David R. Gillespie

The functions of plant feeding in the omnivorous predator Dicyphus hesperus: water places limits on predation

1. Dicyphus hesperus Knight (Heteroptera: Miridae) nymphs were fed from egg hatch to the adult stage on Ephestia kuehniella eggs provided either alone or in combination with tomato leaves or with a supplementary water source.

Does mother really know best? Oviposition preference reduces reproductive performance in the generalist parasitoid Aphidius ervi

The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations.

Plant preference in relation to life history traits in the zoophytophagous predator Dicyphus hesperus

Dicyphus hesperus Knight (Heteroptera: Miridae) is an omnivorous predator used to control pests of greenhouse vegetables. Plant preferences and life history traits were studied using nine plant species: Lycopersicon esculentum Mill. (Solanaceae), Capsicum annuum L. (Solanaceae), Verbascum thapsus L. (Scrophulariaceae), Nepeta cataria L. (Lamiaceae), Stachys albotomentosa (Lamiaceae), Nicotiana tabacum L. (Solanaceae), Vicia sativa L. (Fabaceae), Zea mays L. (Gramineae), and Chrysanthemum coronarium L. (Asteraceae). Plants were selected from among potential target crops, natural hosts, plants used for mass rearing, and plants on which D. hesperus has not been reported. Plant preference was measured by multi‐choice host plant selection and oviposition assays. Development and reproduction were measured on each of the plant species on both a plant diet alone and on a plant diet supplemented with Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) eggs. Dicyphus hesperus females and nymphs expressed a preference for some plants over others. Plant preference ranged from low preference plants, such as Z. mays, V. sativa, C. coronarium, and C. annuum, to high preference plants such as V. thapsus, N. tabacum, and S. albotomentosa. When E. kuehniella eggs were supplied, there were few differences in the development time and fecundity of D. hesperus among plants, with the exception of corn and broad bean, where fecundity was lower. On a plant diet alone, nymphs were able to complete their development on V. thapsus, C. annuum, and N. cataria. However, mortality and development time were much lower on V. thapsus than on C. annuum and N. cataria. On most of the plant species D. hesperus did not lay any eggs when fed on a plant diet alone. On V. thapsus, females laid a few eggs and lived longer than when fed on prey. Dicyphus hesperus females tended to prefer host plants on which nymph survival without prey was greatest.

The effects of mullein plants (Verbascum thapsus) on the population dynamics of Dicyphus hesperus (Heteroptera: Miridae) in tomato greenhouses

The response of Dicyphus hesperus Knight (Heteroptera: Miridae) to whitefly populations in tomato greenhouses was measured in the presence and absence of mullein (Verbascum thapsus L.) as an alternative host plant. The dynamics of the D. hesperus population on tomato (Lycopersicon esculentum Mill.) and on mullein plants were followed through an entire growing season. In houses with mullein plants, more predators occurred on mullein when whitefly density was low on tomato. A mark-release-recapture experiment where rabbit IgG was used as an external marker showed that D. hesperus adults moved from mullein plants to tomato plants. D. hesperus was always more abundant in houses with mullein than in the houses with tomato plants alone. Movements between tomato and mullein plants are discussed as a strategy to optimize predator foraging. The use of mullein as an alternative host plant may contribute to the establishment of D. hesperus and help to preserve the predator population when prey on tomato crops is scarce.

Prey feeding increases water stress in the omnivorous predator Dicyphus hesperus

The effects of water stress (produced by water deprivation and prey feeding) on plant feeding were investigated in the omnivorous predator Dicyphus hesperus Knight (Hemiptera: Miridae). The objective was to determine if prey feeding aggravated water deficits and thus increased plant feeding. We measured plant feeding in a factorial experiment where female D. hesperus were prepared for experiments by providing or withholding water and/or prey for 24 h. We then evaluated the amount of plant feeding on Nicotiana tabacum seedlings by the direct observation of insects at three different densities of the prey, Ephestia kuehniella eggs. The amount of plant feeding, as measured by frequency of plant feeding bouts and time spent plant feeding during observation, was significantly greater for water‐deprived individuals than for those that had been provided with water. Individuals that had been provided with prey fed on plants at a significantly higher frequency than prey‐deprived individuals at two of the prey densities used in the experiment. These results support the hypothesis that plant feeding in zoophytophagous Hemiptera facilitates prey feeding by providing water that is essential for predation.

HOST-RANGE EVOLUTION IN APHIDIUS PARASITOIDS: FIDELITY, VIRULENCE AND FITNESS TRADE-OFFS ON AN ANCESTRAL HOST

Abstract The diversity of parasitic insects remains one of the most conspicuous patterns on the planet. The principal factor thought to contribute to differentiation of populations and ultimately speciation is the intimate relationship parasites share with hosts and the potential for disruptive selection associated with using different host species. Traits that generate this diversity have been an intensely debated topic of central importance to the evolution of specialization and maintenance of ecological diversity. A fundamental hypothesis surrounding the evolution of specialization is that no single genotype is uniformly superior in all environments. This “trade-off” hypothesis suggests that negative fitness correlations can lead to specialization on different hosts as alternative stable strategies. In this study we demonstrate a trade-off in the ability of the parasitoid, Aphidius ervi, to maintain a high level of fitness on an ancestral and novel host, which suggests a genetic basis for host utilization that may limit host-range expansion in parasitoids. Furthermore, behavioral evidence suggests mechanisms that could promote specialization through induced host fidelity. Results are discussed in the context of host-affiliated ecological selection as a potential source driving diversification in parasitoid communities and the influence of host species heterogeneity on population differentiation and local adaptation.

Covariance of phenotypically plastic traits induces an adaptive shift in host selection behaviour

Flexibility in adult body size allows generalist parasitoids to use many host species at a cost of producing a range of adult sizes. Consequently, host selection behaviour must also maintain a level of flexibility as adult size is related to capture efficiency. In the present study, we investigated covariance of two plastic traits—size at pupation and host size selection behaviour—using Aphidius ervi reared on either Acyrthosiphon pisum or Aulacorthum solani, generating females of disparate sizes. Natal host was shown to change the ranking of perceived host quality with relation to host size. Parasitoids preferentially attacked hosts that corresponded to the size of the second instar of their natal host species. This resulted in optimal host selection behaviour when parasitoids were exposed to the same host species from which they emerged. Parasitoid size was positively correlated with host size preference, indicating that females use relative measurements when selecting suitable hosts. These coadapted gene complexes allow generalist parasitoids to effectively use multiple host species over several generations. However, the fixed nature of the behavioural response, within a parasitoids lifetime, suggests that these traits may have evolved in a patchy host species environment.

Host‐adapted parasitoids in biological control: Does source matter?

It has been hypothesized that the success of a biological control introduction is, in part, dependent on the ability of the control agent to become established in its new environment or to its new population of hosts through local adaptation. Despite this, few studies have investigated the influence of the recent coevolutionary history of pest species and natural enemies on the efficacy of biological control agents, especially for agents that are mass-reared for release in agriculture. We investigate the evolutionary potential of a biological control agent Aphidius ervi to adapt to a key pest species, the foxglove aphid Aulacorthum solani, through components essential to the evolution of parasitoid virulence. We explored (1) the influence of genetic variation from natural source populations on the ability to parasitize natal and non-natal host species; (2) the heritability of key traits related to parasitoid fitness; and (3) the efficacy of parasitoid host-selection lines in a greenhouse system. Source populations maintained genetic variation in the ability to utilize natal and non-natal host species; however, only some of the traits sampled suggested local adaptation of parasitoid populations. The ability to parasitize a host was found to be genetically determined and strongly heritable, irrespective of host species. The greenhouse study demonstrated the potential of parasitoid selection lines to substantially increase performance of parasitoids for target pest species. This research provides insight into novel techniques that can be used to increase the quality of biological control agents through the development of lines of natural enemies adapted to particular pest species.

Compatibility of an entomopathogenic fungus with a predator and a parasitoid in the biological control of greenhouse whitefly.

Abstract An ongoing debate in biological control consists of whether interference between biological agents can disrupt pest control. This study investigated the outcome of interactions between the entomopathogen Beauveria bassiana with the whitefly predator Dicyphus hesperus and the parasitoid Encarsia formosa, as well as their effect on the control of the greenhouse whitefly Trialeurodes vaporariorum on greenhouse tomato crops. Our objective was to determine whether the generalist B. bassiana would disrupt biological control by interfering with D. hesperus or E. formosa. In experimental greenhouses, whitefly, parasitoid and predator populations were established, and over 27 days, tomato plants were sprayed with three applications of the B. bassiana based product BotaniGard® (5.13×103 conidia/mm2) or water (control). Populations of greenhouse whitefly and biological control organisms were regularly monitored in control and B. bassiana-treated compartments. Overall, 10.6% of all whiteflies in treated compartments were infected, and 0.98% were both infected and parasitized. There were 31.7 and 22.3% fewer immature and adult whiteflies, respectively, on B. bassiana-treated plants relative to controls. Parasitism by E. formosa and predation by D. hesperus occurred at rates of 7.5 and 2.5%, respectively, in B. bassiana-treated compartments, and 5 and 6%, respectively in control compartments. Our study suggests that applications of B. bassiana for short-term biological control of greenhouse whiteflies are compatible with the concurrent use of E. formosa and D. hesperus on greenhouse tomato crops.

The impacts of extreme and fluctuating temperatures on trait-mediated indirect aphid–parasitoid interactions

1. Global climate change models predict an increase in the frequency and magnitude of extreme temperature events. These temperature events, heatwaves for example, will impact a wide range of physiological and behavioural processes, particularly in ectotherms, and may therefore influence interactions between species.