol and nerolidol. Subsequent bioassays with adult males provided convincing evidence that these substances are components of the spider mite sex attractant. The silken webbing laid down by the females prior to becoming quiescent appears to facilitate mate-finding by males. Penman and Cone (1972, 1974) reported that the pre­ sence of female webbing induces increased linearity in male searching patterns and increases male attraction to quiescent females. They suggested that female web may serve as a sub­ strate for the sex attractant. These studies, however, used only a few mites per trial, and it is questionable whether webbing is important for mate-finding on crowded, heavily webbed leaves. Spider mite silk is produced by a pair of palpal silk glands and is secreted through a pair of palpal eupathidia (Grandjean 1948, Alberti and Storch 1974). The webbing, a fibrous protein (Hazan et al. 1975b), is produced by all active life stages (Hazan et al. 1974). In addition to its 5 proposed role in facilitating mate-finding, silk is of im­ portance in many other aspects of spider mite life. Ewing (1914) recognized its role in the dispersion of colonizers, in facilitating movement about the host plant, in anchoring and protecting eggs, and in attaching the claws to the sub­ strate prior to a molt. Hazan et al. (1975a) proposed that webbing may have hygroscopic properties and function as a humidity regulating mechanism, increasing egg hatchability at very low or very high relative humidity. Webbing may also interfere with the movement and feeding of predaceous Acari (McMurtry et al. 1970). A suite of interesting behavioral adaptations is ex­ pressed during the period when males attend a quiescent fe­ male deutonymph. In an early, but excellent account of spider mite natural history, Perkins (1897) reported the occurrence of vigorous agonistic encounters between competing males. Quoting from the notes of a co-worker, Dr. J.H. Worchester, Perkins provided the following account of this behavior: "For the most part the males are very peaceable, even in the presence of the female, whose favors both are seeking. But occasionally I have seen one male chase away another which did not think it best to contend, but did not run far, as there was no pursuit. Sometimes, however, there was apparently a fight. At first I noted only that they stood opposite each other, very near, face to face. Then one would suddenly dart at the other, then as suddenly dart back. I looked to see them grapple and bite each other, but there was nothing of the sort. Indeed I do not suppose that they have any apparatus to bite with. Presently I caught sight of a sudden momentary flash of lightening passing between them; and finally I made out that they were using their tongues or suction tubes, which 6

gleamed with a sudden flash as they were quickly thrust out and as quickly withdrawn. Finally I saw them appar­ ently grapple by their tongues, and pull back as if each were trying to pu-11 the other's tongue by the roots. But I could never see that any lasting harm resulted, though sometimes one of the two would seem to have had enough of it, and would withdraw as it were for repairs, thrusting out and drawing in his tube by himself, as if it gave him some pain and needed a little nursing. In a short time he was running about again. Sometimes two spiders seemed to have a special spite against each other, and whenever they met a more or less decided contest would ensue. But these were only occasional phenomena. In general, the communi­ ties of spiders, young and old, male and female, seem to dwell together in perfect harmony." Concurring with Worchester's final impression, Perkins himself concluded that "the usual disposition of the red spi­ der is friendly and peaceable. They are obviously fond of each other's society and rarely exhibit any temper or differ­ ence of opinion." Ewing (1914) questioned the accuracy of these observa­ tions, dismissing Perkins' description of male aggression as being "without foundation". Despite brief mention by sever­ al other authors (Smith et al. 1967, Cone et al. 1971b), the adaptive significance of this intriguing behavior has remained unstudied. My observations indicate that aggressive interactions between competing males are common, and determine the ultimate success or failure of males in mating. I present here an analysis of precopulatory guarding and aggression in male spider mites, and examine the role of this behavior in the spider mite mating system. This dissertation is divided into four interrelated chapters, each in the format of a separate paper. In Chap­ ter 1, I provide a detailed description of agonistic encoun­ ters between male mites, discuss factors that determine the outcome of aggressive disputes and present data that demon­ strate the importance of this behavior for male mating suc­ cess. In Chapter 2 I examine a number of previously unrecog­ nized factors contributing to the intense sexual selection inherent in this mating system, and discuss how certain com­ ponents of spider mite precopulatory behavior contribute to the overall success of these mites as colonizing organisms. Chapter 3 concentrates upon the frequency and consequences of interrupted matings and the effectiveness of second in­ seminations. These data are interpreted in light of fertili­ zation and male precopulatory behavior. Chapter 4 considers how differences in male density and in the nearness to recep­ tivity of the guarded female affect the frequency and inten­ sity of male aggression, the responsiveness of guarding males to intrusions and the outcome of contests involving guarders. The dissertation is concluded with a general discussion. CHAPTER 1 REPRODUCTIVE BEHAVIOR AND SEXUAL SELECTION IN TETRANYCHINE MITES

In this chapter I describe the precopulatory guarding and aggressive behavior of male spider mites and demonstrate its consequences in terms of male reproductive fitness.

Methods

The initial observations in this study were based on a large greenhouse culture of T. cinnabarinus (OSU strain) main­ tained on kidney bean plants. Infested leaves, when detached and pressed onto water-soaked cotton in 10cm x 20cm plastic trays, provided a convenient means of observing courtship behavior and male aggression in situ. Mites from this colony have been the object of previous investigations (Mitchell 1973, Wrensch and Young 1975) and their fecundity and growth characteristics are known. In my studies, T. cinnabarinus were used primarily in those experi­ ments requiring an intact population on host plants. In order to evaluate any reproductive advantages attained by guarding and aggression, it was necessary to distinguish between male competitors and to determine with certainty which

8 males were successful in mating. To facilitate this, I uti­ lized colonies of sexually compatible albino and wild-type T. urticae (Sambucus strain). These were maintained on de­ tached leaf cultures in the laboratory. T. cinnabarinus and T. urticae are sibling species with virtually identical reproductive behaviors. Generalizations based on experiments with one species are therefore applica­ ble to the other. Most laboratory experiments were conducted on 12 or 17mm bean leaf discs, cut with a cork borer and pressed onto wet , o cotton. These were incubated at 27 - 1 C. under 24 hr. fluor­ escent illumination. By this technique as many as 128 tiny arenas could be maintained in a single 25 x 32cm plastic con­ tainer. Females were always transferred as active, rather than quiescent deutonymphs, and allowed to enter quiescence on the test discs. This permitted their positions to be un­ disturbed and their webbing intact, and ensured that male response would be as normal as possible.

Results

Before proceeding with other experiments, it was neces­ sary to determine if matings between the mutant and wild-type T. urticae stocks would be assortative or non-assortative. A total of 70 active albino deutonymphs was isolated on 17mm leaf discs, 10 females to a disc. Each disc was provided with 10

5 albino and 5 wild-type males. An equal number of discs was prepared using wild-type females. The males were removed after 3 days and the females allowed to oviposit. Because albino is a simple recessive, examination of female pro­ geny from albino mothers revealed the genotype of the fa­ ther. For crosses with wild-type females all daughters were wild-type, but some heterozygous and some homozygous. These were removed as quiescent deutonymphs and placed individual­ ly on leaf discs. If they produced all wild-type male pro­ geny they were descendents of a wild-type father,' while both wild-type and albino male progeny indicated the mother was descended from an albino male. Mating between the two geno­ types is non-assortative (Table 1). Therefore, any differen­ tial mating success between two males of opposite genotypes can be attributed to physical or behavioral differences be­ tween the individuals, and not to some inherent advantage of one genotype over the other. These data are in contrast to those of Dielman and Overmeer (1972) , who found that in two other strains of T. urticae, White eye-1 and Kelthane-R, fe­ males showed a strong mating preference for males of their own genotype, and that even when matings between opposite strains were accomplished, the efficiency of actual fertili­ zation was low. Guarding and Aggression: Male response toward quies­ cent female deutonymphs was described briefly by Ewing (1914), Cone et al. (1971a) and more completely by Potter 11

Table 1. Tests of assortative vs. non-assortative mating in wild-type (+) and albino (a) t . urticae.

Parental Genotypes no. of no. of no. of no. of +

TEST I 0 70 35 35 TEST II 70 0 35 35

Mating Results____ no. of 99 fertilized no. of 99 fertilized by + male by a male observed (expected) observed (expected) x (1 d.f.)

I 29 (35) 41 (35) 2.06 (n.s.) II 26 (31) 36 (31) 1.62 (n.s.) 12 et al. (1976a,b). A male who discovers an unattended deuto­ nymph usually initiates characteristic guarding behavior, remaining motionless beside the female (Fig. 1) or crawling on top and assuming a resting position (Fig. 2). The guarder contacts the mid-dorsal region with his palps and intermit­ tently taps and strokes the dorsal cuticle with his front tarsi.

Although guarding males may occasionally leave quiescent deutonymphs, they seldom wander more than 2 mm away and usu­ ally return in a short time to re-establish the guarding position. These excursions sometimes include brief periods of feeding. The ability to distinguish between females that have only recently become quiescent and those on the verge of ecdysis would enable a male to avoid waiting many hours be­ fore each mating opportunity. A male who searched out fe­ males nearing emergence would be more efficient than a non- selective male, and would mate more often and have a

relatively higher fitness. To investigate the consistency of male attraction early and late in the quiescent period, 66 active female deuto­ nymphs (T. cinnabarinus) were placed on leaf discs with 3 males each. Once pharate, females were checked for guarders every half hour until ecdysis. During the first 90 minutes of quiescence females were found to be unguarded during 87 of the 198 individual checks, but during the final 90 13

Fig. 1. Male T. urticae (left) in guarding position alongside quiescent female deutonymph. 14

Fig. 2 Male T. urticae in typical guarding position on top of quiescent female deutonymph. Note position of male's mouthparts and front tarsi. 15 minutes were left unguarded for only 8 of the 198 observa­ tions. These data are consistent with an earlier study by Cone et al. (1971b) in which male response was erratic early in the female's quiescent period but became more constant as ecdysis approached. In a mite population, male choice is not restricted to a single female. To investigate the incidence of guarding under more realistic conditions, I placed 5 fertilized T. cinnabarinus females on each of twenty-two 17 mm leaf discs, and allowed their progeny to accumulate until there were large numbers of quiescent female deutonymphs, adult males and other life stages on each disc. The locations of 220 quiescent females were mapped, and then checked for male guarders every half hour until ecdysis. The data (Table 2) indicate that a quiescent female becomes increasingly at­ tractive to male guarders as she grows older. At any given observation the older deutonymphs are nearly always guarded, while younger ones are left unguarded or guarded inconsis­ tently, supporting the hypothesis that males pass over the newly quiescent females and search out the older ones to guard (Potter et al. 1976a). Further evidence supporting this hypothesis will be presented in Chapter 2. It is likely that in addition to perceiving the sex pheromone, the guarding male's stroking and tapping of the female provides information concerning the condition of the cuticle and the nearness of ecdysis. Table 2. Proportion of female T. cinnabarinus guarded by males as a function of time before ecdysis. An arcsin transformation was applied prior to fitting the linear regression.

Hours until ecdysis 24-28 20-24 16-20 12-16 8-12 4-8 0-4

n 12 47 83 113 134 178 209 Females guarded .583 .596 .590 .717 .694 .746 .756 y= -.504(x) + 61.893 r= -.946 p< .01

i—1 cr> 17

Although there was an excess of males on each disc, 22 females, or roughly 10% of the total, remained unguarded throughout the quiescent period. Whether or not these failed to produce the. sex attractant or were overlooked for other reasons is unknown. Intense fighting may occur when two or more males at­ tempt to guard a single female. When a newcomer is attracted to an attended deutonymph, he may, upon detecting the resi­ dent guarder, leave without incident. More often he attempts to assume a guarding position or tries to dislodge the guarding male. Should the original male fail to respond, the two competitors may become coguarders, vying for a favorable position on top of or alongside the female. Commonly, how­ ever, the intruding male is threatened or attacked. Fights generally begin when the guarding male backs down from the quiescent female and confronts the intruder with outspread front tarsi and extruded cheliceral stylets. The threatened male either retreats or responds by adopting a similar stance (Fig. 3). The combatants circle and rush at each other, flailing their forelegs and jousting with the extruded stylets. When contact is made there is considerable grappling and pushing. Although fights are usually brief, some­ times vigorous sparring may continue for up to 1 or 2 minutes. In such lengthy encounters the actual fighting may be inter­ rupted and resumed several times. 18

Fig. 3. Aggressive encounter between T. urticae males for possession of a quiescent female deutonymph (left). Note the typical fighting stance of the

lower male. 19 Despite the rugged nature of these encounters, injuries are rarely inflicted. On several occasions, however, I ob­ served a male puncture the integument of his opponent with the extruded stylets. Mites so injured appeared crippled and usually died within a short time. Although very few lethal encounters were observed in progress, dead, deflated males were not uncommon in the vicinity of guarded females, sug­ gesting that fatalities may occur more frequently than direct observations would seem to indicate. After most fights, the loser wanders off and the winner returns to the female. However, as reported previously by Cone et al. (1971b), the apparent winner may occasionally move away from the site of the contested female. It is possi­ ble that such males become disoriented during a fight and are unable to relocate the deutonymph. If neither male is able to drive away the other, the two may guard the female jointly. Additional fighting between these males or with other intruders may break out at a later time. On numerous occasions in this study, male mites were ob­ served to use their palps to apply strands of silk directly to the gnathosoma and forelegs of the opponent. Sometimes an individual becomes so badly entangled that his movements are impeded and he is forced to retreat to clean himself. After removing the silk by vigorously rubbing the palps and tarsi, the vanquished male may initiate another challenge. These 20 later encounters are typically more brief, and the victor is usually the same. The Analysis of Male Behavior: Steadfastly guarding a pharate female until ecdysis or wandering about the leaf challenging other guarders represent two alternative tactics by which male spider mites may successfully mate. Selective pressures should favor males whose behavioral patterns yield the maximum reproductive return for the effort expended. An analysis of male fitness leads to the following questions: 1. Does becoming the first guarder ensure, or significantly increase the chances that a male will mate? 2. By how much does the presence of a coguarder reduce the chances that the first guarder will mate? 3. Of what importance is male size in aggressive encounters? 4. Does age, and thus experience, yield a competitive advantage? Because prolonged precopulatory guarding reduces the rate at which other females are encountered, for this behavior to persist it would be expected to provide a greater probability of mating than would withdrawal for further searching. To in­ vestigate the relationship between guarding status and mating success, 70 active female deutonymphs (T. urticae) of the al­ bino genotype were placed.on individual 12mm leaf discs. An effort was made to select only larger, mature deutonymphs. Nearly all became quiescent within 6 hours. Twelve hours af­ ter introduction of the females, a single adult male, 21 alternately wild-type or albino, was placed on each disc. Discs were sufficiently small that discovery of the female within a short time was inevitable. Twenty-four hours after the start of the experiment, a second male of the opposite genotype was introduced to each disc. The relationship between the two males and the quies­ cent female was checked every half hour until the female's emergence, and all instances of guarding and coguarding were recorded. After ecdysis, the females were left to oviposit and the males were removed. The progeny were reared to the deutonymph stage, enabling positive differentiation of the sexes. Albino daughters indicated a mating with an albino male; wild-type daughters resulted from a wild-type father. . On nearly all the discs (59/62), the first male to be introduced quickly discovered the quiescent female and be­ came the initial guarder. Aggressive activity was at its highest level shortly after the introduction of the second male, resulting in guarder-deutonymph relationships that were relatively unstable. However, on about half (28) of the discs, by the time ecdysis was imminent one male had estab­ lished dominance and remained as the sole guarder. Ninety- three percent (26/28) of the males who guarded alone during the final hour of quiescence successfully inseminated the female. On the remaining discs, neither male was able to estab­ lish dominance and drive away the other. In most cases 22 of coguarding, the male who had been the last single guarder occupied the preferred position on top of the female. Sixty- three percent (17/27) of the males in this position just prior to the female's emergence were successful. Being the first male to discover and guard a quiescent female appears to be advantageous. Seventy-one percent (39/55) of the females were ultimately inseminated by their first guarder. While a male who keeps coguarders away and guards alone at ecdysis nearly always mates (9 3% success), the chances of a male who tolerates or is unable to repel a coguarder are considerably reduced [mating success of single guarders does not differ significantly from an expectation of 100% (x2= 0.91, d.f.= 1, p= .340); that of coguarders who occupied the preferred position did not differ significantly 2 from an expectation of 50% (x = 0.13, d.f.= 1, p= .718)]. When more than one male is present at ecdysis, there is a scramble as each attempts to maneuver into copulatory po­ sition beneath the female. Sometimes the mating act itself may be disrupted. I have observed a second male harass a male in copula until he disengages from the female and backs away. If the interruption occurs at or shortly after the onset of copulation, the intruder may replace the original male and mate successfully (see Chapter 3). A guarder who is forced to engage a coguarder in combat just as the female is emerg­ ing may in the meantime lose the female to a second intruder. Therefore, it seems a male may maximize his chances of 23 inseminating the quiescent female he is guarding by estab­ lishing his dominance over other interested males before the female ecdyses. To investigate the relationship between male size and fighting success in spider mites, heavily infested bean leaves from the greenhouse culture of T. cinnabarinus were scanned for fighting males. When a male was observed to score a decisive victory in an aggressive encounter, both comba­ tants were removed and mounted in Berlese's medium on micro­ slides. This procedure was continued until 20 pairs of males had been collected. These specimens were examined under oil immersion and measured with an ocular micrometer. Slide-mounting unavoidably distorts specimens to vary­ ing degrees; therefore simple measurements of body length and width may be misleading. To provide more reliable criter­ ia, characters selected for size comparisons were length of tarsus 1, length of tarsus 4 and the distance between the second pair of prodorsal setae. The data are summarized in Table 3. Based on these criteria, winning males were con­ sistently larger (Potter et al. 1976a,b). Finally, experiments were conducted to determine if old, experienced males are able to outcompete younger, virgin op­ ponents. Large numbers of 1-2 day old virgin albino and wild- type T. urticae males were obtained by rearing eggs from unfertilized females. These were paired with older (6-7 days) experienced males taken from crowded, mixed-sex cultures, and Table 3. Size and success of male Tetranychus cinnabarinus in aggressive encounters.

No. of fights in which winner was: X2 (1 d.f.)* Character Larger Equal Smaller size Length tarsus 1 13 2 3 6.25 (p=.012) Length tarsus 4 16 1 1 13.24 (p=.0003) Distance between 11 2 3 4.57 (p=.033) prodorsals II

Ignoring cases of equal size 25 placed together on 12 mm leaf discs containing a single qui­ escent female deutonymph (albino). The study consisted of 110 replications, 55 in which the older male was wild-type and 55 in which he was albino. After ecdysis and mating, the males were collected and measured and the progeny reared to determine which male had been successful. The data, summarized in Table 4, indicate that older, experienced males had a greater overall rate of success than did their younger, virgin adversaries. Size was again found to be important, with the larger of the two males inseminat­ ing the female in the majority of cases. The non-significant interaction chi-square value suggests that the effects of age and size are additive. Males who possessed both of these at­ tributes, that is, older experienced males who were also the larger of the pair, were overwhelmingly more successful. Smaller males who are older and more experienced, however, can apparently compete equally with larger, but virgin op­ ponents . Discussion In males of polygynous animals, those characters that have epigamic importance or that function in intrasexual rivalry are often particularly well developed (Williams 1975) . Commonly, selection has favored a marked increase in male size, and examples of the relation between size and social dominance may be drawn from all classes of vertebrates (Collias 1944). 26

Table 4. Mating success of old, experienced vs. young, virgin male Tetranychus urticae. Numbers represent success­ ful inseminations. The length of tarsus 1 was used as a size criterion.

Old Young Exper. Virgin Z

Larger 32 24 56

Smaller 23 13 36

£ 55 37

Old, Experienced vs. 4.50 (p= 0.034) Young, Virgin

Larger vs. 5.56 (p= 0.018) Smaller

Interaction 0.06 n.s . 27 Despite the relative paucity of analytical studies of the invertebrates, it is evident that agonistic interactions between males occur within many groups of arthropods (Rich­ ards 1927, Wilson 1975). In a noteworthy study on aggression in field crickets, Alexander (1961) determined that when other factors (such as age and previous experience) are not involved, larger males usually dominate smaller ones in en­ counters. Size has also been shown to be of importance in agonistic encounters between male crayfish (Bovbjerg 1956), tenebrionid beetles (Hamilton et al. 197 6) and anthophorid bees (Alcock et al. 1977). The discovery that larger male spider mites are, on the whole, able to defeat smaller ones is not surprising, given the rugged, physical nature of the conflicts. However, in light of previous studies on fitness in spider mites, this finding, and the conclusion that selection should favor an increase in male size are paradoxical. Sexual dimorphism in Tetranychus is striking, males being only about one-sixth as large as fe­ males by weight. Mitchell (197 3) reported that males barely grow after emergence, a phenomenon he attributes to a system in which males that spend their time searching for females father more offspring than those that take time to feed and grow. It is also likely that small size permits faster devel­ opment, enabling a smaller male to emerge and begin guarding even while larger competitors are still undergoing the final molt. The optimum size for a male spider mite, and the amount 28 of energy he must invest in feeding and growth to maximize his fitness are undoubtedly determined by a trade-off between these and other factors. Being a guarder seems to endow a male with some degree of increased aggressiveness and tenacity in fighting, which enhances the probability of his successfully defending the female. However, threats and mild, one-sided aggression are often adequate to repel an intruder, enabling many contacts between male mites to be terminated short of actual combat. It seems probable that within a spider mite colony, male aggressive behavior does not generate any specific dominance order per se, but rather gives rise to a random series of initial confrontations. The guarding of pharate females re­ presents an ephemeral form of territoriality, and the pro­ curement and retention of such a territory is to a large ex­ tent dependent upon first discovery, male size, age and ex­ perience, and the ability to repel other male competitors. CHAPTER 2 FUNCTIONAL SEX RATIO IN SPIDER MITES

The evolution of prolonged precopulatory guarding and male aggression is generally associated with conditions of high male density and intense competition for matings (Par­ ker 1970, 1974). In most spider mite populations, however, females outnumber males by about 3:1 (Laing 1969, Overmeer 1972, Wrensch 1979). It is important, therefore, to address the question of why male aggression should be selected for in a mating system where potential mates are seemingly abundant. Although male spider mites are sexually capable through­ out their adult lives (up to ca. 3 weeks), females are gener­ ally available and attractive only during their final quies­ cent period (1-1.5 days), after which they are mated and no longer receptive. Unlike females which migrate after mating (Mitchell 1973), older males usually remain on the host plant (Hussey and Parr 196 3, Potter, unpubl. data) where they com­ pete with younger males. This led to the hypothesis that males would accumulate during a colonizing episode until they great­ ly outnumbered the available females, making the "functional sex ratio" (no. sexually capable males : no. available fe­ males) increasingly skewed towards males.

29 30 In the present study, changes in the functional sex ra­ tio of a population of Tetranychus cinnabarinus were monitored throughout an episode of colonization and host destruction. These data are compared with a model constructed from life table data. The influence of functional sex ratio changes on male guarding frequency and on the fertilization status of dispersing adult females is then discussed. Methods Mites were obtained from a greenhouse culture maintained on kidney bean plants at 22°-30°C, 35-45% RH. I transferred one hundred 3-5 day old fertilized females to each of 3 pots of beans (48-50 plants per pot), placing the mites on the pri­ mary leaves. The pots were arranged linearly, with the foliage contiguous and the bases surrounded by individual water bar­ riers. Starting with the emergence of the adults (11 days after initial infestation), the plants were successively sam­ pled at 2-3 day intervals over a 30-day period. Six leaves were randomly selected from each of the 1° (primary) through 4° (quaternary) leaf levels; these were gently pressed onto water-soaked cotton in plastic trays so that, for each leaf level, 3 upper and 3 lower surfaces were exposed. As adult males were counted at each sampling period they were classified into 1 of 3 categories: singly guarding a quiescent female; multiply guarding (guarding a female jointly with one or more other males); or wandering (not engaged in guarding). In addition, the numbers of singly guarded, 31 multiply guarded and unguarded quiescent deutonymphs were recorded. Relationships between male density and functional sex ratio (independent variables) and frequency of multiple guarding (dependent variable) were analyzed by stepwise mul­ tiple linear regression. On a second set of similarly infested bean leaves, tene- ral (newly emerged) females were collected from the leaf sur­ faces, and dispersing females were collected from clusters at the leaf tips and water-filled pans beneath the plants. To determine their fertilization status, these were isolated on individual 17mm leaf discs and maintained in an incubator at 27°C. Fertilized females were recognized by the presence of daughters among their progeny. Results and Discussion A M°del of Functional Sex Ratio Changes: Using daily egg production data for T. cinnabarinus from Hazan et al. (1973), a predictive model of functional sex ratio changes through a hypothetical colonizing episode was arithmetically generated. I assumed an initial infestation of 5 fertilized females, constant temperature (24°C) and humidity (38%), a tertiary sex ratio of 3 females : 1 male, an unlimited resource, and no dispersal of immatures or adult males. Male developmental time was set at 10 days, followed by a 2 week adult life span. Ten days were allowed for the females to reach the quiescent deutonymph stage, followed by a 1 day preoviposition period. These times are consistent with previously reported 32 developmental rates for T. cinnabarinus (Hazan et al. 1973, Wrensch and Young 1975).

♦ * Combined egg production, number of immature progeny, and total number of adult males and quiescent female deutonymphs were computed and tabulated for each day of a 48-day episode of host destruction (4 generations). Summing across over­ lapping generations, I determined the daily functional sex ratio. The model (Fig. 4) predicts that as the colonizing epi­ sode progresses, the functional sex ratio oscillates with de­ creasing amplitude and finally approaches unity. With the mat­ uration of the first males (day 11 from initial coloniza­ tion) , the functional sex ratio reflects the tertiary sex ra­ tio with about 3 quiescent female deutonymphs for each adult male. Males accumulate between days 11-19 until they outnum­ ber available females by almost 4:1. The first F 2 progeny ma­ ture on days 20 and 21, and within this pulse of progeny, female deutonymphs again exceed males by about 3:1. These relatively great numbers of newly quiescent females cause the functional sex ratio to drop below unity (days 20-22). This basic pattern is repeated twice more within the 48-day time limit. Each time, the functional sex ratio rises with the accumulation of adult males (days 23-30, 36-41), then drops when the next pulse of female progeny approaches ma­ turity. With each subsequent generation, the overall number of mites increases, hence, the daily summation of males 33

Fig. 4. Changes in functional sex ratio (= no. sexually capable males : no. available females) of Tetra- nychus cinnabarinus through a hypothetical 4 8-day

colonizing episode. 13 (f) -n 5 - 1 — I — Days 4 U> 35 changes the functional sex ratio at a reduced rate, and the curve peaks at a lower value. Functional Sex Ratio in an Actual Colonizing Episode: At each successive sampling of the infested plants, data from the 4 leaf levels were summed and the ratio of total adult males to total quiescent female deutonymphs computed. For a- bout 30 days after the initial infestation, these ratios (Fig. 5) follow closely the changes predicted by the model. A lag between the experimental and predicted curves suggests that developmental rate was slower than was assumed for the model. To clarify how changes in their respective numbers af­ fected the functional sex ratio, the counts of adult males and quiescent female deutonymphs were plotted (Fig. 6) . As expected, available females initially outnumber males (day 11), reflecting the tertiary sex ratio of the F^ progeny. Functional sex ratio (Fig. 5) rises steadily, reaching unity on day 13 and peaking at about 3.3 males/available female on day 21. The initial rise (days 11-13) is due to the accumula­ tion of males; however, male number remains nearly constant between days 13-24, and the continuing rise in functional sex ratio is due instead to a decrease in available females (Fig. 6) . Egg production is highest early and tapers off as an ovi­ positing female ages (Wrensch and Young 1975). This explains why, for each generation, the number of female deutonymphs maturing daily will decline after an initial pulse of progeny. 36

Fig. 5. Changes in functional sex ratio of T. cinnabarinus during a colonizing episode on bean plants. 6

5

4

3

2

1 — U

Days

OJ 38

Fig. 6. Numbers of adult males and quiescent female deuto- nymphs during a colonizing episode on bean plants. The periods when sexually capable males outnumber available females are indicated by shading. Number of Mites 300 400 200 500 100 vial Females Available dl Males Adult

Days 40 Each day, the number of males added to those already present will also decline. It is likely that mortality and migration upward onto petioles and stems may be balancing this accumu­ lation, resulting in the fairly constant number of males pre­ sent until the maturation of the F2 adults. As the second generation approaches maturity, great numbers of deutonymphs enter quiescence on the leaves, caus­ ing the functional sex ratio to drop below 1:1 (days 24-26).

With the accumulation of F 2 males, the male : available fe­ male ratio again rises; however, instead of continuing to oscillate as in the model, the curve (Fig. 5) climbs sharply upward (days 32-40). This coincides with a time when the resource base was deteriorating rapidly, leaves were becoming extensively web­ bed and chlorotic, and adult females were dispersing from clusters at the leaf tips. Unlike the model resource, which was assumed to be unlimited, the real plants could not sup­ port a third generation of mite progeny. Under such condi­ tions, larval and nymphal spider mites may abandon the host plant before maturing (Hussey and Parr 1963, Potter, unpubl. data). Those males remaining on the leaves increasingly out­ number the few remaining available females, and the function­ al sex ratio rises abruptly. Functional Sex Ratio and Guarding Frequency: Changes through time in the number of singly guarding, multiply guarding and wandering males are shown in Fig. 7. 41

Fig. 7. Numbers of singly guarding, multiply guarding and wandering males and of quiescent female deutonymphs during a colonizing episode on bean plants. Number of Mites 400 200 300 100 L, i ------adrn Males Wandering ------igy urig Males Guarding Singly utpy urig Males Guarding Multiply vial Females Available • — _ 7 fO 43 It is of interest that early in the colonizing episode (day 11) when quiescent deutonymphs are plentiful, only 31% of the males are engaged in guarding. Although there are enough males to guard 52% of the available females in the samples, only 16% of these deutonymphs are in fact guarded. On day 13, when adult male and available female numbers are equal, 6 0% of the males on the leaves are engaged in wandering, and only 40% of the quiescent deutonymphs are guarded. Despite the small proportion of quiescent female deuto­ nymphs guarded at one time, 74 of 75, or over 9 8% of the new­ ly emerged teneral adults collected during days 11-12 were found to be fertilized. The scarcity of unfertilized teneral females, and the high percentage of non-guarding males sug­ gest that guarding is directed predominantly at those deuto­ nymphs that are nearing emergence. Previous studies (Cone et al. 1971b, Potter et al. 1976a) demonstrated that quiescent female deutonymphs become increasingly attractive as they near ecdysis, and that males preferentially guard more mature deutonymphs. Therefore, even though quiescent female deuto­ nymphs outnumber males early in a colonizing episode, males are competing for only a small subset of these available fe­ males at any one time. Although becoming the initial guarder gives a male an ad­ vantage toward ultimately inseminating a given female (Potter et al. 1976a,b), guarding young deutonymphs would be a poor investment of time early in a colonizing episode when females 44 are abundant. As functional sex ratio rises, the probability of a wandering male finding an unguarded, nearly mature deu- tonymph is reduced, and guarding a broader spectrum of the females encountered would be expected to yield increasing reproductive returns. In support of this, the proportion of females guarded was positively correlated with functional sex ratio (r= .581, p<0.05). However, the most successful males will still be those whose behavior maximizes the fre­ quency of matings, either by finding and guarding or coguard- ing females near ecdysis, or by attacking and displacing males ales in possession of such females. Until maturation of the F^ progeny, mite density was relatively low (Fig. 6 , days 11-24) . Despite competition for the more mature deutonymphs and a functional sex ratio ex­ ceeding unity throughout most of this period (Fig. 9), multi­ ple guarding was very rare (Fig. 7 ). Throughout the coloniz­ ing episode, however, the proportion of multiply-guarded fe­ males was positively correlated with the functional sex ratio (r= .662, p<0.05). Multiple regression analysis shows that functional sex ratio and male density together account for 50.3% of the variance in multiple guarding (Table 5), with the majority of this (44%) contributed by the functional sex ratio. The residual variability may be due in part to the age-dependent differential attractiveness of quiescent deu­ tonymphs . 45

Table 5. Multiple regression of the proportion of multiply guarded females on male density and functional sex ratio.

Source Degrees of Mean Square freedom Regression 2 .005* Residual 10 .001 Partial Variable regression Standard coefficient error

Male density .00850 .00724 Functional sex ratio .02089 .00724

R2 = 0.503 *p < 0.05 46

Most multiple guarding occurred later in the colonizing episode (days 28-34) when mites were extremely numerous, the number of available females had peaked and was declining re­ lative to males, and the functional sex ratio was rising. In many instances the cuticle of the multiply guarded females had taken on the wrinkled, silvery appearance characteristic of the period just prior to ecdysis. Male density during this period was high, ranging from 2.5-6.0 males/sq. cm. on the upper leaves, yet a large proportion (39-52%) of the available females were found to be fertilized. It appears, then, that both single and multiple guarding are selectively directed at the more mature females, and that males will continue to search or take positions as coguarders in preference to guard­ ing the remaining, less mature deutonymphs. The frequent tactic of coguarding deserves further com­ ment. In the preceding chapter, it was shown that males occu­ pying positions as secondary guarders have a 37% chance of inseminating the female at ecdysis. This, together with the possibility of a disrupted mating (see Chapter 3) indicates that coguarders are often successful in taking over the fe­ male. By selectively attending females close to emergence, a coguarder would minimize the time between subsequent mating opportunities. Under high density, when the probability of encountering unguarded, mature female deutonymphs is low, it is likely that such behavior would yield more matings than would prolonged unselective guarding or continued searching 47 for unguarded, older females. Functional Sex Ratio and the Fertilization of Disper- sants: Deterioration of the host plants was accelerated by the rapid increase in mite numbers between days 24-28, so that by the time of peak abundance of quiescent females (day 28) the upper leaves had become heavily webbed and se­ verely damaged. Upon emergence, adult females congregated at the leaf tips in jostling masses from which they abandoned the plant in great numbers by dropping off or descending on strands of silk. This multitude of female dispersants matured and aban­ doned the host plants at a time when the functional sex ratio was male-skewed and rising. It was of interest to determine what percentage of them had been inseminated prior to leaving. Between days 30-33, I collected 75 adult female dispersants hanging from the strands at the leaf tips and 75 from the surface of water-filled trays placed beneath the plants. Seventy-two of the 73 surviving females from the strands, and all of the dispersants from the water were found to have been inseminated. If fertilized, a single female who arrives at a new re­ source can quickly give rise to a new colony. Therefore, it is important for female dispersants to mate before migrating. Mitchell (1970, 1973) considered the female-skewed tertiary sex ratio in spider mites as part of a suite of adaptations by which ovipositing females on a declining resource can 48 maximize the number of potential colonizers among their pro­ geny. He reported (Mitchell 1973) that immatures feed and grow within a "family territory", a small area delineated with silk by the mother, and that this afforded an increase in mate-finding efficiency and permitted a reduction in the proportion of male progeny. He reasoned that, because of their reduced numbers, males would be obliged to mate repeat­ edly to ensure the fertilization of all female dispersants. Mitchell's studies on family territory were conducted on fresh resources; however, the great majority of female dis­ persants abandon the host plant only after the leaves become extensively damaged and thickly covered with webbing (Hussey and Parr 1963). My observations suggest that under the high density conditions near the end of a colonizing episode egg distributions overlap and family territories are non-existent. The skewed functional sex ratio at the end of an episode of host destruction provides an alternative explanation for the efficiency of fertilization of female dispersants. It is the abundance of sexually capable males relative to available females and the focusing of male courtship behavior on the more mature deutonymphs that ensure that virtually all ten­ eral adult females are inseminated at ecdysis and quickly be­ come potential colonizers. The accelerated development of male spider mites and their guarding and defense of quiescent females appear to have evolved as a means of appropriating an optimal share of 49 potential mates by monopolizing access to them until they be­ come receptive. The intense competition for females close to ecdysis is compounded by the male-skewed functional sex ratio during most of a colonizing episode. The result is a mating system in which direct male-male encounters and aggression are common. CHAPTER 3 INTERRUPTED MATINGS AND THE EFFECTIVENESS OF SECOND INSEMINATIONS IN THE TWOSPOTTED SPIDER MITE

Although sole possession of an ecdysing female virtually ensures a male spider mite of successfully mating (Potter et al. 1976a,b), single guarding becomes difficult when condi­ tions are crowded and intrusions are frequent. As the ratio of sexually capable males to available females rises, the in­ cidence of multiple guarding increases (Potter 1978), so that late in a colonizing episode it is common for more mature fe­ males to be surrounded by several males. Insemination nearly always occurs at or shortly after emergence (Potter 1978) , and usually the male occupying the position on top of the female just prior to ecdysis is the first to mate (Potter et al. 1976a,b). At the onset of copula­ tion, other males commonly climb over or pull at the mating pair, attempting to maneuver into position between the coupled mites. This agitation sometimes disturbs the female, or causes the mating male to back away or attack the intruder. I have observed numerous intrusions in which the copulating male dis­ engaged from the female and was replaced by another male. In spider mites, sperm from a complete first mating generally precludes later inseminations (Boudreaux 1963,

50 51 Helle 1967). Little is known, however, of the consequences of interrupted matings or of their frequency in natural popu­ lations. I investigated the effectiveness of second insemina­ tions in the twospotted spider mite, Tetranychus urticae Koch, and related this to the time of interruption of the first copulation. In addition, I studied the effect of second mat­ ings on older, previously inseminated females and determined the frequency of double inseminations under crowded condi­ tions. In this chapter I report on the results of these in­ vestigations, and discuss their consequences in terms of fer­ tilization and male precopulatory behavior. Methods All experiments were conducted on detached kidney bean leaves or on 12mm or 17mm bean leaf discs maintained in the manner described in Chapter 1. The evaluation of double in­ seminations was facilitated by using the sexually compatible stocks of albino (white-eyed) and wild-type (red-eyed) T. urticae. Because albino is a simple recessive, examination of female progeny from an albino mother will reveal the fa­ ther's genotype. Double inseminations involving males of op­ posite genotypes result in mixed female progeny, which per­ mits the relative effectiveness of each mating to be deter­ mined . To ensure that the interpretation of double inseminations would not be distorted by differences in the compatibility of gametes, I compared the hatchability of eggs fertilized by 52 males of each genotype when mated with albino females. Twenty quiescent female deutonymphs were placed on each of two de­ tached bean leaves. Fifty adult albino males were added to one leaf, and 50 wild-type males to the other. Adult females were removed after 48 hours, isolated on individual leaf discs, and allowed to lay eggs for 3 days. These were reared to the deutonymph stage and sexed. The mean number of female progeny resulting from these matings was not significantly different between the groups (x = 12.71 for wild-type fathers, x = 12.33 for albino fa­ thers, t = .222). Overall, females comprised 72% of the hatched progeny from the wild-type matings and 73% from the albino matings; these proportions are not significantly dif­ ferent (x^= 0.12). Therefore, the number of female progeny of each genotype is a valid criterion for comparing the ef­ fectiveness of first and second matings involving albino and wild-type males. Results Time of Interruption and the Effectiveness of a Second Mating : To examine the relationship between the time of interruption and the effectiveness of second matings, newly emerged, virgin albino females were mated to 3-6 day old virgin albino males. The paired mites were either separated after 30, 60, 120 or 150 sec., or left undisturbed until the mating was completed. The females were then transferred to leaves containing large numbers of 3-6 day old virgin wild-type males, and observed until they had mated again. Fi­ nally, each doubly inseminated female was placed on an indi­ vidual leaf disc for oviposition, and transferred daily to a fresh disc. The outcome of those double matings resulting in mixed progeny is summarized in Fig. 8. There is a significant nega­ tive correlation between the duration of the initial mating and the effectiveness of the second insemination (r= -0.6419, P<0.01). It is interesting that when matings were interrupted after as long as 120 or 150 seconds, the mean percent of fe­ male progeny fathered by the second male was still high, 46.8 and 31.4% respectively. These data are in disagreement with those of Boudreaux (1963) , who reported that second matings were totally ineffective unless the first mating lasted less than 60 seconds. Nevertheless, some incomplete matings did to tally preclude second inseminations. For females in which the first mating was interrupted after 30 or 60 seconds, only 1 of 25 second matings yielded no progeny, but when first ma­ tings lasted 120 or 150 seconds, respectively 8 of 15 and 3 of 10 second matings were totally ineffective. The mean duration of uninterrupted first matings was 286 - 8 seconds (x - S.E., n= 12, T= 22°C). In all cases, second matings that followed a complete first mating were totally ineffective. This supports the earlier findings of Boudreaux (1963) and Helle (1967). 54

Fig. 8. Relationship between time of interruption of the first copulation and the effectiveness of the sec­ ond mating. Each point represents, for an individual female, the proportion of the total diploid eggs from the first 6 days of oviposition which were fertilized by sperm from the second male. Sample means are indicated by open circles, and only those cases in which both matings were effective are in­ cluded. An arcsine transformation was applied prior to fitting the linear regression. A test for de­ parture from linearity was not significant (p= 0.11). % % of Female Progeny Resulting from Second Mating uain f is Mtn Pir o nerpin (seconds) Interruption to Prior Mating First of Duration

Y=-0.i997X+ 63.8504 Y=-0.i997X+ rnfre Pretg (rsn ) (arcsine Percentage Transformed 56 Sperm Precedence in Doubly Inseminated Females: When matings are interrupted and followed shortly thereafter by a second insemination, spermatozoa from both males must be pre­ sent simultaneously within the lumen of the sperm receptacle. By comparing the female progeny of each genotype produced during the first few days with those produced later in ovi- position, I sought to determine if sperm from the first mat­ ing would take precedence in fertilizing the earlier eggs. The results of this analysis are summarized in Table 6. Except when the initial copulation was very brief (30 sec.), the proportion of progeny resulting from the first mating was significantly greater during the first three days of oviposi- tion than it was during the later 3-day period. Despite this priority in fertilization, however, the early preclusion of sperm from the second insemination was in most cases incom­ plete. In 27 of the 31 double inseminations studied, the first 10 female eggs deposited yielded some' progeny of each genotype. Effect of Second Matings on Old, Previously Inseminated Females; Following the same procedure used in the analysis for Table 6, the proportion of male progeny produced early vs. late in oviposition was compared. For 3 of the 4 sets of doubly inseminated females (those in which the first mating was interrupted after 30, 60 and 120 sec.), a significantly greater proportion of males was produced during the later period. This is consistent with an earlier study (Wrensch, Table 6. Comparison of effectiveness of first (albino) and second (wild-type) matings early vs. late in oviposition period of doubly inseminated females. Numbers of progeny represent a 3-day sum for the N ovipositing females within each set.

Early ? Later ¥ progeny progeny Time of a : + a : + 2 Interruption N (days 1-3) (days) X (1 df)

30 sec. 7 45 : 155 44 : 142 .08 (n.s.) (9-11)

60 sec. 10 193 : 176 54 : 149 35.35 (p<.001) (9-11) 120 sec. 7 143 : 89 72 : 82 8.33. (p<- 01) (7-9)

150 sec. 7 111 : 34 118 : 64 5.33 (p<.05) (4-6)

U1 58 unpubl. data) in which fully inseminated females, after 10 - 14 days of oviposition, deposited increased numbers of un­ fertilized eggs. The suggestion that the sperm supply from matings occur- ing shortly after emergence may be insufficient late in ovi­ position made it of interest to study the outcome of second matings involving older females. One hundred and thirty-four newly emerged albino females were permitted to mate undis­ turbed with albino males, and then isolated on individual leaf discs for 9 days. On the 10th day, 3 virgin wild-type males were placed on each disc. Numerous mating attempts were observed at this time. After 3 days the females, now about 2 weeks old, were transferred to fresh discs. Oviposition was allowed to continue for 5 days, after which the females were destroyed and the progeny reared to adulthood. The mean sex ratio of these progeny, based on a sample of 16 discs, was 2.31 males/female. This abundance of haploid progeny further supports the hypothesis that by the time of the second matings, the sperm stores of the older females had been partially depleted. Nevertheless, none of the 92 surviv­ ing females produced any red-eyed daughters, indicating that in all instances the second matings had been totally ineffec­ tive . Frequency of Double Inseminations Under Crowding; During the latter part of a colonizing episode, the upper leaves of a mite-infested plant become extensively damaged 59 and thickly covered with webbing. Density may exceed 6 males/ 2 cm of leaf surface, and adult males may outnumber quiescent female deutonymphs by more than 5 to 1 (Potter 1978). Com­ petition for females is intense under such conditions, and the opportunity for interrupted matings seemingly would be great. To estimate the frequency of effective double insemina­ tions under crowding, I allowed 3 albino female deutonymphs to enter quiescence on each of fifty 17mm leaf discs. Seven albino and 7 wild-type males were then added to each disc to provide a density of 6.17 males/sq. cm and a male: quiescent female ratio of 4.67. One day after emergence, females were transferred to individual discs and left to oviposit for 5 days. Doubly inseminated females were detected by mixed pro­ geny. Of the 126 surviving females, 51 had all red-eyed and 66 had all albino daughters, and 9 produced female progeny of both genotypes. Doubling this number to provide for se­ cond matings by males of the same type, it is estimated that 14.3% of the females in crowded populations will receive 2 effective inseminations. DISCUSSION The seminal receptacle in spider mites is an oblately spheroidal sac connected to the outside via a cuticle-lined duct (Smith and Boudreaux 1972, Pijnacker and Drenth-Diephu- is 1973) . The evident lack of a connection between the 60 receptacle and the vagina (Smith and Boudreaux 1972) has prompted speculation on the means by which spermatozoa reach the eggs for fertilization. Pijnacker and Drenth-Diephuis (1973) reported that within 30 sec. of copulation, sperm migrate to and enter the cells forming the walls of the recep­ tacle. Passage through the walls occurs within one day, and transport of sperm to the ovaries is apparently via the hae- molymph. In other arthropods for which double inseminations have been reported, those sperm deposited closest to the point of fertilization (usually those from the last male) generally take precedence (Parker 1970). This may explain why in spider mite double inseminations, sperm from the first male were relatively more effective early in oviposition. It is likely that by the time of the second mating these earlier sperm had already migrated to the walls and begun to penetrate through the epithelial lining of the lumen. That sperm from the very short first matings (30 sec.) did not take prece­ dence would suggest that not enough had been deposited to prevent those from the second male from immediately reach­ ing and penetrating into the receptacle walls. The mechanism by which complete first matings preclude later inseminations remains obscure. The existence of a "mat­ ing barrier" was proposed by Boudreaux (1963). Helle (1967) hypothesized instead that the sperm supply from the first mating determines the success or failure of the second. 61 Overmeer (197 2) suggested that a female accepts only a cer­ tain amount of sperm and is saturated for some time there­ after.

The discovery that incomplete first matings of suffi­ cient duration (2-2.5 min.) commonly prevent successful se­ cond inseminations rules out the possibility of a postcopu- latory "plug" or other such barrier, and suggests that there is a threshold for the volume of ejaculate needed for pre­ clusion. Although the negative correlation between the time of interruption and the effectiveness of second matings sup­ ports Helle's hypothesis, the ineffectiveness of second mat­ ings with old, sperm-deficient females is disturbing. Evi­ dently, even when the original sperm stores become depleted, females that have received a full first insemination cannot effectively mate again. The precise means by which later matings are precluded remains uncertain. Helle (1967) estimated that in natural populations about 6% of the mated females receive 2 effective inseminations. His study, however, used an experimental population in which quiescent female deutonymphs outnumbered males by more than 2 to 1, conditions that prevail only at the very beginning of an infestation (Potter 1978) . Although my data suggest that double inseminations occur more frequently than has previously been recognized, it should be emphasized that even in a crowded population only 1 female in 7 received 2 effective inseminations. It is of interest to consider 62 possible reasons for this unexpectedly low rate of disruption. When a mating is interrupted, the displaced male incurs a reduction in fitness from both the loss of progeny and the waste of time spent in precopulatory guarding. Any morpholo­ gical or behavioral feature providing increased resistance to interruption would therefore confer a considerable selective advantage to a mating male. Male spider mites mate from under­ neath, grasping the female with the anterior 2 pairs of legs and arching the tip of the opisthosoma upward to accomplish coupling (Fig. 9). It is probable that this unusual mating posture makes it difficult for intruders to dislodge the male, and that the clavate shape of the aedeagus aids in keeping the sexes together during copulation. These adaptations may ex­ plain why, even under high male density, the incidence of disrupted copulations is low. The studies described in this chapter underscore the im­ portance for males of being the first to mate. Disruption is difficult, and even when a usurper is successful, his repro­ ductive returns will be meager or nil unless the initial mat­ ing was of short duration. In addition, sperm from the first male will take precedence early in oviposition. These diffi­ culties explain the tenacity with which intruders attempt to engage guarding males in combat prior to the female's emer­ gence, and the vigorous jostling and scrambling for position that occur when more than one male remains at ecdysis. 63

Fig. 9. Male and teneral female T. cinnabarinus in copula. Note the female's freshly shed ex­

uviae . CHAPTER 4 FACTORS AFFECTING FREQUENCY AND INTENSITY OF FIGHTING

With the exception of Alexander's (1961) classic study of aggression and territoriality in field crickets, the adaptive significance of agonistic behavior among arthropods other

than crustaceans has received little quantitative analysis. Because of their extreme tractability, the spider mites pre­ sent an ideal system for such an investigation. If one watches a series of interactions between male spider mites, it becomes evident that the encounters vary considerably as to the intensity of aggression attained. As was suggested for crayfish (Bovbjerg 1953) and crickets (Alexander 1961) , agonistic exchanges may be classified ac­ cording to the level or intensity of the behavior displayed. It is possible to recognize four such levels for spider mites which, in order of increasing intensity of aggression, may be defined as follows: First Level- Encounter terminated after mild one-sided threat, usually by guarding male, with retreat of intruder. Such confrontations involve one male raising and spreading apart his first pair of legs and contacting the op­ ponent with his front tarsi. Second Level- Similar to First Level, except that threatening male also extrudes his

64 65 cheliceral stylets. Third Level- Encounter terminated after brief (<5 sec.) interaction involving aggressive behavior by both males; such interactions range from reciprocal threats to short bouts of intense sparring. Fourth Level- Encounter terminated after intense 2-sided fighting that continues for 5 or more seconds. In this chapter I examine how the incidence and intensity of male aggressive behavior are affected by differences in male density and in the nearness to receptivity of the guarded female. In addition, quantitative data concerning the outcome of aggressive encounters involving male guarders are present­ ed. My purpose is to delineate the releasing and motivational factors underlying male aggression, and to apply these data to an understanding of the behavioral patterns by which male spider mites may maximize their reproductive fitnesses. Methods The study consisted of 60 independent one-hour observa­ tion periods or "time trials" that were conducted over a per­ iod of several weeks. All trials took place on fresh 12mm bean leaf discs, using wild-type T. urticae males and females. To facilitate analysis of variance, the experiment was designed in a 3 x 4 factorial layout, each of the 12 cells composed of data from 5 trials. The four "treatments" corresponded to four different sets of conditions under which male agonistic be­ havior was studied. These were: 1) Controls: fresh, uninfested leaf discs. 2) Exuviae: discs containing the webbing and 66 exuviae from 2 newly ecdysed adult females. 3) Young Females: discs holding 2 quiescent female deutonymphs and their we- bing, each female 8-10 hours from ecdysis. 4) Old Females: discs holding 2 quiescent female deutonymphs, each within 2 hours of ecdysis. Trials were run with either 3, 5 or 8 males. These dif­ fering densities constituted the "blocks", and were selected as realistic approximations of the male/available female ra­ tio occurring during different stages of a colonizing episode. In general, females were introduced to the freshly cut discs as active deutonymphs on the day before a trial. By maintaining the discs at a constant temperature (27°C), the introductions could be timed to provide quiescent females (or exuviae) of the proper age for the following day's trials. All males were 4-7 days old and were taken at random from moderately crowded mixed-sex cultures. Different males and females were used for each trial. Males were introduced to the test arena 10 minutes prior to the start of each trial. During this preliminary period, any males injured during the transfer process were replaced. For each one hour observation period, I recorded All in­ stances of guarding and all aggressive interactions (by level) that occurred between males. Whenever a threat or fight in­ volved a solitary guarder or a male in the primary guarding position, the outcome of the encounter (whether or not the 67 guarding male retained the female) was also recorded. All en­ counters were scored by the maximum level of aggression at­ tained; for example, a one-sided threat that escalates into extended 2-sided combat is a level 4 encounter. Because the within-cell sample variances were found to be significantly heterogeneous when tested by Bartlett's me­ thod (Sokal and Rohlf 1969) , a square root transformation (/x+1) was applied prior to analysis (Table 7). To detect significant differences between the treatments (female states) and blocks (male densities), the transformed data were then tested by 2-way analysis of variance. Significantly different means were identified using Student-Neuman-Keul's procedures (Sokal and Rohlf 1969). Results Frequency and Intensity of Aggression; The data from the 60 time trials were subjected to four separate analyses to determine if the treatments and/or blocks differed in the following factors: a. Total number of aggressive behaviors b. Total aggressive behaviors per male c. Total weighted aggressive behaviors (encounters weighted by multiplying them by their respective levels) d. Total weighted aggressive behaviors per male Table 8 summarizes the outcome of these analyses. The actual data sets for each of the above, including the results of the mean separation procedures, are presented in Tables 9-12. 68

Table 7. Effects of square root (/x+1) transformation on Bartlett’s x statistic for homogeneity of sample variances. The within sample variances (sj_2) be­ fore and after transformation are represented respectively by the upper and lower numbers within each cell. The 4 treatments are, from the left, Controls, Exuviae, Young and Old 9?. The 3 blocks represent densities of 3, 5 and 8 males. There are 4 degrees of freedom associated with each cell.

Total Total Aggressive Aggressive Behaviors Behaviors/cT

1.30 38.50 10.30 2.30 .14 4 .37 1.14 .26 .04 .81 . 47 .25 .01 .23 .11 .04

6.50 46.30 82 . 70 13.20 .26 1.85 3.31 .53 . 10 . 82 1.28 . 17 .02 .13 . 20 .02 14. 80 97. 20 57.70 72.50 .23 1.52 .90 1.14 .18 1.47 .95 .57 .02. .12 .08 .06

n X prior to transforma­ X prior to transforma tion = 29.06 (p<.05) tion = 22.09 (p<.05) X2 after transforma­ X2 after transforma­ tion = 19.13 (n.s.) tion = 19.48 (n.s.)

Total Weighted Total Weighted Aggressive Behaviors Aggressive Behaviors/o* 26. 80 299 .7 87.00 27.20 2.97 33.31 9.68 3.02 .44 2. 68 2 . 00 1. 38 .13 .82 .53 .30 125.5 279.7 588.7 179.2 5.02 11.18 23.55 7.17 . 78 2. 86 4.10 .48 . 14 .47 .73 . 09 91.30 1476. 2 341.3 839 .8 1.42 23 .05 5.32 13.13 .46 8.05 3.02 2 . 22 .05 .84 .29 .24 2 2 X prior to transforma­ X prior to transforma tion = 27.81 (p<.05) tion = 16.55 (n.s.) 2 2 X after transforma­ X after transforma­ tion = 16.45 (n.s.) tion = 14.23 (n.s.) Table 8. Analysis of variance: summary table for 60 one-hour observation periods. A square root transformation (/x+1) was applied prior to analysis.

MEAN SQUARES Source of Total Total Total Weighted Total Weighted Variation df Aggression Aggression/cf Aggression Aggression/cT

* * Treatments ($?) 3 2.30* 0.26* 18.43** 2.09 ★ ~k Blocks (cfcf) (2) 29.19** 1.74** 82.86 5. 34 * ★ Linear 1 44.94 0.60* 128.52** 2 - 4 5 * * Quadratic 1 13.33** 12.81** 34 .88** 8.22** ** Interaction 6 3.89* 0.53 13.71** 2.51** Error 48 0. 56 0. 09 2.37 0.39

*p<.05 **p<.01

CTl 70

Table 9. Total munber of aggressive behaviors. Those means not significantly different at the 5% level are bracketed.

Blocks Block Young $$ Exuviae Control Old $$ Means:

6 19 5 0 5 4 7 1 3 cfcf 10 6 6 3 x— 5.30 5 6 8 0 1 5 6 3 27 14 19 36 8 12 19 28 18 13 13 23 x= 19.80 24 24 18 33 7 5 16 34

6 13 15 22 14 5 21 41 12 17 20 38 x— 20.55 7 23 17 24 25 31 25 35 Treatment Means: 11.67 13.13 14.33 21.73 --- « SS= 913 71

Table 10. Total aggressive behaviors per male. Those means not significantly different at the 5% level are bracketed.

Blocks Block Young ?? Exuviae Control Old $$ Means: 2.00 6.33 1.67 0 1.67 1. 33 2.33 0.33 3 db* 3.33 2.00 2.00 1.00 x= 1.77 > 1.67 2.00 2.67 0 ) 0.33 1.67 2 .00 1.00 5.40 2.80 3.80 7.20 1.60 2.40 3.80 5.60 ) 5 dcT 3.60 2.60 2.60 5.60 x= 3.96 > 4.80 4.80 3.60 6.60 / 1.40 1.00 3.20 6.80 0.75 1.63 1.88 2.75 1.75 0.63 2. 63 5.13 ) 8 dtf 1.50 2.13 2.50 4.75 x= 2.57 i 0.88 2.88 2.13 3.00 j 3.13 3.88 3.13 4.38 Treatment Means: 2.25 2.54 2. 66 3.61 i i 72

Table 11. Total weighted aggressive behaviors. Those means not significantly different at the 5% level are bracketed.

Blocks Block Young +$ Exuviae Control Old $$ Means: 14 50 12 0 16 11 16 4 3 db* 26 14 8 8 x— 12.85 8 12 22 0 1 9 14 12 69 28 54 101 14 27 44 87 5 cTd1 38 32 29 77 x= 50.80 55 53 28 105 15 6 45 109 13 16 41 72 36 9 42 124 a 00 21 46 49 122 x= 54.60 9 52 35 66 54 106 60 119 Treatment Means: 25.93 31.40 33.27 67.07 • 73

Table 12. Weighted aggressive behaviors per male. Those means not significantly different at the 5% level are bracketed.

Blocks Block Young $$ Exuviae Control Old ?? Means: 4.67 16.67 4.00 0 5.33 3.67 5.33 1.33 3 db* 8.67 4.67 2.67 2.67 x= 4.2 8 > 2. 67 4. 00 7.33 0 0. 33 3.00 4 .67 4.00 13. 80 5. 60 10 . 80 20. 20 2 . 80 5.40 8.80 17.40 x= 10.16 > 5 dtf 7.60 6.40 5.80 15.40 / 11.00 10 . 60 5.60 21.00 3.00 1.20 9.00 21.80 1.63 2 . 00 5.13 9.00 4.50 1.13 5.25 15. 50 8 dcf 2.63 5.75 6.13 15.25 x= 6.83? 1.13 6.50 4 .38 8.25 / 6.75 13.25 7 .50 14.88 Treatment Means: 5.09 5.99 6 .16 11.11 74

As can be seen from Table 9, a total of 913 agonistic encounters was observed during the study. While some aggres­ sion occurred in all but 2 of the 60 trials (both involving 3 males and old females), the incidence of fighting was not even­ ly distributed between the blocks or the treatments (Table 8). Concerning the different male densities, it is evident that aggressive behaviors occurred less frequently at 3 males/ trial than with either 5 or 8 males (Table 9). Although this would be expected on the basis of fewer overall male-male con­ tacts alone, the same result holds true for the incidence of aggression per male (Table 10). Interestingly, at low male density the presence of old females tends to reduce fighting. •In this combination, the release of males onto a disc general­ ly resulted in one of two situations: a) after a brief period of instability, two of the males established guarding while the third continued to search or wander about the edges of the leaf disc, or b) one female was guarded singly and the other guarded jointly by two males. While the total weighted aggressive behaviors was also lowest with 3 males (Table 11), this is a consequence of the overall scarcity of fights. Those fights that did occur in the 3 male/old female combination were of high intensity (mean level= 3.43) and comparable in this respect to those with 5 males (x= 3.01) and 8 males (x= 3.14). As reflected by the significant quadratic component in every analysis (Table 8), the incidence of aggression is not 75 entirely a simple linear function of male density. In fact, the trials at the intermediate and highest male density did not differ significantly in either the overall number of ag­ gressive behaviors (Table 9) or in the total weighted aggres­ sion (Table 11). On a per male basis, however, significantly more fighting occurred at the intermediate density (Tables 10, 12). Since there are undoubtedly more contacts between males at higher density, there appears to be a reduction in the proportion of these that escalate to aggression. Interpretation of the treatment data is complicated by the significant interaction term in the analyses. Comparisons between the overall treatment means do, however, indicate a number of important trends. Interestingly, males continued to threaten and fight with one another even on the bare leaf discs, and neither the presence of fresh exuviae and webbing nor that of younger quiescent females elicited any increase in this behavior (Tables 9, 10). Male response toward young deutonymphs tended to be erratic at all densities, with the females commonly being left unguarded. Aggressive behaviors were most prevalent around older females, with significantly more encounters occurring in those trials than with either exuviae or young females. Although falling short of signifi­ cance at the 5% level (p<.075), the presence of older females also caused a substantial increase in total aggressive beha­ viors over the controls (Table 9). Trends consistent with these are also evident in the analysis for total aggressive behaviors per male (Table 10), although here the differences 76 between the means of the transformed data are not sufficient­ ly great to be statistically significant. Nonetheless, the presence of nearly mature females has a clear and important effect upon male response. When the data were weighted by level or intensity, significantly more aggression (overall and per male) occurred with old females than with any of the other treatments (Tables 11, 12). This indicates that in addition to increasing the incidence of male-male encounters, the presence of older females caused an increase in the intensity of fighting. This effect can be seen more clearly in Fig. 10. As was noted earlier, aggression was uncommon in the trials involving older females and 3 males. In contrast, placing 5 or 8 males in this situation characteristically re­ sulted in frequent and intense bouts of fighting. It is this important density-related discrepancy that is largely res­ ponsible for the significant interaction term in the analyses. The results of the foregoing experiments may be summar­ ized as follows: Even in the absence of females, male spider mites frequently respond aggressively to one another. Under these circumstances, however, most encounters involve only threats or short bouts of sparring and terminate short of extended fighting. No measurable change in male response oc­ curs with freshly shed exuviae and webbing, or around newly quiescent females. At a low male/available female ratio old females tend to attract males, elicit guarding, and thereby 77

c o 'H uj U5

Cntr Ex 0 n = 215 197 175 326

Figure 10. Mean intensity of aggressive encounters in controls, and in presence of exuviae, young females and old females. 78

reduce the rate at which males encounter one another. At a medium to high functional sex ratio, however, females close

to ecdysis incite vigorous male competition and elicit a marked increase in both the incidence and intensity of ag­ gression . Success of Guarding Males in Aggressive Encounters; It is extremely uncommon for an intruding male to replace a guarder via a one-sided threat. Only 3 such instances were observed during the entire study, and 2 of these involved a guarder who had been attending a young deutonymph. Almost as rare are cases in which an intruder replaces a resident guarder without aggression by climbing onto the female and pushing the guarder aside. Only 14 such "passive" takeovers were recorded. These findings indicate that in most cases an intruding male must engage a guarder in combat in order to replace him. During the trials with quiescent females a total of 228 encounters was observed in which one of the combatants was a guarding male and in which the threat or fight ended with one male in sole possession of the female. The outcome of these encounters is summarized in Table 13. Here, the level of aggression refers to the maximum intensity of behavior displayed by the guarder (one-sided threats by intruders were uncommon, had little effect, and are not included in the table), and the percent retained is a measure of how of­ ten the guarded female was successfully defended. Table 13. Success of Guarders in Aggressive Encounters Over Old vs. Young Females

Fights Over Old Fights Over Younger Deutonymphs Deutonymphs No. Males Level of on Disc Aggression Won Lost % Retained Won Lost % Retained

1 0 0 0 0 3 2 2 0 100 0 0 - 3 0 0 - 0 0 -

4 2 1 67 0 0 -

E 4 1 80 0 0 -

1 9 0 100 12 0 100 5 2 19 0 100 13 1 93 3 21 5 81 4 1 80 4 35 20 64 4 6 40 E 84 25 77 33 8 80

1 3 0 100 4 0 100 8 2 7 0 100 8 1 89 3 5 4 56 2 1 67 4 14 16 47 2 6 25 E 29 20 59 16 8 67 80

Table 14. Overall Fighting Success of Guarding Males

Level of Aggression Females Females % Successfully by Guarder Retained Lost Defended

1 28 0 100 2 49 2 96 3 32 11 74 4 57 49 54 81 A consistent trend within each cell of Table 13 is that the percentage of successful defenses by guarders declines as the level of aggression increases. The consequences of esca­ lation can be seen more clearly in Table 14, in which the overall success of guarders is summarized. Note the very low risk involved with threats, and the increased probability of losing the female when escalation occurs. The data in Table 18 also suggest that guarders are less likely to fight when crowded. Although it is probable that they faced more intrusions, guarding males responded aggres­ sively less than half as often at the highest density than at the intermediate density. For both groups of females, the success of guarders who were aggressive (those who engaged in 2-sided fighting) declined as density increased. It appears, then, that fighting is more risky under crowded conditions. At all densities there were clearly more fights for pos­ session of old females than for younger ones. A comparison of the frequency distributions for these encounters (Fig.11) sup­ ports an important conclusion: escalation is much more likely to occur with females close to ecdysis. Threats by the guard­ ing male were sufficient to settle most encounters over young females, while the great majority of encounters over older fe­ males were terminated only after 2-sided fighting. Figure 11. Frequency distributions ofencounters involving males guarding young vs.

l females.old Note increased tendency toescalate with older deutonymphs. No. of Encounters 70 - 90 40 50 60 - 80 - 10 20 30 - • - FEMALES OLD mnr, n rm • • • • ••• ► • • • • ► • >• •• >• •• • >• ••• »* • • • »• •• • ►• ••• • • • • ••• • • •• • •• • • • • • • * •• •• •• •• •• •• •• • • •• • •• • I I IV III II Level JL

50 60 70 80 90 10 20 30 40 I II IV III II I FEMALES YOUNG • ••••« • ••••« • ••••« • • • • • • •• •• # •• •• • »••••< .••••• • • • •• •• • • •

• • • • • « • • • • • • • • • • •• •• •• • • •• • • • • • • • • • •••• • • • • Level t i 1 1

»••••< • •»••••< • • • »••••< »••••< »••••< »••••< >••••' • •• •• • •• • • • • • • •• • • • « • • • • • • • • •• •• • • • 4 oo M 83 Discussion Although crowding increases exponentially the rate at which individuals encounter one another (Wilson 197 5), it is not always accompanied by corresponding increases in fighting. In many mating systems, increases in male density beyond a certain point result in lowered male aggressiveness, loss of territoriality and increased social tolerance. This phenome­ non has been reported for field crickets (Alexander 1961), dragonflies (Pajunen 1966) , crayfish (Bovbjerg and Stephen 1971) and numerous groups of vertebrates (Sale 1972, Wilson (1975) . Ghiselin (1968) argued that as the number of competitors increases, a male in possession of a defensible resource will experience diminishing returns from aggression. Under high density, a guarding male mite who responded aggressively to every intrusion would have to fight nearly continuously, and would repeatedly risk being defeated and replaced. In addi­ tion, while engaged in fighting he might leave the female open to takeover by another male. In the present study, guard­ ing males who remained on the female were only rarely dis­ lodged, while those who engaged in fighting at high density were frequently replaced. These findings suggest it may be selectively advantageous for guarders to avoid confrontations under crowded conditions. There was, in fact, a reduced ten­ dency for guarding males in the high density trials to res­ pond aggressively to intrusions. 84 Since fighting occurs in the absence of females, the motivating and releasing factors for aggression are appar­ ently not entirely female-based. It is clear, however, that the presence of quiescent females close to ecdysis increases both the incidence and intensity of male-male encounters. Whether this increased aggressiveness is a direct or indirect result of olfactory perception of the sex pheromone is open to question. Since pharate females become increas­ ingly attractive as they approach ecdysis (Cone et al. 1971b, Potter et al. 1976a, Potter 1978), the increased fighting may simply reflect a higher incidence of male-male contacts. This, however, would not explain why the level or intensity of aggression increases. This increase seems to indicate a motivational change in the pheromone-stimulated males. A similar phenomenon has been observed in male cockroaches (Bell and Sams 1973). Much of the fighting in the trials with older females involved secondary guarders- males who had been attracted to an attended deutonymph but who had failed to gain the primary guarding position, and who then took positions alongside the quiescent female. By jostling and maneuvering for posi­ tion, these males were often able to gain temporary tarsal contact. That they could clearly perceive the female's pre­ sence but were unable to establish guarding may explain their readiness to fight. Penman and Cone (1974) proposed that the webbing laid down by a female^ deutonymph prior to becoming quiescent may serve as a substrate for release of the sex attractant. Based on the present study, this hypothesis appears questionable. Older females themselves clearly stimulated male aggres­ siveness, but their freshly shed exuviae and webbing eli­ cited no increase in male response over that obtained on bare leaf discs. Given the low incidence of "passive" takeovers and the risks associated with escalation, it remains to be explained why guarders do not always cling tenaciously to the female instead of attacking and fighting with intruding males. It must be remembered, however, that failure to respond also entails certain risks. When more than one male remains at ec­ dysis there is a scramble as each tries to maneuver into cop- ulatorv position. In earlier experiments (Potter et al. 1976a) guarders who gained sole possession of an ecdysing female nearly always mated, but those who tolerated or were unable to repel a coguarder had their chances greatly reduced. The presence of intruders also increases the risk that the mating act itself will be disrupted (Potter and Wrensch 1978). Wheth­ er a guarding male attacks or tolerates an intruder is un­ doubtedly determined by a number of interacting factors, in­ cluding the frequency of previous intrusions, the tenacity of the opponent, how long the guarder has remained with that female, and how close she is to ecdysis and receptivity. 86 Many of the behavioral patterns of male spider mites appear to have arisen from selective pressures for optimum time investment in guarding. This is why male response towards younger females is weak. Clearly, selection will act against a male genotype that guards indiscriminately, expending time and effort with females far from ecdysis, if a greater fre­ quency of mating can be had by withdrawal for further search­ ing. As a consequence, males guard selectively, discriminat­ ing between deutonymphs on the basis of apparent pheromonal cues and competing most vigorously for those closest to receptivity. GENERAL DISCUSSION

In polygynous mating systems, the ability to conquer or intimidate other males is an important component of male fit­ ness (Williams 1966). Among the arthropods, male aggression is a common phenomenon, having been studied extensively in the crustaceans and the insects. Often, males in these groups possess specialized morphological adaptations that function in intrasexual competition, or display elaborate patterns of precopulatory behavior (Richards 1927, Parker 1970, Wilson 1975) . In contrast, behavioral studies of the Acari have been hindered by the small size and cryptic habits of most mites. As a consequence, aggression between conspecific males has been observed in only a few scattered instances. Among these are Woodring's (1968) account of fighting between male San- cassania, Costa's (1967) report that older male Macrocheles kill newly emerged males in rearing cells, and unpublished observations of aggression in Tarsonemus (D. McClymont, pers. comm.) and Oligonychus (D. Mague, pers. comm.). To my know­ ledge, the present study is the first comprehensive analysis of agonistic behavior in an acarine mating system.

87 88

Parker (1974) in a thoughtful treatise on the evolution of fighting behavior, suggested that guarding and aggression will be selectively advantageous when competitors outnumber available resources, and where an immediate fitness gain may be had by forcibly ousting a rival. He argued that the more discrete the resource (that is, the easier it is to guard) and the higher its probable yield in increased fitness, the more intense will be the selection for aggression. These conditions are characteristic of spider mite mat­ ing systems. Precopulatory guarding, in fact, has been ob­ served in several genera other than Tetranychus, including Panonychus (Beavers and Hampton 1971), Oligonychus (D. Mague, pers. comm.), and Eutetranychus (Siddig and Elbary 1971), and I expect that guarding and aggression are general features of Tetranychine biology. The idea that local mate competition (competition be­ tween genetic relatives) can account for female-biased sex ratios was proposed by Hamilton (1967) . He noted that among small arthropods in which sib-matings are the rule, and in which control of the sex ratio is determined by the phenotype of the mother, there is often an extreme reduction in the proportion of male progeny. Comparing the mating systems of a large number of insects and mites, Hamilton (1967) compiled the following list of characteristics commonly associated with local mate competition: a) the primary sex ratio is spanandrous, i.e. females greatly outnumber males; 89 b) reproduction is arrhenotokous; c) development of siblings from egg to adult is gregarious; d) every batch of offspring contains at least one male; e) adult males eclose first and can mate many times; f) females are mated immediately after ecdysis; g) males generally do not disperse, and h) females store sperm, and one mating serves to fertilize all the eggs. Hamilton (1967) argued that because a female's fitness is a function of the number of fertile dispersants she can produce from a host, it is in her interest to produce only as many sons as are necessary to ensure that all her daughters are inseminated. In spider mites, the female-skewed tertiary sex ratio appears to be an adaptation by which ovipositing females maximize the number of potential colonizers among their progeny. In this respect, the accumulation of males during a colonizing episode and the intense competition for females close to ecdysis are of great importance, since they allow the proportion of female progeny to be increased with­ out a loss in fertilization efficiency. The reproductive success of a parent is a function not only of the number of offspring it leaves, but of the quali­ ty or probable survival of these offspring as well (Fisher 1930). Because a female's fitness is determined in part by the genetic contribution of her partner, selection should favor females who discriminate among potential mates and choose the fittest male available (Orians 1969). 9° In spider mites, however, teneral females are inevita­ bly mated at or shortly after emergence, and almost always by the male in control just prior to ecdysis. As a conse­

quence, they have no direct control over the male by which they are inseminated.

In a thoughtful analysis of this problem, Cox and LeBoeuf (1977) discuss a strategy by which females can max­ imize their chance of pairing with a superior genotype with­

out discriminating among males or the resources they control. They report that in elephant seals, females protest loudly $ when mounted, attracting other nearby males and activating the dominance hierarchy. Hence, by inciting male competition and then mating with the winner, a female increases her chances of mating with a mature, high-ranking male.

Female incitation of male competition appears to be a widespread phenomenon in both vertebrate and invertebrate mating systems, and may involve either specialized behavioral signals or hormonally-controlled morphological changes (Cox and LeBoeuf 1977). In this context, the sex pheromone pro­ duced by pharate female spider mites may be seen as serving several functions. First, as a sex attractant it facilitates mate-finding and ensures that virtually all females are in­ seminated at ecdysis. This enables them to quickly become potential colonizers. In addition, by inciting intense intra­ male competition, it assures that females, which exert no direct choice of mates, are inseminated by a male of high 91 reproductive fitness. The question of male mating competitiveness in spider mites is of more than academic interest. Particularly in the Netherlands, much attention has been paid in recent years to the possibility of controlling glasshouse populations through the release of sterilized or genetically incompatible males (Helle 1969, Nelson and Stafford 1972, Overmeer and Van Zon 1973, 1977, Overmeer 1974, Smith 1975, Feldmann 1977). Re­ cognizing that the success of such a program would require that the treated males be competitive with those in the popu­ lation to be suppressed, numerous investigators have attempted to measure male competitiveness in mating. At least 2 of the above-mentioned studies (Feldmann 1977, Overmeer and Van Zon 1977) must be invalidated on the grounds of a flawed experi­ mental design, in which males were allowed to compete for ac­ tive rather than pharate adult females. This procedure elimi­ nates the main elements of male precopulatory behavior, so that the data are more a reflection of random encounters be­ tween the sexes than a true measure of male competitive abil­ ity . The present findings (see Chapter 1) suggest that young mites from all-male cultures may be unable to compete equally with older males from crowded populations. This would repre­ sent a potential deterrent to the success of genetic control measures. There are so many other problems associated with a genetic control program for spider mites that, in my opinion, this approach has little potential. Certainly, the mechanics and cost of rearing, treating and releasing large numbers of sterile mites seems prohibitive. Because of the aggregative character of field populations, their distribution generally assumes a negative binomial (e.g. Nelson and Stafford 1972), i.e. one leaf may bear a large population although adjacent leaves have very few or no mites. Nelson and Stafford (1972) demonstrated mathematically that this would necessitate the release of tremendous numbers of males to attain any measura­ ble benefit. Although feeding by males is minimal, the release of many thousands would be likely to result in unacceptable feeding damage. Finally, females will produce normal males even after mating with a sterile male, thereby reducing the ratio of treated : untreated competitors. The development of complex, predictive models of mite populations appears to be a far more promising trend in in­ tegrated pest management (see Hoyt and Burts 1974, Croft 1975, Tanigoshi et al. 1976). By allowing the storage of large quan­ tities of data, computers have the potential to provide a his­ torical perspective and to aid in sorting out the multiplicity of factors determining population fluctuations in complex agro-ecosystems. In addition, computer simulation promises to facilitate the testing of a large number of management proce­ dures . Such strategies demand a thorough understanding of the bionomics of the pest species concerned. Without the input of complete and accurate biological data, the decisions reached by analysis or computer simulation are destined to be mean­ ingless. Although the recent renewed interest in tetranychid biology has generated a number of important basic studies (e.g. Huffaker et al. 1963, Helle 1967, Cone et al. 1971a,b, Mitchell 1973, Regev and Cone 1975a,b, 1976, Wrensch and Young 1975, 1978, Wrensch 1979) it is likely to be many years before the internal and external factors regulating spider mite populations are fully understood. Certainly a more in­ timate knowledge of tetranychid behavior will be of great value in understanding and predicting spider mite outbreaks. LIST OF REFERENCES

Alberti, G. and V. Storch. 1974. Uber Bau und Funktion der Prosoma - Drusen von Spinnmilben (Tetranychidae, Trom- bidiformes). Z. Morph. Tiere 79: 133-53. Alcock, J., C.E. Jones and S.L. Buchmann. 1977. Male mating strategies in the bee Centris pallida. Amer. Natur. Ill: 145-55. Alexander, R.D. 1961. Aggressiveness, territoriality, and sexual behavior in the field crickets (Orthoptera: Gryllidae). Behavior 17: 130-223. Beavers, J.B. and R.B. Hampton. 1971. Growth, development, and mating behavior of the Citrus Red Mite (Acarina: Tetranychidae). Ann. Entomol. Soc. Am. 64: 804-06. Bell, W.J. and G.R. Sams. 1973. Aggressiveness in the cock­ roach Periplaneta americana (Orthoptera, Blattidae) Behav. Biol. 9~i 581-93 . Boudreaux, H.B. 1963. Biological aspects of some phytophagous mites. Annu. Rev. Entomol. 8: 137-54. Bovbjerg, R.V. 1953. Dominance order in the crayfish, Orcon- ectes virilis (Hagen). Physiol. Zool. 26 : 173-73. Bovbjerg, R.V. 1956. Some factors affecting aggressive beha­ vior in crayfish. Physiol. Zool. 29: 127-36. Bovbjerg, R.V. and S.L. Stephen. 1971. Behavioral changes in crayfish with increased population density. Bull. Ecol. Soc. Am. 52: 37-38. Chant, D .A. 1966. Integrated control systems, pp. 193-218- In Scientific Aspects of Pest Control. NAS-NRC Publ. 1402, Nat. Acad. Sci., Washington, D.C. Collias, N.E. 1944. Aggressive behavior among vertebrate animals. Physiol. Zool. 17: 83-123.

94 95

Cone, W., L.M. McDonough, T.C. Maitlen and S. Bordajewicz. 1971a. Pheromone studies of the two-spotted spider mite. 1. Evidence of a sex pheromone. J. Econ. Entomol. 64: 355-58. Cone, W., S. Predki, and E.C. Klostermeyer. 1971b. Pheromone studies of the two-^spotted spider mite. 2. Behavioral response of males to quiescent deutonymphs. J. Econ. Entomol. 64: 379-82. Costa, M. 1967. Notes on macrochelids associated with manure and coprid beetles in Israel. II. Three new species of the Macrocheles pisentii complex, with notes on their biology. Acarologia 9: J04-29. Cox, C.R. and B.J. LeBoeuf. 1977. Female incitation of male competition: a mechanism in sexual selection. Amer. Na- tur. Ill: 317-35. Croft, B .A. 1975. Tree fruit pest management, pp. 471-507. In R.L. Metcalf and W.H. Luckman {ed.} Introduction to Insect Pest Management. J. Wiley & Sons, New York. Dielman, J. and W.P.J. Overmeer. 1972. Preferential mating hampering the possibility to apply a genetic control method against a population of Tetranychus urticae Koch. Z. Angew. Entomol. 71: 156-61. Ewing, H.E. 1914. The common red spider or spider mite. Oreg. Agric. Exp. Sta. Bull. 121: 95 pp. Feldmann, A.M. 1977. Mating competitiveness and the effect of X-rays and aging on males of Tetranychus urticae in relation to genetic control. Entomol. Exp. Appl. 21: 182-91. Fisher, R.A. 1930. The Genetical Theory of Natural Selection. Clarendon Press, Oxford; reprinted 1958, Dover, New York. 291 pp. Garman, P., and B.H. Kennedy. 1949. Effect of soil fertili­ zation on the rate of reproduction of the two-spotted spider mite. Jour. Econ. Ent. 42: 157-53. Ghiselin, M.T. 1974. The Economy of Nature and the Evolution of Sex. University of California Press, Berkeley. 346 pp. Grandjean, F. 1948. Quelque characteres des Tetranyques. Bull. Mus. Nat. Hist. Natur., Paris 20: 517-24. 96 Hamilton, W.D. 1967. Extraordinary sex ratios. Science 156: 477-88. Hamilton, W.J., R.E. Buskirk and W.H. Buskirk. 1976. Social organization of the Namib desert tenebrionid beetle Onymacris rugatipennis. Can. Entomol. 108: 305-16. Hazan, A., U. Gerson and A.S. Tahori. 1973. Life history and life tables of the carmine spider mite. Acarologia 15: 414-40. Hazan, A., U. Gerson and A.S. Tahori. 1975a. Spider mite webbing. II. The effect of webbing on egg hatchability. Acarologia 17: 270-73. Hazan, A., A. Gertler, A.S. Tahori and U. Gerson. 1975b. Spider mite webbing. III. Solubilization and amino acid composition of the silk protein. Comp. Biochem. Physiol. 51: 457-62.

Helle, W. 1962. Genetics of resistance to organophosphorus compounds and its relation to diapause in Tetranychus urticae Koch (Acari). Tijdschr. PlZiekt. 68: 1-41. Helle, W. 1965. Resistance in the Acarina: mites, pp. 71-93. In Advances in Acarology, vol. II. Cornell University Press, Ithaca. Helle, W. 1967. Fertilization in the two-spotted spider mite (Tetranychus urticae: Acari). Entomol. Exp. Appl. 10: 103-10. Helle, W. 1969. New developments towards biological control of the two-spotted spider mite by incompatible genes. Pub. Organ. Eur. Plant Protect., Paris Ser. A. 52: 7-15. Hoyt, S.C. and E.C. Burts. 1974. Integrated control of fruit pests. Annu. Rev. Entomol. 19: 231-52. Hueck, H.J., D.J. Kuenen, P.J. Van Den Boer, and E. Jaeger- Draafsel. 1952. The increase of egg production of the fruit tree red spider mite under influence of DDT. Physiol. Comp. Oecol. 2: 371-77. Huffaker, C.B. and C.H. Spitzer. 1950. Some factors affect­ ing red mite populations on pears in California. J. Econ. Ent. 43: 819-31. Huffaker, C.B., K.P. Shea and S.G. Herman. 1963. Experimen­ tal studies on predation: Complex dispersion and levels of food in an acarine predator-prey interaction. Hil- gardia 34: 305-30. 97 Huffaker, C.B., M. van de Vrie and J.A. Mcmurtry. 1970. Ecology of tetranychid mites and their natural enemies: a review. II. Tetranychid populations and their possi­ ble control by predators: an evaluation. Hilgardia 40: 391-458. Hussey, N.W. and W.J. Parr. 1963. Dispersal of the glasshouse red spider mite.Tetranychus urticae Koch (Acarina, Tetra­ nychidae) . Entomol. Exp. Appl. 6: 207-14. Laing, J.E. 1969. Life history and life table of Tetranychus urticae Koch. Acarologia 9: 32-42. McMurtry, J.A., C.B. Huffaker and M. van de Vrie. 1970. Ecol­ ogy of tetranychid mites and their natural enemies: a review. I. Tetranychid enemies: their biological charac­ ters and the impact of spray practices. Hilgardia 40: 331-90. Mitchell, R. 1970. An analysis of dispersal in mites. Am. Na- tur. 104: 425-31. Mitchell, R. 1973. Growth and population dynamics of a spi­ der mite (Tetranychus urticae K.). Ecology 54: 1349-55. Nelson, R.D. and E.M. Stafford. 1972. Effects of gamma radia­ tion on the biology and population suppression of the two-spotted spider mite, Tetranychus urticae Koch. Hil­ gardia 41: 299-342. Orians, G.H. 1969. On the evolution of mating systems in birds and mammals. Amer. Natur. 103: 589-603. Overmeer, W.P.J. 1972. Notes on mating behavior and sex ratio control of Tetranychus urticae Koch (Acarina: Tetrany­ chidae) . Entomol. Ber. 32: 240-44. Overmeer, W.P.J. and A.Q. van Zon. 1973. Studies on hybrid sterility of single, double and triple chromosome muta­ tion heterozygotes of Tetranychus urticae with respect to genetic control of spider mites. Entomol. Exp. Appl. 16: 389-94. Overmeer, W.P.J. 1974. Genetic control of spider mites, pp. 45-56. In R. Pal and M.J. Whitten {ed.} The Use of Genetics in Insect Control. Elsevier Publ. Co., Amster­ dam. 98 Overmeer, W.P.J. and A.Q. van Zon. 1977. A comparative study on mating competitiveness of males from normal and chro­ mosome mutation strains of the red spider mite Tetrany­ chus urticae Koch (Acari: Tetranychidae) Z. angew. Ento­ mol. 84: 31-37. Pajunen, V.I. 1966. The influence of population density on the territorial behavior of Leucorrina rubicunda L. (Odonata, Libellulidae). Ann. Zool. Fenn. 3: 40-52. Parker, G.A. 1970. Sperm competition and its evolutionary consequences in the insects. Biol. Rev. 45: 525-67. Parker, G.A. 1974. Assessment strategy and the evolution of fighting behavior. J. Theor. Biol. 47: 223-43. Penman, D.R. and W.W. Cone. 1972. Behavior of male twospotted spider mites in response to quiescent female deutonymphs and to web. Ann. Entomol. Soc. Am. 65: 1289-93. Penman, D.R. and W.W. Cone. 1974. Role of web, tactile stimu­ li, and female sex pheromone in attraction of male two­ spotted spider mites to quiescent female deutonymphs. Ann. Entomol. Soc. Am. 67: 179-82. Perkins, C.H. 1897. The red spider, pp. 75-86. In Report of the Entomologist, 10th Annu. Rep. Vt. Agr. Exp. Stn. Pijnacker, L.P. and L.J. Drenth-Diephuis. 1973. Cytological investigations on the male reproductive system and the sperm track in the spider mite Tetranychus urticae Koch (Tetranychidae, Acarina). Neth. J. Zool. 23: 446-64. Potter, D.A., D.L. Wrensch and D.E. Johnston. 1976a. Guarding, aggressive behavior and mating success in male two­ spotted spider mites. Ann. Entomol. Soc. Am. 69: 707-11. Potter, D.A., D.L. Wrensch and D.E. Johnston. 1976b. Aggres­ sion and mating success in male spider mites. Science 193: 160-61. Potter, D .A. 1978. Functional sex ratio in the carmine spider mite. Ann. Entomol. Soc. Am. 71: 218-22. Potter, D .A. and D.L. Wrensch. 1978. Interrupted matings and the effectiveness of second inseminations in the two­ spotted spider mite. Ann. Entomol. Soc. Am. In Press. Regev, S. and W.W. Cone. 1975a. Evidence of farnesol as a sex attractant of the twospotted spider mite, Tetranychus urticae Koch (Acarina: Tetranychidae). Environ. Entomol. 4: 307-11. 99

Regev, S. and W.W. Cone. 1975b. Chemical differences in hop varieties vs. susceptibility to the twospotted spider mite. Environ. Entomol. 4: 697-99. Regev, S. and W.W. Cone. 1976. Analysis of pharate female twospotted spider mites for nerolidol and geraniol: Evaluation for sex attraction to males. Environ. Entomol. 5: 133-38. Richards, O.W. 1927. Sexual selection and allied problems in the insects. Biol. Rev. Cambridge Philos. Soc. 2: 298- 364. Rodriguez, J.G. 1951. Mineral nutrition of the two-spotted spider mite, Tetranychus bimaculatus Harvey. Ann. Ento­ mol. Soc. Am. 54: 512-18. Rodriguez, J.G. 1958. The comparative NPK nutrition of Pano- nychus ulmi (Koch) and Tetranychus telarius (L.) on apple trees. J. Econ. Ent. 51: 369-73. Sale, P.F. 1972. Effect of cover on agonistic behavior of a reef fish: a possible spacing mechanism. Ecology 53: 37-38. Selander, R.K. 1972. Sexual selection and dimorphism in birds, pp. 180-230. In B. Campbell {ed. } Sexual Selection and the Descent of Man 1871-1971. Aldine Publishing Co., Chicago. Siddig, M.A. and E.A. Elbadry. 1971. Biology of the spider mite Eutetranychus sudanicus. Ann. Entomol. Soc. Am. 64: 806-09. Smith, F.F., A.L. Boswell and R.F. Webb. 1967. Segregation between strains of carmine and green twospotted spider mites, pp. 155-59. In Proc. 2nd Int. Congr. Acarol. (1969). Akademiai Klado, Budapest. Smith, J.W. and H. B. Boudreaux. 1972. An autoradiographic search for the site of fertilization in spider mites. Ann. Entomol. Soc. Am. 65: 6 9-74. Smith, J.W. 1975. Spider mites: population suppression by in­ terspecific hybridization. Environ. Entomol. 4: 588- 90. Sokal, R.R. and F.J. Rohlf. 1969. Biometry. W.H. Freeman and Company, San Francisco. 776 pp. 100

Tanigoshi, L.K., R.W. Browne, S.C. Hoyt and R.F. Langier. 1976. Empirical analysis of variable temperature regimes on life stage development and population growth of Tetra­ nychus mcdanieli (Acarina: Tetranychidae). Ann. Entomol. Soc. Am. 69: 712-16. van de Vrie, M., J.A. McMurtry and C.B. Huffaker. 1972. Ecol­ ogy of tetranychid mites and their natural enemies: a review. III. Biology, ecology and pest status and host- plant relations of tetranychids. Hilgardia 41: 343-432. Williams, G.C. 1966. Adaptation and Natural Selection: a Critique of Some Current Ecological Thought. Princeton Univ. Press, Princeton. 307 pp. Williams, G.C. 1975. Sex and Evolution. Princeton Univ. Press, Princeton. 200 pp. Wilson, E.O. 1975. Sociobiologv. Belnap Press of Harvard University Press, Cambridge. Woodring, J.P. 1969. Observations on the biology of six spe­ cies of acarid mites. Ann. Entomol. Soc. Am. 62: 102-08.

Wrensch, D.L. and S.S.Y. Young. 1975. Effects of quality of resource 'and fertilization status on some fitness traits in the two-spotted spider mite, Tetranychus urticae Koch. Oecologia (Berl.) 18: 259-67. Wrensch, D.L. and S.S.Y. Young. 1978. Effects of host quality on developmental rate, survivorship and sex ratio in the carmine spider mite. Environ. Entomol. 7: 499-501. Wrensch, D.L. 1979. Components of reproductive success in spider mites. In Recent Advances in Acarology. Academic Press, New York. In Press.