Natural Enemies of Spider Mites (Acari: Tetranychidae) on Cotton: Density Regulation Or Casual Association?

Natural Enemies of Spider Mites (Acari: Tetranychidae) on Cotton: Density Regulation or Casual Association?

L. T. WILSON,' P. J. TRICHILO,' AND D. GONZALEZ2 Department of Entomology, Texas A&M University, College Station, Texas 77843

Environ. Entomol. 20(3): 849-856 (1991) ABSTRACT This study addresses the potential impact of natural enemies on the abundance of spider mites, Tetranychus spy., on cotton in the San Joaquin Valley of California. These natural enemies are omnivorous predators, and include the big-eyed bug, Geocoris pallens StAl and G. punctipes (Say), the minute pirate bug, Orius trfsticolor (White),and the western flower thrips, Frankliniella occidentalis (Pergande). Simple linear regression suggested that omnivorous predators were potentially effective in delaying the buildup of spider mites, with the highest rz (0.65) recorded for adult F. occidentalis. Geocoris showed the potential to suppress the rate of spider mite population increase (rl = 0.73). All three tested predator species exhibited the capacity to suppress early season spider mite abundance, with the highest r2 (0.62) recorded for Ceocoris and Orius. Predators were also potentially able to suppress mid- to late-season spider mite populations. Multiple regression analysis indicated a significant negative correlation between mid- to late-season spider mite abundance and early season predators. Results from a second year were less conclusive, suggesting that the reduced range of spider mite abundance limited our ability to discern potentially significant interactions during that year.

KEY WORDS Insecta, Cossypium, Tetranychus, predators

SPIDERMITES, Tetranychus spp. can cause serious Several mechanisms have been suggested to ex- economic injury to cotton, Gossypium hirsutum plain the spider mite-insecticide phenomenon. One L., in the San Joaquin Valley of California (Leigh explanation is that insecticides decimate large 1963; Leigh & Burton 1976; Wilson et al. 1983, numbers of natural enemies, thereby reducing pre- 1985). Barring insecticidal intervention, natural dation pressure and allowing the herbivore to in- enemies of spider mites, including omnivorous ar- crease rapidly in abundance (Huffaker et al. 1970, thropod predators, are believed to maintain spider DeBach et al. 1976, Stoltz & Stern 1978, Trichilo mites below economic injury levels (van den Bosch & Leigh 1986b). Another view is that insecticides & Hagen 1966, Gonzalez & Wilson 1982, Gonzalez cause increased spider mite reproductive rates, ei- et al. 1982). Therefore, methods of pest manage- ther directly, or indirectly through the plant (Bart- ment that preserve and enhance the natural enemy lett 1968, van de Vrie et al. 1972, Iftner & Hall component should be considered desirable. 1984). A third view is that synthetic pyrethroids During 1982 and 1983, we engaged in a field such as permethrin have a repellancy effect on study to determine the impact of spider mites on spider mites, increasing the rate of dispersal to cotton yield, which indicated that economic dam- areas which have lower spider mite densities and age from spider mites occurs only when the rate higher food quality (Hoyt et al. 1978, Hall 1979, of population increase exceeds a critical Infestation Penman & Chapman 1988). Lower densities and rate (Wilson et al. 1985). At rates progressively improved food quality generally lead to higher . greater than the critical rate, there is a precipitous spider mite reproductive rates (Wrensch & Young loss of yield. Infestations that exceeded the critical 1978, Iftner & Hall 1983). rate occurred only when permethrin, methyl para- While many investigators have long believed that thion, or both were applied. Thus, consistent with arthropod predators exert significant regulation of other reports (Bower & Kaldor 1980, McWhorter herbivore densities, it has been difficult to dem- 1982, Botha et al. 1986, Bentley et al. 1987), eco- onstrate quantitatively this effect for spider mites nomically damaging levels of spider mites were on a field crop such as cotton. Unlike parasitoids, closely associated with the application of insecti- predators of spider mites leave no easily recognized cide. evidence of their activity. Thus, assessment of predator effectiveness in the field on a large scale is I Department of Entomology.-. University of California, Davis. relegated to inducing and observing changes in Calif. 65616. Division of Biological Control. University of California. Riv- predator frequencies and any associated fluctuation erside, Calif. 92521. in populations of their prey.

0046-225X/91/0849-0856$02.00/0@ 1991 Entomological Society of America 850 ENVIRONMENTALENTOMOLOGY Vol. 20, no. 3

Although a variety of arthropod predators, such curred on 8 and 22 June, 6 and 20 July, and 3 as nabids, lacewings, predatory mites, and preda- August. Experimental differences between blocks ceous thrips, are found on cotton, they appear to were based solely on release dates of T. turkestani, be insignificant in controlling spider mite popu- and accompanying applications of permethrin. lations during critical infestation periods in the San A sixth block, representing the modified part of Joaquin Valley due to low numbers and normally the design, was subdivided into six plots, of which late season occurrence. The major natural enemies five received inoculative release of T. turkestani of spider mites on cotton in this region are gen- (but no permethrin), corresponding to the dates of eralist, omnivorous predators, and include the the first five blocks. The sixth plot was untreated western flower thrips, Frankliniella occidentalis and allowed for testing of spatial homogeneity (see (Pergande), the big-eyed bugs, Geocoris pallens Trichilo et al. 1990). This design translated into a StAl and G.punctipes (Say), and the minute pirate five block by five treatment factorial, consisting of bug, Orius tristicolor (White) (van den Bosch & the following five treatments: (1) no mites released, Hagen 1966, Gonzalez & Wilson 1982, Gonzalez and no pesticides, (2) mites released, permethrin, et al. 1982, Trichilo & Leigh 1986a,b). and dicofol at 0.2 P(I), (3) mites released, per- If natural enemies suppress spider mite densities, methrin, and dicofol at 0.8 P(Z), (4) mites released, we would expect to observe a negative association permethrin, but no dicofol, and (5) mites released, between snider mite abundance and that of their but no pesticides. Timing, application rates, and predators over a given block of time, thus, at least experimental designs for 1982 and 1983 are de- partially explaining why spider mites dramatically scribed in detail by Trichilo et al. (1990). increase in abundance after insecticides are used. Sampling. Spider mite (immature and adult) When predators are at normal (i.e., maximum) lev- and predator (egg through adult) seasonal abun- els throughout the season, overall levels of spider dance was estimated using the proportion of sam- mites should be relatively low, and vice versa. ple units infested, P(I), with one or more individ- However, there is little quantitative evidence that uals of a given species (Wilson et al. 1983). Plots effectively demonstrates the impact of natural en- were sampled weekly, beginning on 26 May and emies on populations of spider mites on cotton fields. ending on 14 September in 1982, which yielded Our objective was to assess the relationship between 17 samples. In 1983, weekly sampling began on 25 spider mites and the natural enemy complex dur- May and ended on 8 October, which yielded 20 ing the cotton season. samples. Physiological time (degree-days (DD) after planting) was calculated, based on an approx- imate minimum developmental temperature of Materials and Methods 12OC for spider mites, as extrapolated from devel- Experimental Design. This study represents the opmental data from Carey (1980). analysis of data collected on cotton during 1982 In each year, a sample consisted of 40 mainstem and 1983 at the University of California, Westside leaves, one leaf per plant; each plant was chosen Field Station. The 1982 study was a factorial ex- at random from a given plot. The leaf chosen rep- periment based on a split-plot design of 6.7 ha, in resented the main stem node leaf most likely to which each of four blocks representing four levels have spider mites or predators (see Wilson et al. of permethrin was divided into six subplots or treat- 1983). Insufficient levels of Geocoris adults and ments. Permethrin was used as an insecticide to nymphs were detected to warrant including these suppress natural enemies, while the acaricide di- stages in the statistical analysis. cofol was used to suppress spider mites. Methyl Statistical Analysis. Simple linear regression was parathion was used as a pesticide to contrast di- used to test the influence of major predator species cofol. The six treatments included: (1) no pesti- on spider mite population dynamics. Analyses were cides, (2) dicofol applied (at half label rate) when also performed using multiple linear regression, to the proportion of infested leaves,.P(Z), reached 0.2, assess the effects of early season predator popula- (3) dicofol applied (at full rate) at 0.2 P(Z), (4) tions on early and mid-to-late season spider mites. dicofol applied (at full rate) at 0.8 P(Z), (5) methyl For all regression analyses, significant differences parathion applied at half rate, and (6) methyl para- were assumed to exist at P < 0.05. thion applied at full rate. To guarantee a minimum Spider mite infestation rate (dp = change in level of spider mite infestation, a small number of proportion [infested leaves]/100 DD), was used as cotton leaves from another field, with damage a dependent variable. A second dependent vari- symptoms resembling that of Tetranychus turke- able, initiation of rapid spider mite increase, was stani Ugarov and Nikolski, was distributed to all represented by the sample date (in degree-days) plots on 4, 9, 15, and 22 June. immediately preceding the phase in which spider The 1983 study was based on a modified factorial mites most rapidly increased in proportion of in- design, also in the form of a split-plot, in which fested leaves. The third and fourth dependent vari- each of five blocks was subdivided into four treat- ables were generated by computing the area under ments. Each block represented five separate dates the spider mite P(I) curve, as a function of phys- of permethrin application, followed by inoculative iological time for a given treatment, from sample release of T. turkestani. Spider mite releases oc- 1 (~300DD > 12OC from planting) through sam- June 1991 WILSONET AL.: NATURAL

0.2 - . EARLY SEASON , I SEASON , 0.0 1.0 - B I I---- m~arvae I- .- .- .-. WFT Adults I------Orius Nympha I 0.8 - ;! ;! 1- Geocorii Eggs 1

PHYSIOLOGICAL TIME PHYSIOLOGICAL TIME (DD>12OC From Planting) (DD>lZÂ° From Planting) Fig. 1. Seasonal infestation levels, P(,I),for (A) spider Fig. 2. Estimated seasonal density of (A) spider mites, mites, Tetranychus spp., and (B) three major arthropod Tetranychus spp., and (B) three major arthropod pred- predators in the cotton agroecosystem. ators in the cotton agroecosystem. pie 8 (ss850 DD) and from sample 9 (~851DD) 200 DD before spider mites (Fig. 1 and 2). Adult through sample 16 (a1622 DD). These variables and larval F. occidentalis usually exhibited a sharp were designated early season and mid-to-late season increase in abundance, immediately following their spider mite proportion infested-degree-days (P(I)- appearance on cotton, followed by an abrupt drop DD), respectively. The fifth and sixth dependent 150-200 DD later. Beyond this point, their popu- variables were derived by converting proportion lations fluctuated in a random and unpredictable infested leaves into estimated absolute densities (see manner during each year. Geocoris eggs and Onus Wilson et al. 1983), and then calculating spider nymphs were first observed around 550 DD, often mite degree-days (DD) for early and mid-to-late in conjunction with the initiation of spider mite season. infestations (Fig. 1 and 2). However, occurrence Predator independent variables, P(1)-DD and of eggs and juveniles implies that Geocoris and predator-DD, were derived similarly to spider mite Orius adults had entered cotton fields at an earlier variables for the early and mid-to-late season pe- date. riods. Predator impact was also analyzed in terms Simple Associations. In 1982, the greater the of the population immediately following applica- abundance of early season F. occidentalis adults, tions of permethrin in 1982. These predator vari- Geocoris eggs, and Onus nymphs, the later the ables comprised the areas under the P(1) and den- estimated time when the spider mites initiated the sity curves, between sample dates 4 through 7, period of rapid population increase (Table I), sug- designated early season post=insecticide predators. gesting that predators can delay the onset of spider Because applications of permethrin were staggered mite infestation. The subsequent rate at which the throughout the season in 1983, the post-insecticide spider mite population increased was negatively variable was not derived for that year. correlated only with early season Geocoris eggs. Treatments from each year that involved appli- Early season spider mite P(1)-DD was negatively cations of dicofol were omitted from the analyses, correlated with F. occidentalis adults, Geocoris because spider mite abundance would be artifi- eggs, and Orius nymphs and adults (Table 2). cially low in relation to the frequency of predators. Mid-to-late season spider mite P(1)-DD was neg- Because all treatments in 1983 involved inoculative atively correlated with early season Geocoris eggs, releases of T. turkestani, except the control (no early season Orius nymphs and adults, and mid- mites released, no pesticides), control treatments to-late season F. occidentalis larvae and adults (Ta- were omitted from the analysis. ble 2). There was also significant negative correlation between mid-to-late season spider mite-DD (i.e., absolute numbers) and early season Geocoris Results eggs, early season Onus nymphs, and mid-to-late Population Phenology. Frankliniella occiden- season F. occidentalis adults. talis was usually observed around 350 DD, 150- Regression on post-insecticide F. occidentalis Vol. 20, no. 3

Table 1. Coefficients of determination (r2), for spider the first relationship in Table 5. All coefficients mite infestation rate and initiation of rapid spider mite increase, regressed on early season predator abundance were negative, suggesting that all of these predators for 1982 had a negative impact on early season spider mites.

- The potential impact of early season predators on Early season Spider mite abundance variables" mid-to-late season spider mites is depicted in the predator Initiation of rapid second function of Table 5. All coefficients, except P(7)-DD ppulation inCree Infestation rate for F. occidentalis adults, were negative, suggest- WFT* larvae (+) 0.02 NS (-) 0.01 NS ing that early season activity of natural enemies WFT adults (+) 0.65** (-) 0.17 NS can negatively affect spider mite abundance later Geocoris eggs (+) 0.52** (-) 0.73*** in the season. Removing the effect of F. occiden- Orius nymphs (+) 0.45* (-) 0.17 NS talis adults from the relationship resulted in the Orius adults (+) 0.11 NS (-) 0.01 NS third function of Table 5. The fourth function was

a Parentheses enclose sign of the regression slope; *, P < 0.05; produced using estimated spider mite and predator **, P < 0.01;***. P < O.Ool;NS, P > 0.05;n = 12. densities. Coefficients from this function suggest * WFT, western flower thrips, F. occfdentalfs. that all early season predators exhibited some potential for impact on densities of mid-to-late season spider mites. larval populations (samples 4-7) indicated a slight- In general, results from 1983 show little or no ly significant negative association between early early season potential effectiveness by Geocoris or season F. occidentalis larvae and mid-to-late season Onus. There were no significant multiple associ- spider mites (Table 3), suggesting a potential early ations between predators and early season spider season impact by F. occidentalis larvae on later mites, and only a few significant or nearly signif- season spider mites. Using the post-insecticide time icant associations between early season predators period also supported earlier findings on other :ind mid-to-late season spider mites (Table 5). -Fhe predators as well. i absence of early season activity by these predaitors In 1983, there was a significant negative corre- i1s in sharp contrast to the results of 1982, wl-~ich

*h/^*Ãˆ-^th^'m tn hft..,., ff-m."" ../^t^yitlnI ^>/Ãˆ-l. "-A lation between mid-to-late season spider mite P(Z)- SJIJIVTT uu UAW~S.~ I" A~CTV- JLA WAA~~VLGJUL~~J~ Gal 1y 3c;aSOn DD and early season F. ocddentalis adults and impact on spider mite populations. larvae (Table 4). This effect was also true for es- Mean spider mite P(Z)-DD was lower in 1983 timated numbers of snider mites. There. were.-- no (596) than in 1982 (651), which constituted an 8% significant ci orrelations between snider mite abu drop. However, estimated spider mite-DD were on dance and Ceocoris eggs, but there was a significant average 34% lower in 1983. The range between positive correlation with Orius nymphs (Table 4). high and low values of spider mite P(Z)-DD was Multiple Associations. Multiple regression anal- also much less in 1983 (196) than in 1982 (385), yses were conducted for all possible combinations representing a drop of 49%. Similarly, the range of spider mite and predator variables. However, in estimated spider mite-DD was 70% less in 1983 many were not statistically significant, and there- than in 1982. fore, only those analyses which exhibited significant individual and overall effects are presented here. Discussion Regressing early season spider mite P(Z)-DD Population Phenology. The time of initiation of against early season predators in 1982 produced rapid spider mite increase occurs relatively early

Table 2. Coefficients of determination (r2), for spider mite abundance, regressed on early season and mid-late season predator abundance for 1982

Spider mite abundance variables"'* Predator Time Spider mite P(1)-DD Spider mite DD Early Mid-late Early Mid-late WFT" larvae early (-) 0.00 NS (-) 0.00 NS (-) 0.01 NS (-) 0.02 NS mid-late - (-) 0.67** - (-) 0.03 NS WFT adults early -) 0.57** -) 0.25 NS (-) 0.20 NS (-) 0.28 NS mid-late - (-) 0.56** - (-) 0.77- Ceocoris eggs early (-) 0.62** (-) 0.60** (-) 0.44* (-) 0.65** mid-late - (-) 0.01 NS - (-) 0.15 NS Ortus nymphs early (-) 0.62** (-) 0.76*** (-) 0.45* (-) 0.60** mid-late - (-) 0.01 NS - (+) 0.11 NS Orius adults early (-) 0.37* (-) 0.49* (-) 0.29 NS (-) 0.22 NS mid-late - (+) 0.24 NS - (+) 0.15 NS

a Parentheses enclose sign of the regression slope; *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, P > 0.05; n = 12. *Spider mite P(1)-DD regressed on Predator P(1)-DD, and Spider mite DD on Predator DD. WFT, western flower thrips, F. occfdentalfs. June 1991 WILSONET AL,: NATURALENEMIES OF SPIDER MITES 853

Table 3. Coefficients of determination (s),for spider mite abundance, regressed on early season post-insecticide reda at or abundance for 1982

Spider mite abundance variables"'^ Predator Spider mite P(1)-DD Spider mite DD Early Mid-late Early Mid-late WFTc larvae (-) 0.11 NS (-) 0.36* (-) 0.00 NS (-) 0.03 NS WFT adults (-) 0.22 NS (-) 0.11 NS (-) 0.01 NS (-) 0.01 NS Ceocoris eggs (-) 0.58** (-) 0.46* (-) 0.55;- (-) 0.66** Orius nymphs (-) 0.51- (-) 0.38* (-) 0.35* (-1 0.4V Orius adults (-) 0.33 NS (-) 0.49* (-) 0.26 NS (-) 0.22 NS

a Parentheses enclose sign of the regression slope; *, P < 0.05; **, P < 0.01; *#, P < 0.001; NS, P > 0.05; n = 12. Spider mite P(1)-DD regressed on Predator P(1)-DD, and Spider mite DD on Predator DD. WFT, western flower thrips, F. occidentalis. in the season, and in heavy mite infested treat- Predator-Spider Mite Associations. All preda- ments, P(I) can reach 1.0 by midseason. During tors exhibited the potential to suppress some aspect this period, spider mites become established on of spider mite population growth. The early season cotton plants and expand their distribution (i.e., potential of Geocoris, Onus. and F. occidentalis disperse). to affect spider mite abundance was supported by Frankliniella occidentalis is the earliest arthro- 1982 results, but less so by 1983 results. In 1982, pod to colonize cotton and has the potential to be the greater the early season abundance of each of an effective regulating agent .by restricting spider these predators, the later the time at which the mite infestations, delaying them, or both. As later spider mites initiated theil- phase of rapid popu- arrivals, Geocoris and Orius would seem to be less lation increase, and lower the abundance of mid- well positioned in time than F. occidentalis to af- to-late season spider mites. :Fhis effect was obscured fect early spider mite infestations. However, our *by the.. reduced. .. abundance . of spider mites in 1983, 1982 data indicate the potential for early season and possibly by the staggered release of T. turke- impact by both Geocoris and Orius. Being po- stunt, and accompanying applications of permeth- lyphagous, Orius and Geocoris prey on F. occi- rin. dentalis as well as spider mites (van den Bosch & Frankliniella occidentalis has been character- Hagen 1966, Huffaker et al. 1970, Gonzalez & Wil- ized as an opportunist and a passive predator (Tri- son 1982, Letourneau & Altieri 1983, L.T.W., un- chilo & Leigh 1986a). This omnivore was observed published data). Early season F. occidentalis can to feed only on newly laid eggs of spider mites, serve as an alternative food source for these other and has difficulty maneuvering in spider mite web- predators until spider mites become established on bing. Moreover, this species is primarily herbivo- cotton. As an herbivore, a food source for Geocoris rous and only becomes noticeably predaceous at and Orius, and a predator of spider mites, F. oc- high egg densities. Flower thrips do not actively cidentalis is of considerable importance in the cot- seek spider mite eggs, but will feed on them if ton-arthropod food web. encountered on the leaf surface (P.J.T., unpub-

Table 4. Coefficients of determination (r2), for spider mite abundance, regressed on early season and mid-late season predator abundance for 1983

Spider mite abundance variables"^ Predator Time Solder mite P(I)-DD Snider mite DD - -- Early Mid-late Early Mid-late WFTc larvae early (-) 0.13 NS (-) 0.59** (-) 0.00 NS (-) 0.66** mid-late - (-) 0.10 NS - (-) 0.28 NS WFT adults early (-) 0.17 NS (-) 0.69;. (-) 0.15 NS (-) 0.69** mid-late - (-) 0.33 NS - (-) 0.61** Geocorfs eggs early (+) 0.05 NS (-) 0.10 NS (+) 0.02 NS (-) 0.04 NS mid-late - (+) 0.01 NS - (-) 0.19 NS Ortus nymphs early (+) 0.42* (+) 0.01 NS (+) 0.52* (+) 0.00 NS mid-late - (+) 0.52* - (+) 0.30 NS Ortus adults^ early - - - - mid-late - (+) 0.02 NS - (+) 0.18 NS

a Parentheses enclose sign of the regression slope; * P < 0.05; **, P < 0.01; ***, P < 0.001; NS, P > 0.05; n = 10. Spider mite P(1)-DD regressed on Predator P(1)-DD, and Spider mite DD on Predator DD. WFT, western flower thrips, F. occidentalis. ^ No Orius adults observed in early season samples. 854 ENVIRONMENTALENTOMOLOGY Vol. 20, no. 3

Table 5. Coefficients and corresponding statistics, resulting from multiple linear regression of spider mite abundance, on early season predator abundance for 1982 and 1983

Dependent Independent variables statistics variable Y-intercept TLes TAes GEes 0% 0'4- r2 F P 1982 SMm P(1)-DD 189.622 -0.094 -0.415 -2.608 -1.102 -4.020 0.87 8.336 0.0113 SMmh P(1)-DD 742.507 -1.168 +1.905 -13.691 -9.671 -27.185 0.94 19.911 0.0011 SMmk P(1)-DD 946.229 -0.602 - -10.512 -10.454 -20.318 0.91 16.675 0.0011 SMmk DD 7.143.777.135 -7.069 -31.498 -1.002.866 -366.156 -1.155.033 0.89 9.894 0.0073

Regression equations of the form: SM = fSy + ffi TL + flv TA + fti GE + b4 ON + Bs OA. es, early season; mls, mid-late season;SM, spider mite; TL, flower thrips, F.occidentalis, larva, TA, flower thrips adult; GE, Ceocoris egg; ON, Orius nymph; OA, Orius adult. Spider mite P(1)-DD. regressed on Predator P(1)-DD, and Spider mite DD on Predator DD. lished data). Consequently, their effectiveness as and predator frequencies to allow the impact of predators should increase as they become more predators to be expressed. Lower average spider abundant. Significant negative associations be- mite abundance, and especially the greatly re- tween spider mites and mid-to-late season F. oc- duced range of variation in the numbers of spider cidentalis in 1982 suggest that they have the po- mites, undoubtedly contributed to the difficulty in tential to affect spider mite abundance throughout demonstrating significant correlations between spi- the season. Frankliniella occidentalis had potential der mites and their natural enemies in 1983. impact during mid-season, when spider mites were Greater spider mite abundance in permethrin typically increasing" in density. treated plots might be the result of increased spider If all 1iredators were equally abundant, we woi lid mite dispersal (Iftner & Hall 1983, Penman & not expt;ct F. occidentalis to have as dramatic an Chapman 1988). Increased dispersal should be ob- effect as Geocoris or Onus. By virtue of their larg ;er served as increased infestation rates (i.e., increase size, Geocoris and,A. urws could... be expected to take in proportion of infested leaves), as spider mites more prey per unit time than F. occidentalis. move away from heavily or moderately infested Moreover, these two larger predators appear to be leaves to uninfested leaves. Reduced competition more aggressive, and probably search more ac- and higher food quality associated with less crowd- tivel!y for- prey than does F. occidentalis. Data fr()m ed leaves is generally believed to stimulate higher 1982 support this prediction, with smaller sim]?le spider mite reproductive rates (Wrensch & Young and multiple linear regression coefficients for F. 1978). ...* ,. - - occidentalis. than tor the. other. two predators. How- Negative correlations between natural enemies ever, early season F. occidentalis often outnum- and their prey do not demonstrate cause and effect. bered early season Geocoris by more than 20:1, Absence of consistent correlations from one year and therefore could have greater overall impact. to another could suggest that the fluctuation in Frankliniella occidentalis was difficult to sup- spider mite densities was coincidental and not nec- press with insecticide for extended periods. Adult essarily the result of predator activity. However, it F. occidentalis primarily inhabit cotton flowers is more likely that the extremely low abundance during peak blooming (Pickett et al. 1988, P.J.T., of spider mites during the second year of the study unpublished data), which could afford some escape was the major reason for the loss of significant from insecticide exposure during this period. Adults effects that year. Flower thrips and spider mites are highly mobile and can invade insecticide-treat- share the same photosynthetic resource, although ed plots relatively soon after the chemical has dis- F. occidentalis adults acquire a major portion of sipated. With a reproductive rate similar to spider their nutrition from pollen (Trichilo & Leigh 1988). mites (Trichilo & Leigh 1985, 1988), larvae can A common food source and difficulty with spider quickly increase in numbers. Rapid replacement mite webbing could constitute reasons for avoid- of larval F. occidentalis undoubtedly interfered ance of spider mites by F. occidentalis. This option with our ability to clearly quantify relationships becomes less likely later in the season when spider between these predators and spider mite abun- mites are increasing in abundance and the resource dance. of fresh new cotton leaves is diminishing. Results of our current study emphasize the dif- The predaceous nature of all three of these nat- ficulty in consistently observing significant associ- ural enemies has been documented (York 1944, ations among populations within a given year and Iglinski & Rainwater 1950, van den Bosch & Hagen from one year to another. The ability to demon- 1966, Huffaker et al. 1970, Trichilo & Leigh 1986a), strate significant associations is dependent upon and there is no evidence to support avoidance of producing sufficient variation in both spider mite spider mites by Geocoris or Onus. Thus, there is - -

June 1991 WILSONET AL.: NATURALENEMIES OF SPIDERMITES

adequate reason to suggest that much of the ob- and permethrin on Tetranychus urticae Koch (Acari: served differences in spider mite abundance be- Tetranychidae) dispersal behavior. Environ. Ento- tween experimental units resulted from variation mol. 12: 1782-1786. in predation pressure. 1984. The effects of fenvalerate and permethrin res- idues on Tetranychus urticae Koch fecundity and rate of development. J. Agric. Entomol. 1: 191-200. Acknowledgment lglinski, W., Jr., & C. F. Rainwater. 1950. Onus insidiosus, an enemy of spider mites on cotton. J. This research was supported in part by a grant to Lo Econ. Entomol. 43: 567-568. T. Wilson from the EPA and USDA funded National Leigh, T. F. 1963. Considerations of distribution, Consortium for Integrated Pest Management, and by a abundance, and control of acarine pests of cotton. grant to D. Gonzalez and L. T. Wilson from the Uni- Adv. Acarol. 1: 14-20. versity of California Systemwide Integrated Pest Man- Leigh, T. F. & V. E. Burton. 1976. Spider mite pests agement Program. We thank R. Friesen and M. P. Hoff- of cotton. Division of Agricultural Sciences. Univer- mann for their help in gathering much of the field sity of California, Berkeley. Leaflet 2888, infestation data. Letourneau, D. K. & M. A. Altieri. 1983. Abundance patterns of a predator, Orius tristicolor (Hemiptera: Anthocoridae), and its prey, Frankliniella occiden- References Cited talis (Th~sanoptera:Thripidae): Habitat attraction in po~yculturesversus monocultures. Environ. Entomol. Bartlett, B. R. 1968. Outbreaks of two-spotted spider- 12: 1464-1469. mites and cotton aphids following pesticide treat- McWhorter, G. M. 1982. Spider mites: a status report, ment. I. Pest stimulation vs. natural enemy destruc- pp. 189-191. In 1982 Proc. Beltwide Cotton Prod. tion as the cause of outbreaks. J. Econ. Entomol. 61: Res. Conf. National Cotton Council of America. 297-303. Memphis, Tenn. Bentley, W. J., F. G. Zalom, W. W. Barnen & J. P. Penman, D. 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