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Abstract: Plants have direct and indirect constitutively produced and inducible defenses against herbivores and pathogens, which can substantially aid in their ability to defend themselves. However, very little is known about the influence of agronomic factors on such defenses. Here, we tested the effects of nitrogen levels and water availability on the ability of cotton plants to deter feeding by Spodoptera exigua through induction of anti-feedants, and to attract Microplitis croceipes through systemic induction of volatile emission. Cotton plants were grown with various nitrogen levels and were either exposed to water stress or normal water before being exposed to S. exigua for 48h for induction of defenses. Dual choices of various nitrogen and water treatments were provided to M. croceipes in flight tunnel bioassays. Dual choices of leaf tissue from the various nitrogen and water treatments were provided to S. exigua larvae. Both water stress and nitrogen levels under and over the recommended levels increased leaf tissue consumption and decreased attraction of M. croceipes to the plants. Analyses of induced volatiles released from herbivore damaged plants indicate that their concentrations differ among the nitrogen levels tested with plants receiving no nitrogen or twice the recommended dose having amounts much lower than plants receiving the recommended dose. Because both direct and indirect plant defense mechanisms are negatively affected by improper nitrogen and insufficient water, we argue that these factors should be considered for a better natural control of pests in cotton and most probably in other crops. [Copyright &y& Elsevier]

1. Among a great diversity of other strategies, insects have evolved polymodal emergence patterns that can increase survival in the face of annual variations in environmental conditions. Delia radicum L. (Diptera: Anthomyiidae) is a polyvoltine pest that attacks several cultivated cruciferous species. At the time of emergence, most populations show a polymodal emergence of type A (i.e. all individuals that enter diapause in the same year terminate diapause the following growing season, and exhibit a bimodal emergence curve). This results in the occurrence of two sympatric phenotypes, early and late, which differ by the timing of adult emergence in both diapausing and non-diapausing generations. 2. in Brittany, D. radicum pupae can be heavily parasitised by three parasitoids, Aleochara bilineata Gyll. (Coleoptera: Staphylinidae), Aleochara bipustulata L., and Trybliographa rapae (Hymenoptera: Eucoilidae). As in all parasitoids species, the successful development of both Aleochara species and T. rapae larvae depends mainly on the amount and quality of the food provided by the host. 3. The relationship between early and late host phenotype and (i) the parasitism efficiency, (ii) emergence patterns, and (iii) host selection behaviour were investigated for the three parasitoid species. 4. Depending on parasitoid species and their different parasitism and development mode, the results reveal that host phenotype can influence (i) survival and development time in T. rapae, and (ii) survival in Aleochara species. Aleochara larvae did not appear to discriminate between early and late host pupae on the basis of phenotype, but rather selected them according to their developmental stage. Furthermore, it was discovered that the same phenological strategy occurred in T. rapae and in D. radicum. However, for D. radicum the results indicate that such a strategy has a cost as the longer development time of late host pupae results in a longer period of time favourable for parasitism. [ABSTRACT FROM AUTHOR]

In this paper, we compare the host selection behaviours of two parasitoids, Aphidius rhopalosiphi and Aphidius picipes, in order to analyse whether behavioural adaptations to the defensive behaviour of their host (the grain aphid Sitobion avenae) could, in part, be responsible for the simultaneous presence of both species in cereal fields. The oviposition behaviour of A. picipes differed from that of A. rhopalosiphi by including a number of ‘fluttering wings’ sequences followed by immobility. It resulted in a 44 times longer host-handling time for A. picipes than for A. rhopalosiphi. Hosts attacked by A. picipes exhibited fewer defensive behaviours than hosts attacked by A. rhopalosiphi. A. picipes and A. rhopalosiphi rejected respectively 0% and 53% of unparasitized hosts presenting cornicle secretions, one of the defensive means of aphids. Furthermore, A. picipes females rejected 100% of the hosts that were already parasitized, whereas A. rhopalosiphi was previously described to reject only 20 to 40% of such hosts. Such differences could be explained by the way the two species deal with the aphid defensive behaviour. Field analyses, showed that A. rhopalosiphi was already present in wheat fields in early April whereas A. picipes appeared later and only achieved a low level of parasitism. However, when both species were present simultaneously, they shared the same resource. [ABSTRACT FROM PUBLISHER]

When attacked by herbivorous insects, many plants emit volatile compounds that are used as cues by predators and parasitoids foraging for prey or hosts. While such interactions have been demonstrated in several host-plant complexes, in most studies, the herbivores involved are leaf-feeding arthropods. We studied the long-range plant volatiles involved in host location in a system based on a very different interaction since the herbivore is a fly whose larvae feed on the roots of cole plants in the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). The parasitoid studied is Trybliographa rapae Westwood (Hymenoptera: Figitidae), a specialist larval endoparasitoid of D. radicum. Using a four-arm olfactometer, the attraction of naive T. rapae females toward uninfested and infested turnip plants was investigated. T. rapae females were not attracted to volatiles emanating from uninfested plants, whether presented as whole plants, roots, or leaves. In contrast, they were highly attracted to volatiles emitted by roots infested with D. radicum larvae, by undamaged parts of infested roots, and by undamaged leaves of infested plants. The production of parasitoid-attracting volatiles appeared to be systemic in this particular tritrophic system. The possible factors triggering this volatile emission were also investigated. Volatiles from leaves of water-stressed plants and artificially damaged plants were not attractive to T. rapae females, while volatiles emitted by leaves of artificially damaged plants treated with crushed D. radicum larvae were highly attractive. However, T. rapae females were not attracted to volatiles emitted by artificially damaged plants treated only with crushed salivary glands from D. radicum larvae. These results demonstrate the systemic production of herbivore-induced volatiles in this host-plant complex. Although the emission of parasitoid attracting volatiles is induced by factors present in the herbivorous host. their exact origin... [ABSTRACT FROM AUTHOR]

1. Female parasitoids that lay their eggs away from potential host insects are supposed to have only a minor influence on the fitness of their own progeny, as they do not select and evaluate directly the quality of hosts for their offspring. Aleochara bilineata Gyll. (Coleoptera: Staphylinidae) females oviposit near to cabbage plants infested by larvae of Delia radicum L. (Diptera: Anthomyiidae). Once the mobile A. bilineata larva emerges from the egg, it must then search the soil to find and select a suitable fly pupa. The beetle larvae have a limited time to found pupae and can only parasitize one host pupa during their lifetime. Moreover, because A. bilineata is a solitary parasitoid whether or not the host pupa is parasitized, has a strong influence on the successful development of the beetle larva. 2. We studied the oviposition behaviour of A. bilineata females in laboratory experiments in which females could choose between a range of several oviposition sites. 3. Our results reveal that A. bilineata females, like hymenopteran parasitoid females, show an adaptive oviposition behaviour in response to the quality of the oviposition site. The female beetles adapt their oviposition decisions and their clutch size based on information associated with the presence of a host. Furthermore, our results reveal that the Aleochara females can discriminate between specific cues related to the stage and parasitization status of the host. Such behaviours may greatly enhance the parasitization success of the larvae, and thereby female fitness. [ABSTRACT FROM AUTHOR]

Abstract: Delia radicum (L. 1758) is a major pest of cabbage crops in northern Europe. Due to more constraining laws relating to insecticide use, new strategies to control this pest are urgently needed. Manipulating insect behavior through infochemicals is a promising approach. The recent identification of dimethyl disulfide (DMDS) as a compound that both attracts the main predators of D. radicum and inhibits oviposition by the fly gives a challenging opportunity to develop such strategy. The aim of the present study was to confirm such potential of DMDS, in the field. Through the 8 weeks of the first egg laying peak of the fly we assessed, the potential of artificially increasing the levels of this molecule in the close vicinity of broccoli plants to 1/attract predators, 2/stimulate predatory activity and 3/limit damage done by the fly. Despite a lower number of D. radicum eggs as food resource, DMDS effectively increased predator catches in treated plots (119 Aleochara bilineata (Gyllenhal, 1810) caught in treated plot, while only 21 in control plots). However, damages done by the fly were of the same magnitude order in treated plots than in control ones. Number of D. radicum larvae and pupae recovered in plant roots were similar, despite the important decrease in eggs laid. This result, together with the observation that the numbers of eggs predated in artificial patches were lowered in the presence of the molecule, seems to indicate that increasing DMDS amounts disturbed the foraging activity of the fly predators. Consequences of these findings for the future of DMDS use in crop protection against D. radicum are discussed. [Copyright &y& Elsevier]