Sentinel Larvae Research Articles

The impact of exotic parasitoids on populations of a native Hawaiian moth assessed using life table studies

The impact of alien species on native organisms is a cause for concern worldwide, with biological invasions commonplace today. Suppression efforts targeting many invasive species have included introductions of biological control agents. The numerous releases of biological control agents in the Hawaiian archipelago have resulted in considerable concern for non-target impacts, due to high levels of non-target parasitism observed to occur in some cases. This study investigated the impact of introduced Hymenoptera parasitoids on a Hawaiian moth. The endemic Hawaiian moth Udea stellata (Butler) has seven alien parasitoids associated with it, two purposely introduced, three adventive, and two of uncertain origin. The objective of this study was to determine the relative contribution of the seven parasitoid species to the population dynamics of U. stellata by constructing partial life tables. Marginal attack rates and associated k-values were calculated to allow comparison of mortality factors between experimental sites. Sentinel larvae were deployed on potted host plants and left in the field for 3-day intervals in open and exclusion treatments. The factors that contributed to total mortality in the open treatment were: disappearance (42.1%), death due to unknown reasons during rearing (16.5%) and parasitism (4.9%). The open treatment incurred significantly higher larval disappearance compared to the exclusion treatment (7.8%), which suggests that in large part disappearance is the result of predation. Adventive parasitoids inflicted greater total larval mortality attributable to parasitism (97.0%) than purposely introduced species (3.0%).

Relative abundance of parasitoids reared during field exposure of sentinel larvae of the leafminers Liriomyza trifolii (Burgess), L. sativae Blanchard, and Chromatomyia horticola (Goureau) (Diptera: Agromyzidae)

In order to examine the relative abundance of indigenous parasitoids against three leafminer species, Liriomyza trifolii, L. sativae and Chromatomyia horticola, kidney bean plants infested with larvae of each species were exposed in vegetable gardens or grassland to different seasonal conditions (spring, summer and fall). Abundant parasitoid species were Chrysocharis pentheus, Diglyphus isaea and D. minoeus in spring or fall, and Neochrysocharis formosa, D. albiscapus and D. pusztensis in summer. Also, abundant parasitoid species varied among the different field conditions; however, within the same season and field the dominant parasitoid species were similar in all three leafminer species tested. Our findings indicate that the dominant parasitoid species did not exhibit any strong preference in the field between the three host species, namely L. trifolii, L. sativae or C. horticola.

Use of Entomopathogenic Nematodes (Steinernematidae) in Conjunction with Mulches for Control of Overwintering Codling Moth (Lepidoptera: Tortricidae)

Mulches were compared to test their utility for enhancing efficacy of entomopathogenic nematodes (EPNs) applied against over-wintering codling moth, Cydia pomonella (L.), larvae. Compared with bare ground, mulches may enhance control by providing cocooning sites for codling moth larvae and a substrate that is easy to treat, maintains moisture, and enhances nematode activity. Steinernema carpocapsae (Weiser) or S. feltiae (Filipjev) were applied at a rate of 2.5 × 109 infective juveniles (IJs)/ha against cocooned sentinel codling moth larvae in cardboard strips followed by 2 h of irrigation in plots that were covered with one of four mulches (clover, shredded paper, grass hay or wood chips) or to bare plots on 29 September 2003. Average mortalities of 97 and 98% were observed in paper-mulched plots treated with S. carpocapsae or S. feltiae IJs, respectively, compared to 80 and 76% mortality in bare plots. Larvicidal activity for S. feltiae against sentinel larvae in cardboard strips that were placed in crevices in the soil was nearly identical in all mulched and bare plots (97–100% mortality), but reduced for S. carpocapsae in wood chip and clover plots (76–79% mortality) relative to paper, grass hay and bare plots (93–97% mortality). A significant portion of sentinel larvae (25 and 14%) that were placed in crevices in the soil in hay- and paper-mulched control plots revealed the natural presence of EPNs (Heterorhabditis sp.). Applications of S. carpocapsae and S. feltiae at a reduced rate of 109 IJs/ha on 20 April 2004, followed by 1 h of irrigation resulted in 13.1 and 7.4% reduction in sentinel larvae in bare plots compared to 36 and 62% in mulched plots, respectively. Applications of S. carpocapsae and S. feltiae at a rate of 2.5 × 109 IJs/ha on 21 September 2004 to bare and wood chip-mulched plots followed by 1 h of irrigation resulted in 21 and 65% reduction in sentinel larvae in bare plots compared to 93 and 85% in mulched plots, respectively. Residual larvicidal activity of EPNs 3 d after applications in treated plots was low, but significant in the nonmulched plots (12–17% mortality) relative to untreated controls (1–2% mortality).

Entomopathogenic nematodes for control of codling moth (Lepidoptera: Tortricidae) in apple and pear orchards: Effect of nematode species and seasonal temperatures, adjuvants, application equipment, and post-application irrigation

Abstract Codling moth (CM), a serious pest of apple and pear in most countries where these fruits are grown, overwinters in cryptic habitats as cocooned diapausing larvae. Control of diapausing CM larvae would reduce or eliminate damage to fruit early in the following growing season. Entomopathogenic nematodes (EPNs) have shown promise as biological control agents of cocooned CM larvae in the Pacific Northwest and elsewhere, but several factors, such as choice of EPN species and other operational factors warrant investigation to provide growers with practical control options. Field trials with Steinernema carpocapsae and S. feltiae were conducted in apple and pear orchards to determine the effects of seasonal temperatures, adjuvants, post-application irrigation, and method of application on control of cocooned CM larvae. In studies conducted in late summer, fall and early spring (1999–2000), EPNs were applied to apple trees (Golden Delicious) with a backpack sprayer at a rate of 10 6 infective juveniles (IJs)/tree plus supplemental wetting to aid survival of IJs. Good control by both EPN species was observed in September (94–95% mortality in sentinel CM larvae). In October, control by S. feltiae was also effective (90% mortality), but S. carpocapsae was less effective (58% mortality), ostensibly due to the cooler conditions. In identical applications the following spring, the efficacy of S. carpocapsae and S. feltiae was reduced during cool windy conditions in March 30 tests, providing 26 and 65% control of sentinel larvae, respectively, but improved during warmer conditions in April 12 tests (71 and 86% control, respectively). In further tests in the same location in mid-October 2001, S. feltiae (10 6 IJs/tree) were most effective for control of sentinel CM larvae cocooned in cardboard strips (≈ 80% mortality) and logs (34–47%) when combined with a wetting agent (Silwet L77) or a humectant (Stockosorb) and the trees were misted for 4 h post-treatment. In the absence of post-application wetting, the addition of either adjuvant (Silwet and Stockosorb) to IJs also increased larval mortality in strips, although it did not significantly improve nematode efficacy on logs. In another test in late summer 1999, the use of a lance applicator (applying 2.0 × 10 6 IJs/tree) did not significantly improve control of cocooned larvae for either EPN species, when compared with a tractor-mounted airblast sprayer. Two further tests in the fall of 2003 with S. carpocapsae and S. feltiae compared post-application wetting with existing and modified irrigation in 4-year-old trellised apple (Gala) and established Bartlett pear orchards. No significant improvements in sentinel larval mortality were observed following application of both EPN species with an airblast sprayer (1–2.5 × 10 9 IJs/ha) when conventional overhead rotator sprinklers were replaced with lower volume microsprinklers.

Host density influences parasitism of the armyworm Pseudaletia unipuncta in agricultural landscapes

Summary Several studies have shown positive responses of parasitism to either host density or landscape complexity. However, no experiments have manipulated host density in landscapes of differing complexity. Here we report the results of a field experiment conducted to determine how host density and agricultural landscape structure jointly affect parasitism and parasitoid diversity of Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae). Parasitism was assessed by experimentally adding P. unipuncta sentinel larvae at low (1 larvae/plant) and high (3 larvae/plant) densities to detect parasitism in commercial cornfields located in a complex and a simple agricultural landscape. The braconid wasps Glyptapanteles militaris (Walsh) and Meteorus spp. accounted for 98.4% of the observed parasitism. Landscape structure did not influence parasitism (80.2% on average) and contrary to expectations, showed a trend towards increased parasitoid richness and diversity in the simple landscape. Increasing host density revealed a trend of increasing parasitoid richness and diversity, and differentially affected parasitism at the parasitoid specific level. G. militaris parasitized a significantly greater proportion of hosts at low host density, while the opposite occurred for Meteorus spp. (primarily M. communis ). These offsetting responses of parasitoids resulted in the lack of an overall host density effect on parasitism. The differential response of these parasitoids to host density is discussed in relation to differences in morphological and life history characteristics. Our results suggest that the specific composition of parasitoid assemblages could significantly alter parasitism at different host densities independently of landscape structural complexity.

Assessment of risk posed by introduced braconid wasps to Pieris virginiensis, a native woodland butterfly in New England

The range of the native butterfly Pieris virginiensis Edwards has decreased in New York and Ontario since the 1940s. Loss of habitat and harm from Cotesia glomerata (L.), a braconid parasitoid introduced to North America in the late nineteenth century for biological control of the invasive pest butterfly Pieris rapae (L.), have been suggested as the causes of this decline. In western Massachusetts, extensive habitat suitable for P. virginiensis remains and its principal host plant, Cardamine diphylla , is common. We found P. virginiensis to be widely present, occurring at 39% of host plant patches. In laboratory tests, we found that both C. glomerata and Cotesia rubecula (Marshall) are able to parasitize and successfully develop in P. virginiensis larvae when these are presented on detached leaves of C. diphylla . However, when we exposed laboratory-reared first or second instars of either P. rapae or P. napi (both suitable hosts for these Cotesia spp.) as sentinel larvae on leaves of either Brassica oleracea or C. diphylla at sites where P. virginiensis was present, no parasitism by either Cotesia sp. was detected. Some sentinel larvae were parasitized by an unidentified ichneumonid in the genus Hyposoter . We conclude that in western Massachusetts, P. virginiensis is widely distributed at low densities and that while it is physiologically an acceptable and suitable host for both C. glomerata and C. rubecula , larvae of field populations of this butterfly are not attacked because these parasitoids do not forage in forested habitats, even when they are locally present in adjacent meadows. Consequently, we report that this butterfly is not threatened by these parasitoids contrary to early suggestions in the literature.

15N‐enrichment of plant tissue to mark phytophagous insects, associated parasitoids, and flower‐visiting entomophaga

AbstractNew techniques are presented on the use of 15N to mark insects. 15N, a stable isotope of nitrogen, was enriched above natural abundance in plant and insect tissues. Two laboratory studies demonstrated that enriched 15N‐concentrations could be tracked from plant to insect using mass spectrometry. In the first study, adult Cotesia plutellae (Kurdjimov) (Hymenoptera: Braconidae) and Hippodamia convergens Guérin‐Méneville (Coleoptera: Coccinellidae) were allowed to feed at the flowers of rapid‐cycling Chinese cabbage plants that had been fertilized with 15N‐enriched potassium nitrate (KNO3‐15NO3). Both insect groups were found to have significantly elevated 15N levels after visiting the flowers of the 15N‐enriched plants for 48 hours. In the second study, 15N‐enriched bean plant (Phaseolus vulgaris L.) tissue was incorporated into an insect diet and fed to navel orangeworms, Amyelois transitella (Walker) (Lepidoptera: Pyralidae). When the navel orangeworm larvae were 4th instars, they were removed from the diet and exposed to the parasitoid, Goniozus legneri Gordh (Hymenoptera: Bethylidae). Results indicated that the enriched 15N‐concentration of the bean plants was transferred to the navel orangeworms and, subsequently, to the parasitoids. This work may provide useful techniques to help establish whether agriculturally important entomophaga visiting 15N‐enriched flowers or parasitizing enriched sentinel larvae in the field can be effectively marked with 15N.

Evaluation ofLagenidium giganteumfor Biocontrol ofMansoniaMosquitoes in Florida (Diptera: Culicidae)

Inoculative releases of Lagenidium giganteum (California isolate) were made against a population of Mansonia dyari during a typical breeding season at one site in eastern Hillsborough County, Florida. Zoospores of L. giganteum were introduced into outdoor caged replicated test pools containing waterlettuce, Pistia stratiotes, and larvae of M. dyari collected from an inactive phosphate pit. First and second instar larvae were added twice a week between August and December, 1993, to simulate natural oviposition by M. dyari. Adult emergence was reduced by >77% in test pools treated with L. giganteum. Sentinel larvae provided evidence of recycling of the fungus approximately 14 days posttreatment. Continuous larval mortality of M. dyari was observed for a period of 46 days. Our data showed that water quality in the test pools and phosphate pit was suitable for fungal growth and zoosporogenesis as long as the water temperature did not exceed 38°C.

Biotic and abiotic parameters associated with an epizootic of Coelomomyces punctatus in a larval population of the mosquito Anopheles quadrimaculatus

Biotic and abiotic parameters associated with an epizootic of the fungus Coelomomyces punctatus in larval populations of the mosquito Anopheles quadrimaculatus were investigated for three mosquito breeding seasons (1986–1988) in two adjacent farm ponds in North Carolina. In the first pond, the prevalence of infected larvae averaged 42% (range 0–85%) for collections made weekly from May 1 to November 20, 1986, but larvae did not occur in this pond in 1987. Infection rates in the adjacent pond, sampled during the mosquito breeding seasons of 1987 and 1988, declined from 10.9% (range 0–27.5%) in 1987 to 2.5% (range 0–14.2%) in 1988. Correlation analyses between the number of female copepods and fungal infection rates in sentinel mosquitoes were significant ( P < 0.01) for Acanthocyclops robustus but insignificant for eight other species. Infections obtained in sentinel larvae placed in the ponds for 3 hr intervals indicated that C. punctatus infected larvae around sundown. Infection rates for field-collected larvae increased with the stage of larval development. However, experiments with sentinel larvae showed that early instars were more susceptible to infection than later instars, suggesting that the higher infection rates in late instars resulted from individual larvae being infected by two or more zygotes during larval development. Standard multiple regression analyses, used to determine the relationship between seasonal infection rates and water chemistry, weather variables, and the abundance of early and late instar larvae, showed that the abundance of late instars was the only independent variable common to linear models. The models only accounted for 20 and 9% of the variation in larval infection rates for 1987 and 1988, respectively. These results indicate that of the parameters examined, the seasonal abundance of the copepod, A. robustus, was the most important factor (or variable) correlated with the prevalence of mosquito infection.