Investigator’s Name(s): Steven E. Naranjo¹, James R. Hagler¹, & Peter C. Ellsworth².

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ and ²Department of Entomology, University of Arizona, Maricopa Agricultural Center, 37860 W. Smith-Enke Road, Maricopa, AZ 85239.

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

Dates Covered by the Report: January 1998 - December 1999

Conservation of Whitefly Natural Enemies in Conventional and IGR-based Management Systems

Studies were continued in 1998 to examine the comparative effect of two new insect growth regulators (IGR) and conventional insecticides on the abundance and activity of native natural enemies of Bemisia tabaci. Overall results were very similar to those observed in 1997. Mean rates of parasitism by Eretmocerus eremicus and Encarsia meritoria rarely exceeded 50 % in any treatment plot on any sample date and differences among treated and untreated control plots were generally insignificant. Parasitism was consistently higher in plots that received no insecticides for control of Lygus hesperus (split-plot treatment) compared with plots that were treated. Results for predators were more definitive. Population densities of most predators examined were significantly lower in plots treated with conventional insecticides compared with the untreated control. The IGRs caused reductions in several predator species compared with untreated controls. Based on seasonal means, densities of Orius tristicolor were depressed about 29% in pyriproxyfen-treated plots and densities of Drapetis spp.(a predatory fly) were depressed about 46-54% in buprofezin and pyriproxyfen plots (compare to 62% reduction in conventional plots). Neither IGR significantly depressed populations of any other predator species. Insecticide applications for Lygus bugs consistently depressed populations of all predator species. Combined results from 2 years of studies suggest that use of IGR conserve populations of important natural enemies that help regulate B. tabaci and other key cotton pests. These studies were conducted for a 3rd year in 1999 and results are pending.

Investigator’s Name(s): M. S. Palaniswami & Binu Antony.

Affiliation & Location: Division of Crop Protection, Central Tuber Crops Research Institute (Indian Council of Agricultural Research), Trivandrum 695 017, INDIA.

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

Dates Covered by the Report: June 1998 - December 1999

Biology and Bionomics of Encarsia Transvena (Timberlake) on Bemisia Tabaci Genn. in Cassava

Encarsia transvena is a predominant aphelinid parasitoid on Bemisia tabaci in cassava ecosystem. Investigations carried for the last one year are embodied in this paper. Host searching behaviour and biology of the parasitoid were studied in detail. Behavioural pathway involves searching, grooming, host encounter, tapping on the host by antenna and arriving over the host, to find suitable site for its oviposition. Time taken from encounter to oviposition was 46 sec.; from insertion to withdrawal 239.2 sec. and feeding time was 216 sec. Studies on E. transvena revealed that females laid fertilized eggs in whitefly nymphs preferably 3rd and 4th instars giving rise to females. Developmental duration of female parasitoid on four instars of B. tabaci were recorded. Female parasitoid passed through three larva instars. The development from egg to adult took 12-15 days. Adult female laid eggs shortly after its emergence. Biometrics of different stages of the parasitoid were also presented. Field observation revealed presence of E. transvena throughout the year. Field parasitisation ranged from 0.73-19.71% and in the net house condition it ranged from 33.3 to 69.1%.

Investigator’s Name(s): C. H. Pickett, G. S. Simmons¹, E. Lozano¹, & J. A. Goolsby².

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, Sacramento, CA; ¹USDA--APHIS--PPMC, Brawley, CA; ² USDA--ARS, Australian Biological Control Laboratory, Private Mail Bag #3, Indooroopilly, Queensland, Australia 4068.

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

Dates Covered by the Report: February 1998 - May 1999

Augmentative Biological Control Using Transplants

We report on a novel approach to enhancing early season field populations of Eretmocerus sp. using cantaloupe transplants. Cantaloupe seedlings prior to placement in fields were inoculated with a highly specific whitefly parasite, an Eretmocerus population recently imported from the United Arab Emirates. We want to determine whether control of whiteflies in fields receiving transplants inoculated with parasites, or "banker plants," is more effective than in fields receiving conventional hand releases. We also want to show that transplants with parasites can be integrated into imidacloprid treated fields at very little additional cost, or at least equal to conventional insecticide costs.

We completed our second field season spring 1999. Parasites were released into commercial farms of cantaloupe in the Imperial Valley. In 1998 and 1999 we conducted a replicated study at an organic grower, where we compared the effect of banker plants (transplants with parasites) against plots receiving hand-releases of parasites, and a no-release control. Treatments were assigned to 1/3 ac (1998) and ½ ac (1999) plots using a randomized complete block design with 4 replicates. The other growers used the conventional product imidacloprid (Admire ® ) for silverleaf whitefly control, where we compared whitefly plant densities in paired 1 acre plots, with and without the addition of banker plants, respectively. We randomly selected 40 leaves bearing early to late instar nymphs, from each treatment plot.

In 1998 and 1999 we measured significant differences in whitefly nymphal populations between the different treatment plots at the organic site. Both years, the whitefly numbers were lowest in the plots receiving parasites. However, no consistent, significant differences were detected when comparing population means between hand-released parasites and those entering fields on transplants. Parasitism means were generally higher in the transplant plots in 1998, but a little lower than those in the hand release plot in 1999; whitefly numbers were generally lowest in the transplant plots in 1998, but mixed in 1999. Whitefly numbers in transplant plots were significantly lower than both the control and hand release plots on the last sample date in 1998.

We were successful in augmenting the parasite population in one conventional field in 1999; the conventional field in 1998 never developed a whitefly population, and we were too late in getting our plants into the other 2 conventional fields in 1999; many of the transplants died in their transfer. Parasitism levels in the 1999 conventional field were several fold greater than in the control; maximums were 16% versus less than 1%. Outside of our plot, the conventional field essentially lacked parasitism. This difference was reflected in the silverleaf whitefly population: a maximum of 4 whiteflies per cm² in the control plot and 1.2 in the treated plots. Furthermore, these results show that the parasites on the transplants not only survived through the imidacloprid treatment but reproduced a new generation. This finding is supported by data from the other two conventional fields treated with imidacloprid. The number of parasitized whitefly on transplants five weeks after placement in these fields was approximately the same as on plants used in the transplanting, but held back and grown under controlled conditions at the USDA field station.

Investigator’s Name(s): C. H. Pickett, E. Lozano¹, & D. Overholt².

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, Sacramento, CA; ¹USDA--APHIS--PPMC, Brawley, CA; ²Pink Bollworm Program, CDFA, Visalia, CA.

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

Dates Covered by the Report: August 1997 - November 1999

Fall Releases of Parasites into Citrus

The silverleaf whitefly (Bemisia argentifolii) was an increasingly important pest of cotton in the San Joaquin Valley from 1994 through 1997 when this project was initiated. Field studies suggested that citrus had become an important overwintering site for this whitefly. Cotton has the highest incidence of whitefly infestations in areas of the Valley with a matrix of both citrus and cotton. We report on large scale releases of Eretmocerus emiratus (M95104), Eretmocerus nr. emiratus (M96076, Ethiopia), E. mundus (M92014), and secondarily E. hayati, and trace numbers of Encarsia transvena (M95107) (a possible contaminant) into four citrus groves. The study has two goals: (1) to determine if exotic parasites released into citrus during the fall will overwinter in this habitat and move into cotton the following spring; and (2) to permanently establish new populations of exotic parasites specific for the silverleaf whitefly.

Three study sites were identified initially, one each in Fresno, Tulare, and Kern Counties. A fourth was added because one of the original growers stopped farming cotton (Kern Co.). Sites consisted of citrus and cotton acreage managed by the same owner. Cotton is grown directly adjacent to the citrus, and growers have had a history of silverleaf whitefly problems. We began releasing parasites in early August or September 1997, 1998, and 1999 when migrating whitefly nymphs were first recorded from citrus leaves. Over 100,000 parasites were released weekly at each location and a total of 4.05 million were released in 1997, over 10 million in fall 1998, and 3.1 in 1999. The dispersal of the released parasites was recorded using sticky cards with identification based on the adult males since they could be readily distinguished from native Eretmocerus while on the sticky cards.

Whitefly migration into orchards was ongoing when we began sampling in August 1997, but didn’t commence until late September in 1998 and 1999. The delay in cotton maturity, as a result of a the cool spring, undoubtedly played a role in forestalling the emigration of whiteflies from this preferred host plant. Parasitism of silverleaf whitefly on citrus was always quite low, even in the second year of releases, rarely exceeding 10%, despite the massive releases of parasites into these trees (see Pickett & Overholt, this volume).

Our weed survey data shows that the released parasites are capable of surviving during winter months on a number of weedy plants common to citrus orchards. We also found parasitized whiteflies on weeds in spring and summer when few if any whiteflies were recorded from citrus. Their abundance appears to be associated with the abundance of weeds in and around the citrus orchard from where we were sampling. The grower in Tulare County had the cleanest orchard and host weeds were extremely difficult, if not impossible on some occasions, to find. He had the lowest presence of overwintering parasites on weeds.

Exotic parasites have been recorded from three of four cotton fields adjacent to orchards, one year after releases. The one exception to date has been the most recently added release site, Cappello’s in the southern end of Kern Co. No whiteflies were recorded from cotton this last summer, thus no whitefly hosts were available for parasitism. Parasitism levels shown at the Tulare and Fresno county farms reflect mixed populations of native and exotics. All recovered and identified parasites at the original Kern County site have been exotic; therefore the parasitism levels resulted entirely from our releases. Parasitism was recorded early in the season at all sites showing that these parasites can rapidly find and attack silverleaf whitefly present at very low numbers. All samples were taken within 100 yards of the orchard.

Investigator’s Name(s): C. H. Pickett & Bill Abel¹.

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, Sacramento, CA; ¹USDA-APHIS PPQ, Shafter, CA.

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

Dates Covered by the Report: August 1997 to November 1999.

Tracking the Impact of Released Parasites using Sentinel Plants

The establishment and impact of silverleaf whitefly (Bemisia argentifolii) has been difficult to track. The whiteflies attack a broad range of annual and perennial plants in a wide range of habitats that include both urban settings and agricultural. Sentinel plants have been used to monitor parasitism of other pests when they were difficult to locate in the environment (Marston, N. 1980. Sampling parasitoids of soybean insect pests, In Kogan & Herzog (Eds.), Sampling Methods in Soybean Entomology). Sentinel plants allow for sampling in a variety of habitats, independent of host plant type, without concern for impacts from insecticides, and independent of the host plant density. We developed a sentinel plant sampling system to measure the change in parasite species composition and the change in magnitude of parasite oviposition by introduced exotics, over and above that attributable by natives.

We started using hibiscus cuttings as sentinel plants in 1998 but switched to cotton plants in 1999 due to contamination problems. Plants were grown from potted seed, isolated in individual cages. After two weeks of exposure to adult whiteflies, one foot tall plants were placed outdoors at 30 protected sites in the southern San Joaquin Valley, two plants per site. After one week of exposure to extant parasites, plants were placed back into individual cages, allowing approximately 2 weeks for additional incubation of insects. Leaves were then stripped from plants allowing for adult whitefly and parasite emergence under controlled conditions. These individuals were counted and recorded for each of four monthly sampling events during the latter part of 1998 and 1999.

Most of the problems encountered during our first two years using sentinel plants were overcome. We eliminated contamination of sentinel plants and were able to produce a cotton sentinel plant that could withstand field conditions. This method is apparently sensitive to low populations of parasites. We picked up native parasites during the first run in May 1998 at 6 of 26 sites. The regional whitefly population in the San Joaquin Valley was exceptionally low in 1999. Parasites weren’t picked up until September when sentinel plants from 10 of 29 sites captured parasites; 4 of these captured exotic Eretmocerus. Exotic Eretmocerus made up 11% of all captured species when including Encarsia spp, and 80% when excluding them. One third of the Encarsia were Enc. inaron, introduced for control of the ash whitefly, Siphoninus phillyreae (Haliday). All of the remaining were En. pergandiella, except for one En. meritoria. Exotic Eretmocerus made up 0.16% of emerged adult insects (parasites + silverleaf whitefly) and natives made up 0.04%.

Investigator’s Names(s): W. J. Roltsch¹, E. R. Andress², K. A. Hoelmer^3, & G. S. Simmons⁴.

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, and 4151 Hwy. 86, Brawley, CA 92227 ¹; USDA-APHIS, Phoenix Plant Protection Center, Brawley²; USDA-ARS, EBCL, Parc Scientifique Agropolis II, Montelier, Cedex 5, France (formerly: USDA-APHIS, Phoenix Plant Protection Center, Brawley, CA)³; USDA-APHIS IS, Tuxtla, Mexico (formerly: USDA-APHIS-PPQ, Brawley, CA 92227)⁴.

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

Dates covered by the report: 1995 to 1999

Establishment of Introduced Parasitoids of the Silverleaf Whitefly in Imperial Valley, CA

Since 1994, a number of exotic Eretmocerus and Encarsia species/strains have been evaluated in field cages, and released in large numbers in commercial fields, refuge nursery plots and urban yards. The most promising Eretmocerus for this desert region include E. emiratus Zolnerowich & Rose, E. nr. emiratus from Ethiopia and E. mundus Mercet. Encarsia sophia (=Encarsia transvena) from Maltan, Pakistan appears promising as well. Identification to species was accomplished using recently published keys and by DNA analysis (RAPD-PCR) by the USDA-APHIS, Mission Biological Control Center, Mission, TX.

Parasitoid population development in long-term refuge nursery plots from 1994-1999: From 1994 through 1997, species of exotic parasitoids were released into long-term refuge field plots on multiple occasions each year. Plots (1/2 to 1 acre) were located at the Imperial Valley Research Center near Brawley, CA, and at an organic farm at the south end of the county. During the warm season, the plots consisted of okra and basil. During the cool season, cole crops (esp. collard) and sunflower were present. Kenaf, roselle and eggplant were also periodically present (1994-1996) along with adjacent plantings of cotton and spring cantaloupe. Leaf samples were taken approximately 6 times during each year to determine the status of parasitoid population increase and persistence. Nether E. tejanus nor E. stauferi (i.e., Eretmocerus spp. from Texas) have been recovered following their release. During 1995, E. melanoscutus was released in large numbers beginning in early August. Recoveries of this parasitoid were rare. Releases of E. mundus, E. hayati and E. emiratus began in April of 1996. Numbers of exotic parasitoids compared to natives were high during early summer, however, the proportion of the sample consisting of exotic species dropped markedly by late July, indicating poor performance (population increase and persistence) during this very warm summer period. During 1997, E. emiratus and E. nr. emiratus were released. The relative performance of exotics was considerably better than in 1996. The proportion of exotic Eretmocerus relative to native Eretmocerus eremicus declined once again during late summer, however, not to the same extent. During 1998, none of the long-term refuge plots were inoculated with exotic whitefly parasitoids. This made possible the assessment of populations released in previous years at these sites, in terms of their ability to overwinter and compete with native species of silverleaf whitefly parasitoids. Overwintering on cole crops was confirmed albeit in low numbers. During the summer of 1998 and 1999, Eretmocerus densities soared on okra, basil and adjacent cotton. By late August there was a greater proportion of exotic Eretmocerus (upwards of 80% on okra and cotton) than native Eretmocerus. The order of dominance of exotic Eretmocerus species is E. nr. emiratus, E. emiratus and E. mundus. Encarsia sophia has reached high densities during the summer and fall of 1998 and 1999 in several of the refuge field plots as well.

Regional surveys: During late summer and fall of 1998, exotic Eretmocerus were collected from numerous ornamental plants in several communities in Imperial Valley. In addition, leaf samples were obtained from three edges of a number of conventionally managed cotton fields during September of 1998 and 1999. The fall samples of ornamental plants at 15 urban sample sites in three communities indicate that exotic Eretmocerus were present in 10 of 15 sites. On average, 25% of the Eretmocerus at the 10 locations was exotic. Among cotton fields, exotic Eretmocerus were detected in 9 of the 23 fields (i.e., 39%) sampled in the fall of 1998 and in 31 of 42 fields (i.e., 74%) sampled in the fall of 1999. In those fields where exotics were detected, 6% of the Eretmocerus were exotics in 1998 and 23% were exotic in 1999. Similarly, an increase in Encarsia sophia was noted as well from 1998 to 1999. Encarsia sophia was detected in only one of 23 cotton fields (i.e., 4%) in 1998 and in 27 of 42 cotton fields (64%) in 1999.

Investigator’s Names(s): W. J. Roltsch.

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, and 4151 Hwy. 86, Brawley, CA 92227.

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

Dates Covered by the Report: 1997 to 1999

Encarsia sophia Reproduction Patterns

Encarsia species have been utilized effectively in numerous biological control projects and have commonly been identified as the primary agent responsible for control. This genus of Aphelinids is well known for its varied and ostensibly unusual reproductive biology. Some species are only known to be uniparental primary parasitoids, whereby males are absent or very rare. Comparatively few known arrenotokous species of Encarsia are characterized by diploid females and haploid males that are both primary parasitoids [e.g., E. inaron/ ash whitefly host]. Many have a heteronomous hyperparasitic mode of reproduction characterized by females that are primary parasitoids of whiteflies, whereas males develop on conspecific female parasitoid pupae or pupae of other species. Representative species such as E. opulenta and E. lahorensis have been instrumental in achieving biological control of the citrus blackfly and citrus whitefly respectively. Theoretically based questions have been raised relative to the potential of heteronomous hyperparasitic species interfering with biological control activities of other parasitoid species targeted for use in biological control programs. The ability to exploit females of another parasitoid species for male production represents an additional means by which a heteronomous hyperparasitoid may compete with other species. This is in addition to such common practices by numerous parasitoid taxa of host feeding, superparasitism, and multiple parasitism; traits that also may play important roles in interspecies interactions.

Four populations of Encarsia sophia (=E. transvena) as well as two other species of heteronomous hyperparasites have been evaluated in field cages in Imperial Valley, Ca for their potential to control Bemisia argentifolii (SLW). Encarsia sophia from Multan, Pakistan demonstrated a distinct ability to rapidly increase its densities during the hottest time of the year, whereas all other populations and species performed poorly during this time. This population of Encarsia sophia has become established and has increased in abundance since 1997. The objectives of this study are to: 1) Determine the generalized temporal patterns of Encarsia sophia sex ratio; 2) Identify E. sophia sex ratio fluctuations relative to whitefly population fluctuations; and 3) Identify uniformity of sex ratios relative to species of whitefly hostplants.

METHODS: Within a home yard in Brawley, CA represented by 19 silverleaf whitefly host plant species (plus multiple varieties), whitefly and parasitoid densities are monitored. Encarsia sophia and Eretmocerus spp. pupae are isolated in individual cells within plastic trays and allowed to emerge to determine the sex ratio. This is primarily done for broccoli, collard and ornamental hibiscus, the most commonly attacked plants on site. Similar methods are employed for monitoring whitefly host plants grown together as strips within ½ to 1-acre field plots. Plant species include okra, basil, collard, sunflower, and cotton.

RESULTS: In 15 of 16 samples obtained from broccoli and collard in 1998 & 1999, the E. sophia population consisted of over 50% females, and commonly exceeded 90% females. Similar results were obtained from field plot samples. Reductions in % females (i.e., extensive male production) were associated with a precipitous decline of whitefly density following peak seasonal whitefly density cycles. These peak activity periods were at times initiated by large influxes of whitefly from remote "outside sources" during late summer. This was observed in both home yard and field plots sites. Sex ratios were usually homogeneous across plant species environments (i.e., basil, hibiscus, collard and broccoli).

DISCUSSION: Encarsia sophia appears to be typified by high rates of female progeny production. Data suggest that widely fluctuating whitefly densities precipitate wide fluctuations in sex ratios. It is further suggested that female offspring production predominate during the initial phases of whitefly increase and during times of low whitefly fluctuations. As suggested by data collected to date regarding the homogeneity of sex ratios across plant types, the production of female offspring typically predominates throughout a vegetationally diverse environment. Encarsia sophia is known to exploit other species including Eretmocerus to produce males of its own species. Based on field cage studies and field releases, the Multan strain of E. sophia has a remarkable capacity for inflicting high levels of parasitism on whitefly populations on certain host plants during the hottest period of the year in Imperial Valley. The tradeoff between its ability to directly impact whitefly populations and its negative impact on other parasitoid species has yet to be fully determined. However, its propensity toward female offspring production and high levels of parasitization suggests that it is a potentially valuable natural enemy.

Investigator’s Name(s): John S. Weaver¹ & Matthew A. Ciomperlik².

Affiliation & Location: ¹University of New Hampshire, Durham, NH, 03824; ²USDA--APHIS--PPQ, Mission Plant Protection Center, Mission, TX, 78573-2140.

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

Dates Covered by the Report: Field Season: July - December 1998

Dispersal of Serangium parcesetosum (Coccinellidae) on Poinsettias Infested with Bemisia argentifolii (Aleyrodidae) in Greenhouse Trials

Releases of Serangium parcesetosum were evaluated for their ability to disperse throughout a greenhouse crop of poinsettias infested with Bemisia argentifolii. Whiteflies were introduced at a rate of 1.25 adult/plant in week 0 (Sep 8) into two separate greenhouses. Plants in each greenhouse were grouped into three quadrats, with 48 plants per quadrat, arranged 4 rows by 12 columns of plants. Single plants were potted in 15 cm pots, spaced 15 cm apart. Leaf samples were collected on a weekly basis and examined with a dissecting microscope. The number of live immature whiteflies were recorded, grouped into three growth stages: egg, small nymphs (= instars1+2), large nymph (= instars 3+4). Whitefly densities are expressed herein as the mean number of individuals per 25 sq cm. Releases of S. parcesetosum were made on weeks 5, 7, and 9. Beetles were tagged with three different fluorescent colored powders, and similarly tagged beetles were released on a central plant in each quadrat. The relative dispersal of S. parcesetosum was observed following releases in the greenhouse areas, and was assessed based on the location of recoveries after 48 hours. Beetle dispersal was grouped into three categories: 1) low, recovered within 0-2 plants from release point, 2) moderate, recovered within 3-5 plants, 3) high, recovered in a different quadrate. Three mark-recapture trials were preformed.

In the first mark-release trail (Oct 19) 270 beetles were released, 9.6% recovered, and the relative dispersal among the recoveries was 64% low, 18% moderate, and 18% high. Mean whitefly densities during this release were 4.21 eggs, 1.00 small nymphs 1+2, 0.70 large nymphs. (Visual observations of beetles during the first trial suggested too much powder was used which interfered with their mobility.) Tagging methods were modified in the following trials, tagging adult beetles with a small quantity of colored powder. In the second trial (Nov 1), 300 beetles were released, 25% recovered, and the relative dispersal among the recoveries was 47% low, 29% moderate, and 37% high. Mean whitefly densities during this trial were 4.68 eggs, 7.38 small nymphs, 0.48 large nymphs. In the third trail (Nov 17), 300 beetles were released, 37% recovered, and the relative dispersal among the recoveries was 65% low, 22% moderate, and 13% high. Mean whitefly densities in the third trial were 12.28 eggs, 13.78 small nymphs, 4.88 large nymphs. Results of the mark-recapture investigation suggest that releases of S. parcesetosum will disperse successfully throughout a greenhouse crop of poinsettias. However, the data indicates that if host densities are high, the beetles may not disperse as readily as when whitefly densities are low. Visual observations indicated that beetles spent the majority of their time host-searching.

Investigator’s Name(s): John S. Weaver¹ & Matthew A. Ciomperlik².

Affiliation & Location: ¹University of New Hampshire, Durham, NH, 03824; ²USDA--APHIS--PPQ, Mission Plant Protection Center, Mission, TX, 78573-2140.

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

Dates Covered by the Report: Field Season: July - December 1998

Biological Control of Bemisia argentifolii (Aleyrodidae) Infesting Poinsettias: Evaluation of Encarsia formosa, Nile Delta Strain, (Aphelinidae) and Serangium parcesetosum (Coccinellidae)

Releases of Encarsia formosa, Nile delta strain, and Serangium parcesetosum were evaluated for their ability to control Bemisia argentifolii infecting a greenhouse crop of poinsettias. Three treatments were used to assess the impact of natural enemies released on B. argentifolii: 1) releases of E. formosa and S. parcesetosum, 2) releases of S. parcesetosum only, and 3) each greenhouse area with 12 plants caged to exclude natural enemies. Whiteflies were introduced at a rate of 1.25 adult/plant in week 0 (Sep 8) into two separate greenhouses, each containing 144 potted poinsettia plants. Weekly releases of E. formosa began on week 4 and continued up to week 11, and releases of S. parcesetosum were made on weeks 5, 7, 9. Both of the natural enemies, E. formosa and S. parcesetosum, were released at a rate of ~1 adult/plant. Leaf samples were collected on a weekly basis and examined with a dissecting microscope. The number of live immature whiteflies were recorded, grouped into three growth stages (egg, instars 1+2, instars 3+4). The number of dead whiteflies was also recorded, distinguishing the cause of death as being either by predation or parasitism.

Whitefly densities within the exclusion cages were considerably greater than those of each of the two natural enemy treatments. In areas receiving natural enemies, whitefly densities increased, reaching maximum densities in week 10, and decreased over the following 3 weeks. At the end of the study (week 13: December 8), in the greenhouse area receiving both natural enemies, the whitefly population was less than 1/100 the size of the population in the corresponding exclusion cage; in the area receiving S. parcesetosum only, the whitefly population was ~1/50 the number in its respective exclusion cage. However, whitefly numbers on poinsettia plants were above acceptable market standards in both areas. Earlier releases of natural enemies may have produced a more acceptable crop. In the area receiving both natural enemies, E. formosa did not become well established (probably due to a high level of predation by S. parcesetosum) and very little parasitism of whiteflies was observed.

Investigator’s Name(s): Jianzhong Zhang¹, Arland E. Oleson¹, Don C. Vacek², Matthew A. Ciomperlik², Juli R. Gould³, Larry J. Heilmann⁴, & Dennis R. Nelson⁴.

Affiliation & Location: Biochemistry Dept., North Dakota State University¹, Fargo, ND 58105; USDA--APHIS, Mission PPC², Mission, TX 78572; USDA--APHIS, Phoenix PPC³, Phoenix, AZ 85040; USDA--ARS, Red River Valley Research Center⁴, Fargo, ND 58105.

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

Dates Covered by the Report: January 1-November 30, 1999

Whitefly Biocontrol Agents: Differentiation of Parasitoid Wasps by Satellite DNA-targeted Hybridization

The whitefly Bemisia argentifolii (Silverleaf whitefly) is among the most serious pests of cotton and horticultural crops around the world. With the increasing resistance of whitefly populations to insecticides and the emphasis on reduced usage of noxious chemicals, biological control of whiteflies in field environments has become a major interest. A longstanding approach to control of whitefly populations in greenhouses employs Encarsia formosa, a natural enemy of the whitefly. Studies have been initiated by USDA--APHIS, on the release of exotic species of another microhymenopteran genus, Eretmocerus. These insects, originating from Pakistan, Spain, United Arab Emirates, and Ethiopia, augment the limited parasitism of native U.S. Eretmocerus species on whiteflies in the field. Effective assessment of these ongoing studies requires that the parasitizing hymenopteran be identified. The small size and similar physiological and anatomical characteristics of the various Eretmocerus species makes morphological identification of the adult insects difficult, and early-stage larvae within the whitefly are nearly impossible to detect and distinguish.

Species-specific DNA probes have been used for the detection of other insects. In this communication, we report progress on the development of specific DNA probes for several species of Eretmocerus. Our approach targets species-specific components of the satellite DNA present in these organisms. Satellite DNA is comprised of a number of different high-copy-number repeated sequences localized in telomeric, centromeric, and intergenic regions of chromosomes; these sequences are widely divergent in eukaryotic organisms. We have cloned and characterized satellite sequences from two foreign species, E. mundus (Spain) and E. hyati (Pakistan), and from one native U.S. species, E. eremicus. We have also developed a probe for the greenhouse biocontrol agent, Encarsia formosa. Typically, each of the characterized sequences comprises 1-3% of the genome. The lengths of the repeat units in the characterized clones are: E. mundus (sequence-3), 172 bp; E. mundus (sequence-7), 348 bp; E. hyati, 251 bp; E. eremicus, 144 bp; and E. formosa, 33 bp. Specificity studies performed with insect squash blots indicate that the DNA probes from E. eremicus and E. formosa are completely specific and the probe from E. hyati is nearly species-specific (3% cross reaction only with E. emiratus). The probe from E. mundus hybridizes with the genome of all foreign species tested, but not with E. eremicus or E. tejanus; thus, it is useful for differentiating between foreign and native U.S. species of the parasitoid. Genomic libraries also have been prepared from the DNA of two foreign wasps (Eretmocerus sp. (Ethiopia) and E. emiratus (United Arab Emirates)), and screening and characterization of the clones is in progress.

We have also evaluated the utility of a DNA probe for detection of parasitism in whitefly nymphs collected from cotton fields where selected numbers of Eretmocerus sp. (Ethiopia) had been released. Whitefly nymphs with no obvious signs of parasitism (i.e., eggs, egg rings, or displaced mycetomes) were squashed on membranes and hybridized with radiolabeled E. mundus DNA probe; other nymphs were examined by standard non-molecular methods used in this laboratory. Both approaches showed similar, low levels of parasitism in the whitefly population.

Future efforts will include the selection of DNA probes from additional U.S. species, E. tejanus and E. transvena, the use of the array of cloned sequences in the development of nonradioactive detection systems for the various parasitoids, and additional validation of the detection methods with field-collected insects.