The role of comptetitors for Chrysomela lapponica, a north Eurasian willow pest, in pioneering a new host plant

The Palaearctic leaf beetle Chrysomela lapponica usually feeds upon willows in the northern region of its occurrence. However, in Central Europe, some populations are known that have specialised on birch. In this study, we investigated the significance of other herbivores occurring together on the same host plants as possible exploitative competitors of C. lapponica. Two populations were studied: a population from Finland specialised on the willow Salix borealis, and a population from the Czech Republic, specialised on the birch Betula pubescens. Abundances of folivorous and suctivorous insects on both host plants were recorded at both population sites. The willow leaf beetle Phratora vitellinae was the most abundant herbivorous insect at both study sites on willow. A field study was conducted to examine the effects of P. vitellinae on the performance of C. lapponica. The presence of P. vitellinae larvae on the same twig upon which C. lapponica larvae were feeding did not affect increase of body weight in C. lapponica larvae. Thus, the high resource availability of both willows and birches suggest that interspecific competition is unlikely to be a selection factor driving the evolution of host shift in C. lapponica

The response specificity of Trichogramma egg parasitoids towards infochemicals during host location

Parasitoids are confronted with many different infochemicals of their hosts and food plants during host selection. Here, we investigated the effect of kairomones from the adult host Pieris brassicae and of cues present on Brussels sprout plants infested by P. brassicae eggs on the behavioral response of the egg parasitoid Trichogramma evanescens. Additionally, we tested whether the parasitoid¿s acceptance of P. brassicae eggs changes with different host ages. The wasps did not discriminate between olfactory cues from mated and virgin females or between mated females and males of P. brassicae. T. evanescens randomly climbed on the butterflies, showing a phoretic behavior without any preference for a certain sex. The parasitoid was arrested on leaf parts next to 1-day-old host egg masses. This arrestment might be due to cues deposited during oviposition. The wasps parasitized host eggs up to 3 days old equally well. Our results were compared with former studies on responses by T .brassicae showing that T. evanescens makes less use of infochemicals from P. brassicae than T. brassicae

Herbivore-induced plant volatiles mediate in-flight host discrimination by parasitoids

Herbivore feeding induces plants to emit volatiles that are detectable and reliable cues for foraging parasitoids, which allows them to perform oriented host searching. We investigated whether these plant volatiles play a role in avoiding parasitoid competition by discriminating parasitized from unparasitized hosts in flight. In a wind tunnel set-up, we used mechanically damaged plants treated with regurgitant containing elicitors to simulate and standardize herbivore feeding. The solitary parasitoid Cotesia rubecula discriminated among volatile blends from Brussels sprouts plants treated with regurgitant of unparasitized Pieris rapae or P. brassicae caterpillars over blends emitted by plants treated with regurgitant of parasitized caterpillars. The gregarious Cotesia glomerata discriminated between volatiles induced by regurgitant from parasitized and unparasitized caterpillars of its major host species, P. brassicae. Gas chromatography-mass spectrometry analysis of headspace odors revealed that cabbage plants treated with regurgitant of parasitized P. brassicae caterpillars emitted lower amounts of volatiles than plants treated with unparasitized caterpillars. We demonstrate (1) that parasitoids can detect, in flight, whether their hosts contain competitors, and (2) that plants reduce the production of specific herbivore-induced volatiles after a successful recruitment of their bodyguards. As the induced volatiles bear biosynthetic and ecological costs to plants, downregulation of their production has adaptive value. These findings add a new level of intricacy to plant¿parasitoid interaction

Plant responses to insect egg deposition

Plants can respond to insect egg deposition and thus resist attack by herbivorous insects from the beginning of the attack, egg deposition. We review ecological effects of plant responses to insect eggs and differentiate between egg-induced plant defenses that directly harm the eggs and indirect defenses that involve egg parasitoids. Furthermore, we discuss the ability of plants to take insect eggs as warning signals; the eggs indicate future larval feeding damage and trigger plant changes that either directly impair larval performance or attract enemies of the larvae. We address the questions of how egg-associated cues elicit plant defenses, how the information that eggs have been laid is transmitted within a plant, and which molecular and chemical plant responses are induced by egg deposition. Finally, we highlight evolutionary aspects of the interactions between plants and insect eggs and ask how the herbivorous insect copes with egg-induced plant defenses and may avoid them by counteradaptations

A hitch-hiker’s guide to parasitism: the chemical ecology of phoretic insect parasitoids

Phoretic arthropods use other animals as vehicles to migrate to new environments. Among insect parasitoids, phoresy is almost exclusively restricted to minute wasp species that develop in or on the smallest and most inconspicuous life stage of their host: the egg. Females of about 35 egg parasitoid species are known to hitch-hike with adult hosts to reach their egg-laying sites. Recent studies suggest that phoretic parasitoids strongly rely on chemical espionage to locate their transporting host. These wasps have evolved intriguing ways to exploit cues that are part of their host ’ s communication system, including sex, anti-sex and aggregation pheromones. Such a ‘chemical-espionage-and-ride’ strategy can be innate but it can also be learned. The extent and mechanisms by which hosts might avoid exploitation are poorly understood. Here we discuss why we expect phoresy to be much more widespread among egg parasitoids than is known so far. It is expected to be adaptive, especially in those species that have limited ability for directed fl ight, have a short time window available for parasitism, have a narrow host range, and parasitize abundant hosts that lay large eggs (or eggs in groups) with a large distance between them. We review some recently published examples of chemical espionage by phoretic egg parasitoids and discuss to what extent phoretic wasps represent a selective force against the use of chemical cues by their hosts. At the end of the chapter we identify unexplored aspects of the chemical ecology of phoretic insect parasitoids that warrant further investigation. In addition to the fundamental interest, research into the ways that phoretic parasitoids have evolved to locate their host may help improve the effi cacy of using parasitoids as biological control agents against insect pests

Phoresy in the field: natural occurrence of Trichogramma egg parasitoids on butterflies and moths

Phoretic insects utilize other animals to disperse to new environments. We recently discovered how egg parasitoids use an exciting phoretic strategy to reach egg-laying sites of their butterfly hosts. In the laboratory, female Trichogramma wasps detect and mount mated female cabbage white butterflies that emit an anti-aphrodisiac pheromone. Hardly any information exists about the natural occurrence of phoresy in wasps of this genus. Therefore, we monitored the presence of phoretic Trichogramma wasps on lepidopteran hosts in the field. Only female wasps were found at low prevalence on six lepidopteran species. Wasps were mostly found on female hosts and mainly on abundant solitary host species. This is the first report of phoretic Trichogramma wasps on butterflies in nature. We suggest that phoresy is only one of several strategies used by these polyphagous egg parasitoids. The evolution of phoresy is discussed in relation to the nutritional ecology of egg parasitoids

The potential of entomopathogenic nematodes to control moth pests of ornamental plantings

Worldwide biodiversity decline is caused by multiple factors, including pesticides. Aside from their applications in agriculture, the uptake of pesticides in urban gardens is widespread. Here, we review the potential of controlling pests of ornamental garden plants, like the boxtree moth Cydalima perspectalis Walker, 1859 (Lepidoptera: Crambidae), using entomopathogenic nematodes (Heterorhabdidae & Steinernematidae). Nematode biocontrol is highly suitable, particularly for small-scale control such as in boxtree plantings. Boxtree (Buxus spp.) is an ornamental shrub widely used in public and private grounds across Europe. Over the past decade boxtree has suffered heavily from the destructive boxtree moth, an invasive and persistent pest species of East-Asian origin. Widespread application of insecticides has been effective, yet resistance to these compounds is accumulating. The dense foliage of boxtree shrubs facilitates the correct tuning of moisture and temperature conditions required for nematode mediated pest control. Warm weather, without direct sunlight, on moist to wet foliage appear to be the most suitable conditions. We conclude that the use of entomopathogenic nematodes for controlling pests, such as the boxtree moth, may limit damage to horticulture and provide a safe and environmentally friendly form of control in urban spaces

The use of oviposition-induced plant cues by Trichogramma egg parasitoids

1. Female parasitoids have evolved various foraging strategies in order to find suitable hosts. Egg parasitoids have been shown to exploit plant cues induced by the deposition of host eggs. 2. The tiny wasp Trichogramma brassicae uses oviposition-induced cues from Brussels sprouts to locate eggs of the cabbage white butterflies Pieris brassicae and Pieris rapae that differ in their egg-laying behaviour. These plant cues are elicited by male-derived anti-aphrodisiac pheromones in the accessory reproductive gland (ARG) secretions of mated female butterflies. However, the closely related generalist species Trichogramma evanescens does not respond to Brussels sprout cues induced by the deposition of P. brassicae egg clutches. 3. Here we showed in two-choice bioassays that T. evanescens wasps respond to Brussels sprout cues induced by (i) the deposition of single eggs by P. rapae, and (ii) the application of ARG secretions from either mated P. rapae females, or from virgin female butterflies in combination with P. rapae's anti-aphrodisiac compound indole. The wasps only associatively learned to respond to Brussels sprout cues after applying indole alone by linking those cues with the presence of P. rapae eggs. 4. Our results indicate that Trichogramma wasps more commonly exploit oviposition-induced plant cues to locate their host eggs. Generalist wasps show less specificity in their response than specialists and employ associative learnin