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Observations on Dictyna Reticulata Gertsch and Ivie

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Observations on Dictyna Reticulata Gertsch and Ivie Observations on Dictyna reticulata Gertsch and Ivie (Araneida: Dictynidae) in Imperial Valley Cotton Fields *>y J. E. Waters ti A thesis submitted in partial fulfillment of the requirements for the degree of Master of Arts in the School of Natural Science. Fresno State College June, 1972 TABLE OF CONTENTS PAGE ABSTRACT 1 INTRODUCTION .... 2 ACKNOWLEDGMENTS . , 3 METHODS AND MATERIALS 1+ RESULTS 6 DISCUSSION 9 REFERENCES CITED . 13 TABLES ... Ik FIGURES 23 PLATES 2k ABSTRACT The incidence and habits of a population of the hackled band weaver, Dictyna reticulata Gertseh and Ivie, in Imperial County, California, were followed through the 1969 cotton growing season. Spider populations were sampled using a backpack vacuum apparatus. Fixed area counts and random sample techniques were utilized to study the relationship of D, reticulata to the cotton agroecosystem. In May and June individuals of D. reticulata enter young stands of cotton and establish webs at various points on the plants to ensnare their prey. The hackled band weaver population present in the field increased in numbers through the end of July and declined after insecti­ cide applications were used to control the cotton leaf perforator, Bucculatrix thurberiella Busck. INTRODUCTION Information on populations of spiders in agricultural crops is scant, tut population studies have been conducted by Chant (.1956) and Dondale (1956, 1953) in orchard crops and by Kagan (19U3), Whitcomb et al. (1963) and Whitcomb (196b) in cotton, Ed Hunter and Thomas Leigh (personal communication) studied spider populations of various species in San Joaquin Valley cotton fields including Dlctvna reticulata Gertsch and Ivie. This hackled band weaver is common in the San Joaquin and Imperial Valleys of California where it is a general predator associated with the cotton agroecosystem. General information on the dictynids has been reported by Coni3tock (,19b0). In this investigation D. reticulata was studied in Imperial Valley cotton plantings by field observation and sampling populations during the cotton growing season of 1969. Populations of D. reticulata were sampled using a suction machine (D-Vac) as described by Dietrick (1961), along with fixed area counts and various random sample techniques currently used to study agricultural insect populations (Southvood, 1962). By mid-June the hackled band weaver was well established throughout the field, greatly increasing in number by the end of July. Dictyna reticulata webs were found at all heights on the plant, usually occurring on the top surface of leaves or at axils. Insects found trapped in these webs included species of ten orders and 2U families. Pteromalid and scelionid wasps were found in the egg sacs of D_. reticulata. ACKNOWLEDGMENTS The author expresses his appreciation to Dr. Donald J. Burdick for his counsel and guidance in this study, to Dr. Thomas Leigh and Edward Hunter of the University of California at Davis for life history infor­ mation relating to Dictgm reticulata, to Richard Jones and Rajender Sharma of the University of California Pink Bollworm Research project for their great aid in collecting tield data, and to Sheri Waters who rendered assistance in all phases of this study. Included among those who identified spiders taken in this investiga­ tion are: Muriel Monteleone, Vince Roth, Edward Hunter, and David W. Hagstrum. Special thanks are due to Dr. Richard E. Rice for his technical assistance and access to various samples which are the property of the University of California, and to Drs. B. E. Rees and K. Waodvick for reviewing the manuscript. METHODS AND MATERIALS From May to October 1969 specimens of D. reticulata were taken from scattered cotton fields near the towns of Niland, Bravley, Calipatria, El Centro, Westmoreland, and Holtville in the Imperial Valley of California. Detailed observations and population studies were made in five acre plots at Elmore Field, near Niland and at Meloland where Perthane - was applied August 23 for cotton leaf perforator control. Population levels in both space and time were followed on the Delta- Pine-lo cotton at Meloland by taking suction machine (one square foot cone) samples at weekly intervals. The field, planted in UO-inch rows, was divided into quadrants within which 50 square feet were sampled. Suction machine samples were extracted by running them through a Berlese funnel for 2h hours. Collected specimens were preserved in alcohol for identification and enumeration. The number of spiders, their webs, the location of webs on the plant and web ensnared insects were determined weekly (July to Sept.) in a fixed area count (Southwood, 1962) of ten feet of a cotton row randomly selected and marked off by stakes. Also within this area a square foot of ground and duff from base of plant to furrow was examined for the spider, its webs and egg sacs. The average plant height was recorded, and the height of each web on the plant was measured from the ground. Web structure was examined and characterized through use of photo­ graphs (Plates 2 and 3). The spiders found in established nests and suction machine samples were sexed and the male:female ratios established. Egg sacs collected at random were opened, observed for parasxtes, &nd the number of eggs counted. Probable prey vera determined by identification of web ensnared insects with the aid of the U. C. Riverside cotton reference collection. Each species was enumerated. Supportive lab studies were undertaken in which certain of the above species were fed to captive D. reticulata held individually in 25 dram plastic vials. Other spiders in the area were identified as to family, and their numbei s tabulated for comparison with the D. reticulata populations. RESULTS The population trends of D. reticulata and the other spiders, including species of Oxyopidae, Salticidae, Lycosidao, Theridiidae, Thomisidae and Pholicidae found in suction machine samples, were followed weekly from 6/5/69 to 10/15/69 (Tables 1 and 2). Specimens representing totals of all spiders other than D. reticulata are graphically considered herein as a single group. Dictyna reticulata increased in number up to 8/20 and de­ clined abruptly in coincidence with Perthane^' applications 8/23 and 8/28 but had again increased by the October defoliation. In total numbers the othei species ox spiders had declined but by 9/20 had approximated previous high density levels (Figure l). The spider, D. reticulata, appeared evenly distributed between quadrants in the cotton field (Table 1), but in relation to the plants the distribution seemed to be in established webs as no dictynids were observed wandering on the plant. In the lho feet of cotton row observed for web construction sites, 32h webs were located on 156 plants, or approximately 2.1 webs per plant. Fluctuations in the mean number of webs per plant varied from 1.15 °u 7/29 to 3.62 on 8/13 (Table 3). These webs were distributed from 5 to Ito centimeters above the ground (Table 5) with the majority of webs lo­ cated on the lower half of the plant, except on the 8/19 sample. At the hase of the plant lh duff samples, each of a square foot area, were taken (Table ^). One adult was found in a web on a recently xallen leal whereas 90 vacant webs were found on old, dry leaves. Of 306 D. reticulata webs noted for position on the plant (Table 6), 67 per cent were constructed on the upper leaf surface, 18 per cent at the Junction of the stem and a branch (primary axil), 11 per cent at the junc­ tion of two branches (secondary axil) and k per cent on squares, between closely associated bolls, between the calyx and bract of bolls, at the junction of the boll and its stem or between the junction of a petiole and lover leaf surface. Spiders taken from suction machine samples {8/13 and 3/20) for sexing numbered 112 males and 128 females. Of l63 adults collected from leaf vebs, 136 or 81» per cent vere female, and 27 or 16 per cent were males. Similarly, of 50 adults collected from axillary vebs, U2 or 8U per cent vere female, and 8 or 16 per cent vere male. The iield reproductive potential was noted by counting the number of eggs per egg sac. In 35 sacs (Table 7) ten to 50 eggs were found with an average of 2b.5 eggs per sac. On three occasions a scelionid wasp was seen cutting a small circular exit hole at the apex of the spiders egg sac. All of the eggs within these sacs were ripped and dehydrated. Of Mi egg sacs collected at random, seven contained one scelionid and two contained three scelionid wasps. Five egg sacs were found to contain one or two pterornalid wasps with one egg sac containing ten pupa. Insects trapped in the D. reticulata webs were identified (Table 8) as far as possible using the University of California cotton insect refer­ ence collection. Specimens belonging to ten orders and 2k families vere found including several important cotton pests. Insects found in numbers (above ten for season) are presented in Table 9* Insects fed success!ully to the D. reticulata laboratory colony are shown in Table 8. Captive spiders produced JQ egg sacs in captivity all of which con tained viable eggs. A mean incubation period of 10.63 days and range of 6 to 13 days was observed. Egg sacs, under construction, were permanently abandoned when females were interrupted. Construction of egg sacs at new locations began several hours after the spiders were disturbed. live ^les were observed to deposit two egg sacs each in the period of a night. as three to five egg sacs in a single web were recorded in field • • .
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