Section D: Natural Enemy Ecology, and Biological Control (Part One) - 1999

Section D: Part Two (1999)

Section D (2000)

Plenary Session Summary:

Authors: Juli Gould

Affiliation & Location: USDA-APHIS, Phoenix Plant Protection Center.

Evaluating the impact of whitefly natural enemies established during the classical biological control program

Classical biological control has been a very successful strategy for several species of non-native pest whiteflies. The impact of the released natural enemies on the target pest has been well documented. Designing methodologies to evaluate the impact of released parasitoids against Bemisia has not been easy. For this presentation I attempted to answer the question: Why was evaluating the impact of parasitoids so straightforward for other classical whitefly biocontrol programs, and why is it so difficult for Bemisia? Evaluations should seek to answer three questions 1) Do parasitoids reduce the average density of the pest, 2) What are the mechanisms behind density reduction, and 3) Which species are responsible for density reduction? For this presentation I concentrated on how best to determine the effect of exotic natural enemies on the average density of the target pest.

Determining the effect of natural enemies on pest species is best addressed using an experimental approach. One creates two types of populations; ones with and ones without the natural enemy. Differences in pest mortality are then correlated to the natural enemy's density and interpreted as the effect of the natural enemy. One can create with and without contrasts both in time (Before and After Contrasts) or through space (Geographic Contrasts). Because there can be variability within a site from year to year and between sites because of specific site characteristics, it is even more powerful to combine the two approaches.

Examples were presented of the successful use of with and without contrasts to demonstrate the success of biological control for seven whitefly species. Before and after contrasts were used for all seven species, with life-tables and multivariate analysis also used for a few species. With and without contrasts do not work well when the density of the pest from year to year or from site to site is highly variable and influenced by factors such as migration, changing cropping patterns year-specific weather patterns, and changing pesticide use. These factors are quite prevalent in the dynamics of Bemisia. One factor that contributes to this high variability is the fact that the sampling units are not stable through time. The other whitefly species mentioned are all pests of perennial plants that can be sampled through time. Bemisia attacks annual crops, and when sampling it is very difficult to control important factors such as proximity to alfalfa or melons and pesticide use against other pests.

To get around this problem, D'Almeida et al. used multivariate analysis to evaluate the impact of two Encarsia species and other biotic and abiotic factors on the spiraling whitefly (Aleurodicus dispersus). They claimed that "with this technique, data becomes quantitatively comparable across vast areas. In particular, the impact of a biological control agent becomes measurable in the situation of multi-cropped farmers' fields, where many factors influence host populations or yield". The results of their study for the spiraling whitefly and two mealybug species were presented. The density of nearby human populations, as well as natural enemy presence, was the most important variables predicting pest density. I would recommend designing and implementing multivariate analysis if we are going to effectively evaluate the impact of exotic parasitoids on Bemisia populations in the United States.


Investigator's Name(s): D. H. Akey & T. J. Henneberry.

Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040-8803.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: June - October 1998

Use of the Entomopathogenic Fungi, Beauveria bassiana, and Paecilomyces fumosoroseus as Biorational Agents Against the Silverleaf Whitefly (SLWF), Bemisia argentifolii, in Field Trials in Upland Cotton

Deltapine NuCOTN 33B was planted and furrow irrigated in plots 109 ft. in length and 12 rows across (40-in. rows). Plots were separated by 4 fallow rows and 20 ft. alleys. Beauveria bassiana as Naturalis®, Troy Biosciences Inc. at 10 oz. Product/ac, 2.3x 107 conidiia/ml was used at full rate as single product applications. Beauveria bassiana as Mycotrol®, Mycotech Corp., 0.5 lbs./ac, 2 x 10 13 spores/lb. was used at full rate as single product applications. Paecilomyces fumosoroseus PFR-97® Thermo Trilogy Corp., 0.025 lbs. / gal., 1x 10 9 CFU (spores)/ gm equivalent 20% product was used. These treatments were part of a 12-treatment random block design that included a "Best Agricultural Practice regime", embedded controls, and a 1-ac block control.

Eggs, small nymphs, and large nymphs were sampled from leaves taken from 5 plants per plot, from the fifth main-stem leaf down from the first expanded terminal leaf. Each sample was counted from a 1-in. disk taken between the main leave stem and the next lateral vein. Adults were sampled from 30 leaves/plot, same location using a binomial decision of counting a leaf as positive if 3 or more adults were present. Weekly sweeps were taken in all plots for predators, parasites, and Lygus. Applications were made by ground with 5 nozzles/row; 1 overhead, and 2 swivel nozzles angled upward on a drop on each side of the row. Sprays were applied at 250 psi and 30 gal./ac.

The 1998 cotton season was a poor production year. The SLWF population reached action threshold (Univ. AZ recommendations) between July 27- Aug.6, 1998. Two sprays were applied (8-day period), then the SLWF population dropped precipitously and did not recover before the end of the cotton season. Against eggs, formulations of Beauveria bassiana had efficacies of 41 and 23 % as Naturalis L®and Mycotrol®, respectively; PFR-97® had an efficacy of 30 %. Against small nymphs, formulations of Beauveria bassiana had efficacies of 6 and 19 % as Naturalis L®and Mycotrol®, respectively; PFR-97® had an efficacy of 24 %. Against large nymphs, formulations of Beauveria bassiana had efficacies of 28 and 8 % as Naturalis L®and Mycotrol®, respectively; PFR-97® had an efficacy of 4 %. Despite these low efficacy rates, the SLWF populations remained below action thresholds. Only egg efficacies were statistically significant by ANOVA

and LSD (P < 05). Buprofezin (ApplaudTM 70 WP, AgrEvo, 0.35 lb. AI/ac) was the first BAP treatment was to be applied. Because of the unusual nature of the SLWF population decline, no other BAP treatments were made. Buprofezin (ApplaudTM 70 WP, AgrEvo, 0.35 lb. AI/ac) was used as a standard and had egg, small nymphs, and large nymph efficacies of 28, 37, 38 % respectively.

Investigator's Name(s): Elizabeth W. Davidson1 & Walker Jones2.

Affiliation & Location: 1Department of Biology, Arizona State University, Tempe, AZ 85287-1501; 2USDA-ARS, Biological Control of Insects Research Unit, Weslaco, TX 78596.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January 1, 1997 - December 31, 1998

Successful Rearing of Parasitoid Wasps on Bemisia argentifolii Cultured on Artificial Diet

We have demonstrated that Encarsia formosa and Encarsia pergandiella parasitic wasps can successfully oviposit and develop in whitefly larvae maintained on liquid artificial diet. Eretmocerus mundus, which oviposit underneath their host, demonstrated typical oviposition behavior on these larvae, but no progeny developed. We presume that oviposition never took place because host larvae were too tightly secured to the artificial substrate, preventing the wasp's ovipositor to access the underside of the Bemisia larvae.

We have subsequently concentrated our efforts on rearing E. formosa as this species is a commercially available parasitoid sold for managing whitefly pests of greenhouse crops. Additionally, E. formosa is uniparental, making it more convenient for experimentation.

E. formosa was obtained from the USDA Mission, TX Plant Protection Center, and used to establish a laboratory colony on B. argentifolii reared on tomato, cotton and collards. Parasitoid pupae were individually removed from leaves and surface sterilized on filters, using a regime of 70% ethanol followed by 3 min in 10% chlorox solution. Pupae were rinsed with sterile water and confined to sterile petri dishes until emergence. Emerged wasps were chilled and moved to feeders containing artificial diet (Jancovich et al., 1997, Feeding chamber and diet for culture of nymphal Bemisia argentifolii. J. Econ. Entomol. 90: 628-633) on which whitefly larvae had attained third or fourth instar. Wasps were confined to the inner chamber containing the whitefly larvae using sterile slides. After 48 hr, chambers were again chilled and wasps removed. Chambers were held at 25 oC and observed for evidence of parasitism.

Parasitism, as indicated by dark oviposition marks, was observed on nearly all exposed hosts after 48 hr. Successful emergence of adult wasps occurred 15-22 days. Wasps continued to emerge over a period of ca. 5 days. On most feeders, nearly all living whitefly larvae of appropriate age (third instar or beyond) were parasitized. As of December 1, 1998, at least 50 E. formosa have been produced on these Bemisia produced on artificial diet, and emerged adult females have been transferred to further feeders and to plants to assess parasitic efficiency of the adults produced on the artificial system. We believe this represents the first report of production of parasitoid wasps on whiteflies on artificial feeding system.

The major problem encountered in this process is fungal contamination. Because the chambers must be handled several times during the parasitism process, they frequently become contaminated with fungi which quickly overgrow the chamber. Subjecting the chambers to sterilizing ultraviolet radiation after each handling procedure reduces contamination somewhat. We are currently attempting to reduce or eliminate fungal contamination, to shorten the development time by improvements in whitefly diet, and to assess the reproductive success of wasps produced by this technique.

Investigator's Name(s): S. M. Greenberg1, Walker A. Jones2, B. C. Legaspi1, Jr., & W. C. Warfield2.

Affiliation & Location: 1Joint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS and Texas Agricultural Experiment Station, Weslaco, TX; 2Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS, Weslaco, TX.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1998

Interaction Between Encarsia pergandiella (Hymenoptera: Aphelinidae) and its Host Bemisia argentifolii (Homoptera: Aleyrodidae): Effects of Parasitoid Densities and Host-Parasitoid Ratios

Laboratory experiments were conducted to measure the functional response of Encarsia pergandiella Howard (Hymenoptera: Aphelinidae) to Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) and the effects of different densities of parasitoids on mutual interference. When host density was held constant (100 third instars) and parasitoid density was varied from at 1, 5, to 15 females, the percentage of total host mortality (parasitized + desiccated nymphs) was significantly lower at the lower parasitoid densities. The number of parasitized host nymphs per parasitoid female decreased 15-fold with increasing parasitoid density from 1 to 15. The interference between parasitoids, which detracts from their searching efficiency, increased as parasitoid density increased. The emergence rate, development time, and body lengths of progeny were significantly greater at parasitoid densities of 1 and 5 than at 15. When the number of parasitoids was held constant (n = 5) and the number of hosts varied (5, 25, 50, 100, and 250), the total percentage of nymph mortality decreased from 91.3% (1 : 1 parasitoid - host ratio) to 19.1% (1 : 50). The data could be described using a Type 11 functional response curve. To summarize and compare the effects of parasitoid - host ratios, we propose a generalized index of efficacy, calculated by multiplying the proportions of total host mortality and emergence of parasitoids under each treatment. The index showed that the most efficient parasitoid - host ratio was 1: 10.

Investigator's Name(s): S. M. Greenberg1, Walker A. Jones2, B. C. Legaspi, Jr.1, & W. C. Warfield2.

Affiliation & Location: lJoint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS and Texas Agricultural Experiment Station, Weslaco, TX; 2Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS, Weslaco, TX.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1998

The Effect of Varying Ratios of Bemisia argentifolii (Homoptera: Aleyrodidae) and Eretmocerus mundus (Hymenoptera: Aphelinidae) on Parasitism

We investigated the effects of different host: parasitoid ratios on the efficacy of the parasitoid Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) attacking the silverleaf whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae). When host density was held constant (100 second instars) and parasitoid density increased from 1 to 15 females, the percentage of total host mortality (parasitized + desiccated nymphs) was significantly lower at low parasitoid densities. In this same experiment, the number of host nymphs killed and female parasitoid progeny per female decreased 3.6 and 20.4 times, respectively. The emergence rate, sex ratio, development time, longevity, and body lengths of progeny were significantly higher at the lowest parasitoid density. When the number of hosts was increased from 5 to 250, and parasitoid density was held constant, the total percentage of nymph mortality decreased 1.6 times. The percentage of desiccated nymphs at the lowest host density (65.7%) was significantly greater than at other densities while the percentage of parasitism at the lowest host density (34.3%) was significantly lower. The data could be described using a Type I functional response curve. We propose a generalized index of efficacy (GIE) to summarize and compare the total effects of parasitoid - host ratios. This index showed that the most efficient ratio was one parasitoid female per ten second instar host nymphs.

Investigator's Name(s): S. M. Greenberg1, Walker A. Jones2, & W. C. Warfield2.

Affiliation & Location: 1Joint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS, and Texas Agricultural Experiment Station, Weslaco, TX; 2Beneficial Insects Research Unit, Kida de la Garza Subtropical Agricultural Research Center, ARS-USDA.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1998

Effects of Host Plant and Whitefly Species on Parasitoid Biology

Biological parameters were measured for the native Encarsia pergandiella Howard and the exotic Eretmocerus mundus Mercet when reared on 3 whitefly genotypes maintained on 2 host plant species. The 3 whitefly cultures were: Bemisia argentifolii Bellows and Perring (= B. tabaci, Biotype B), Trialeurodes vaporariorum (lab culture from Ithaca, NY, designated "A"), and a "wild" culture of T. vaporariorum collected locally from weeds (designated "B"). The 3 whiteflies were each reared on excised leaves of cotton and green bean placed in floral aquapik tubes filled with hydroponic solution, thus producing a 2 x 3 x 2 factorial. Tests were conducted at 25°C, 55% RH, with a 16 : 8 (L : D) h regime. Two female parasitoids were exposed to 100 whitefly nymphs for 3 h (2nd instars for E. mundus; 3rd for E. pergandiella ). The parasitization and emergence rates of both parasitoids were significantly higher on B. argentifolii reared on cotton or bean than those on T. vaporariorum "A"; data for T. vaporariorum "B" were intermediate. These indices for E. pergandiella were higher and the differences between them from host plants and whitefly species were less pronounced than by E. mundus. Developmental time of both E. mundus and E. pergandiella was significantly faster after parasitizing B. argentifolii maintained on cotton than on bean. However, E. pergandiella developed significantly faster when parasitizing T. vaporariorum "A" on bean than on cotton; no other whitefly - plant combinations produced significant differences in parasitoid growth rate. E. mundus produced a significantly higher percentage of female progeny when parasitizing B. argentifolii reared on cotton, but produced significantly fewer females when attacking T. vaporariorum "A" on cotton, compared to the same whitefly hosts maintained on bean. Both parasitoid species progeny lived significantly longer after developing on B. argentifolii maintained on cotton than on bean. However, when both E. mundus and E. pergandiella developed on T. vaporariorum "A" reared on bean, the resulting adults lived significantly longer than those that developed on the same hosts reared on cotton. The other whitefly-host plant combinations yielded no significant differences in progeny longevity. Significant differences in the size of female progeny corresponded directly to the significant differences in longevity. Generally, B. argentifolii was a better host for parasitoids when cotton was the host plant, whereas the laboratory strain of T. vaporariorum was a more suitable parasitoid host if reared on green bean.

Investigator's Name(s): James Hagler, Glen Jackson, & Juli Gould1.

Affiliation & Location: USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ & 1USDA-APHIS, Phoenix, AZ.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: March 1998 - December 1998

Evaluation of Parasitoid Dispersal Patterns by Mark-Release-Recapture

Laboratory studies were conducted to evaluate the efficacy of marking whitefly parasitoids with a protein prior to enzyme-linked immunosorbent assay (ELISA). Data indicated that this marking technique is superior to conventional marking techniques. In the field, adult Eretmocerus from the United Arab Emirates were internally marked by feeding them a honey solution spiked with either rabbit protein or chicken protein. The parasitoids marked with the rabbit protein were released at the center of a cotton field while those marked with chicken protein were released at the center of an adjacent cantaloupe field. The parasitoids were then recaptured every 4 to 6 hours for 72 hours after release using passive suction vacuum traps (100 to 200/collection) located in the two fields. Every parasitoid that was recaptured was assayed by ELISA for the presence of either the rabbit protein or chicken protein marker in order to determine its point of origin. These data are currently being analyzed and summarized to determine the inter-crop dispersal patterns of this parasitoid. The preliminary results look promising.

Investigator's Name(s): K. A. Hoelmer1, C. H. Pickett,2 & W. Abel3.

Affiliation & Location: USDA-APHIS, Phoenix Plant Methods Center, Brawley, CA (current address: USDA-ARS, EBCL, Montpellier, France1), Biological Control Program, California Department of Food & Agriculture, 3288 Meadowview Rd., Sacramento, CA2 & USDA-APHIS-PPQ, Shafter, CA.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January - December 1998

Evaluation of Citrus as an Overwintering Host of Eretmocerus Parasitizing Bemisia

Surveys by the California Department of Food & Agriculture and the University of California identified citrus orchards (especially Washington navel and Valencia orange cultivars) as potential overwintering refuges in the San Joaquin Valley for silverleaf whiteflies which could reinfest field crops the following spring. In order to assess the value of native and introduced parasitoids in helping to suppress overwintering whitefly populations in citrus, studies were initiated in the fall of 1997 and concluded in spring 1998.

A. Evaluation of citrus as a developmental host for exotic and native parasitoids: In replicated outdoor cage studies in Brawley, Washington navel cv. Lane Late seedlings were used as hosts for cohorts of Bemisia argentifolii nymphs. Five 6x6x6 ft cages per parasitoid species, each containing 4 citrus seedlings with new foliage, were inoculated with whiteflies. When populations of 2nd and 3rd instar whiteflies were present, 100 females each of Eretmocerus mundus ex Spain (MBCL #M92014), Eret. hayati (M95012 ex Pakistan), Eret. emiratus (M95104 ex UAE), Eret. sp. ex Ethiopea (M96076) and native Eret. eremicus were introduced into the cages to oviposit and hostfeed on whiteflies. Parasitoids were obtained from cultures maintained by USDA, APHIS Mission Biological Control Center in Mission, TX, the CDFA Biological Control Program in Sacramento and Novartis BCM/Bunting USA. Sampling was begun late in December 1997, and the mean number of F1 progeny per female of each species was determined. All of the Eretmocerus species in the study successfully parasitized and developed in Bemisia nymphs on citrus. The average number of progeny per female was between 2.1 and 5.9, considerably less than produced on favored hosts such as cantaloupe in warmer weather. The between-species differences in mean number of progeny were not significant in this study.

B. Evaluation of citrus in the San Joaquin Valley as an overwintering refuge for exotic and native parasitoids: In late October, after migrating whiteflies from nearby cotton established fall populations on citrus, organdy sleeve cages were placed over whitefly-infested terminal branches of established citrus trees in the San Joaquin Valley orchards. Replicated releases were made at one site each in Tulare and Kern counties. Releases were not made at a third site that had been selected due to high native predation resulting in nearly total loss of whitefly populations. Ten - twenty female Eretmocerus mundus ex Spain (M92014) or Israel (M94120), Eret. hayati ex Pakistan (M95012), Eret. emiratus ex UAE (M95104) or native Eret. eremicus (ex SW desert US) were introduced per sleeve to attack immature whiteflies. Sleeves were removed after 7 days. Reproduction and overwintering survival of the parasitoids was assessed after F1 adults began to emerge in the spring of 1998. Mortality of whitefly eggs and nymphs was very high at each site. The average number of F1 progeny per female parent was between 0.05 and 0.283. No significant differences between species were found due to the low numbers and high variability between sleeves. Our observations suggested that predation of whiteflies in the orchards and developmental mortality on citrus were key factors in reducing whitefly populations over the winter, and predation probably also impacted the survival of parasitized whiteflies.

Investigator's Name(s): Kim A. Hoelmer1 & Alvin M. Simmons2.

Affiliation & Location: USDA-APHIS, Phoenix Plant Protection Center, Brawley, CA (currently at: USDA-ARS, EBCL, Montpellier, France)1 & USDA-ARS, U.S. Vegetable Laboratory, Charleston, SC2.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January - October 1998

Yellow Sticky Trap Catches of Bemisia Parasitoids and Their Relation to Field Populations

Yellow sticky traps are sometimes used to survey for parasitoids, including those attacking Bemisia. Because there is a lack of knowledge regarding the relationship between numbers trapped and actual field populations, studies were conducted in 1997-98 with crop plants to examine the correlation of sticky trap catches with actual parasitoid populations in the associated crop.

Traps were placed in collard (summer-fall 1997) & cowpea (summer-fall 1998) on an experimental farm in Charleston, SC and in commercial organic fields of broccoli (fall-winter 1997-98) and spring cantaloupe & watermelon (spring 1998) in the Imperial Valley, CA. To ensure parasitoid populations would be present, several million exotic Eretmocerus mundus (ex Spain, M92014) were released during a 3-week period early in the season in the commercial broccoli, and several million E. emiratus(ex UAE, M95104) were released for several weeks in the commercial melon fields in the Imperial Valley. Releases were made when susceptible stages of the whitefly were detectable until several weeks prior to first placement of sticky traps. Grids of horizontally-oriented traps were placed at canopy level in crops & replaced weekly, and weekly samples of leaves were collected and numbers of parasitized whiteflies counted.

Steadily declining whitefly numbers throughout the fall-winter broccoli in CA and the summer collard in SC led to extremely low whitefly and undetectable parasitoid populations by the time the remnants of the parasitized generation matured in broccoli in Imperial Valley. Most whiteflies appear to have been lost on decomposing leaves dropped from the plants. Sampling from cowpea was concluded in October 98 and samples were frozen and await processing.

In spring melons in CA, whitefly and parasitoid populations increased throughout the season, and were more numerous on cantaloupe than watermelon. Bemisia adults in cantaloupe were trapped in greater numbers on the lower surface of sticky traps early in the season, but this was reversed during the last weeks as melons became unsuitable as a host. In watermelon, whiteflies were initially trapped on both sides equally but as the crop matured, greater numbers were trapped on top. In contrast, downward-facing trap surfaces consistently captured ca. 5-10 times more Eretmocerus than upwards-facing traps. This suggests that most of the parasitoids trapped probably came from the study site rather than migrating into the fields from elsewhere. Exotic Eretmocerus far outnumbered native Eret. eremicus and Encarsia spp. on the traps. The mean number of Eretmocerus per trap in cantaloupe ranged from ca. 4 early in the season to a high of nearly 50 per trap near crop maturity. The ratio of females to males on traps remained about 2:1 throughout the 7-week sampling period. The trend throughout the season of increasing numbers of Eretmocerus on traps paralleled the increase in numbers of Eretmocerus counted on leaf samples. The mean number of 4th instar whiteflies on leaves peaked at 70.05 / leaf by the end of the season. Throughout the season, 1/4 to 1/2 of all 4th instars were visibly parasitized by Eretmocerus (a conservative estimate which does not include parasitism by young Eretmocerus larvae and unhatched eggs).

The numbers of Eretmocerus caught by yellow sticky traps in melon fields were consistent and similar in trend to numbers on leaf samples, which suggests that sticky traps placed within crops may be used to 1) provide a reasonable estimation of the trend in parasitoid populations at a particular site, and 2) to detect the presence of these parasitoids at specific locations.

Investigator's Name(s): M. S. Hunter, T. R. Collier, & S. E. Kelly

Affiliation & Location: Dept. of Entomology, The University of Arizona, Tucson, AZ, 85721.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: January 1, 1998 - December 31, 1999

Interference Competition Between a Primary Parasitoid and an Autoparasitoid on Bemisia tabaci

Recent theory has revived the controversy surrounding the importance of interspecific competition between natural enemies on regulation of pest populations. In particular, natural enemies that attack both the pest and their competitor (`intraguild predators') are predicted to displace their competitors and reduce regulation of the pest. Autoparasitoids are a unique type of intraguild predator. In these aphelinid wasps, females develop as primary parasitoids of a homopteran host, while males develop as hyperparasitoids, attacking either conspecific females or other primary parasitoids. While autoparasitoids have been used successfully in classical biological control, their unusual life history has made them controversial candidates for introduction.

In laboratory experiments we examined the interactions between two parasitoids of the sweetpotato whitefly. Eretmocerus eremicus Rose & Zolnerowich is a primary parasitoid that is native to the Southwestern U.S., and Encarsia transvena (Timberlake) is an exotic autoparasitoid widely released in the Southwest and now establishing in California. We used the population of E. transvena originally collected in Murcia, Spain. Our experiments demonstrated that E. transvena females did not discriminate among suitable stages of conspecific and E. eremicus immatures as hosts for male eggs. However, the interval during which E. eremicus immatures were suitable for parasitism was much longer, suggesting that the primary parasitoid was more vulnerable to parasitism than conspecific E. transvena. Another set of experiments indicated that E. transvena won competitions with E. eremicus on the whitefly host.

Field experiments were conducted to examine the outcome of competition: caged cotton plants with 1) whiteflies only (Control), 2) E. eremicus alone, 3) E. transvena alone, or 4) both parasitoids together were censused in 1997 and 1998. In both years, the densities of the autoparasitoid, E. transvena were not significantly different in the `E. transvena alone' and the `both' treatments. In contrast, in both years the densities of `E. eremicus' were significantly reduced in the `both' treatment, compared to the `E. eremicus alone.' The effects of interspecific competition on the densities of the parasitoids were consistent with the laboratory experiments suggesting dominance of E. transvena in competition, and were also consistent with the theoretical predictions for autoparasitoids that parasitize their competitors. Theory did not predict the effect of this interaction on whitefly densities, however. The effect of parasitoid introduction on whitefly densities was different in the two years. In 1997, there was no significant treatment effect on whitefly densities; this was due to high whitefly densities at the time of parasitoid introductions. In 1998, there was a highly significant treatment effect, but the difference was entirely due to differences between the parasitoid treatments and the control; no significant differences were found among the three parasitoid treatments. These results suggest that the competitive interactions between the parasitoids had little effect on whitefly control.

Investigator's Name(s): TongXian Liu1 & Philip A. Stansly2.

Affiliation & Location: 1Texas Agricultural Experiment Station, Texas A&M University, 2415 E. Highway 83, Weslaco, TX 785968399; 2Southwest Florida Research and Education Center, University of Florida, 2686 State Road 29 North, Immokalee, FL 34142.

Research & Implementation Area: Section D: Natural Enemy Ecology and Biological Control.

Dates Covered by the Report: 1998

Searching and Feeding Behavior of Nephaspis oculatus and Delphastus catalinae (Coleoptera: Coccinellidae), Predators of Bemisia argentifolii (Homoptera: Aleyrodidae)

The coccinellids, Nephaspis oculatus (Blatchley) and Delphastus catalinae (LeConte), are predators of whiteflies (Homoptera: Aleyrodidae) that have shown potential for biological control of Bemisia argentifolii Bellows & Perring in greenhouses. The searching and feeding behavior of N. oculatus and D. catalinae on hibiscus leaves were observed and analyzed in the laboratory to better evaluate their biological control potential. D. catalinae larvae maintained the entire body in contact with the substrate while moving whereas N. oculatus larvae planted the uropod on the substrate and swept the body in an arc while searching. Even so, the estimated search rate for D. catalinae was greater due to faster motion. Larvae and adults of both coccinellid species responded to prey only after making contact using the mouthparts or front legs. Younger larvae took significantly longer time to consume prey than older larvae or adults. Larvae and adults of the smaller N. oculatus consumed whiteflies at a significantly slower rate than did the corresponding stage of D. catalinae. Comparing stages within species, 1st instar N. oculatus spent 53folds more time consuming an egg than did the adult, and 9folds more to consume a pupa, compared to 18 and 9fold differences, respectively for D. catalinae. First instar N. oculatus preferred 1st instar whitefly nymphs (51%) to eggs (23.6%), or other nymphal instars, and favored pupae least of all. Later larval stages show no significant preferences. D. catalinae larvae preferred whitefly eggs, followed by younger nymphs, and finally, pupae. Adults of both coccinellid species showed a strong preference for whitefly eggs (6077%), followed by nymphs, and finally, pupae. Our results indicate that although feeding preferences of the two beetles are similar, the greater propensity of N. oculatus to accept nymphs as well as relatively efficient search behavior and modest food requirements, may favor this species under conditions of low prey density.

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