Search

We assessed the efficacy of entomopathogenic nematodes (EPN) as biocontrol agents against larvae of the cabbage fly (CF), Delia radicum L. (Diptera: Anthomyiidae) infesting radishes in a laboratory and a greenhouse experiment. In the laboratory experiment, we first assessed the efficacy of the EPN Steinernema feltiae (Filipjev), Steinernema carpocapsae (Weiser), Heterorhabditis bacteriophora (Poinar) and Heterorhabditis megidis (Poinar, Jackson & Klein) when applied in suspensions at a dose of 2000 IJs/radish. Each experimental unit consisted of three radishes artificially infested with 20 CF maggots in pots. Application of S. feltiae and S. carpocapsae was the most effective, leading to the lowest number of CF maggots recovered from the radishes. In the greenhouse experiment we compared the conventional application of EPN in a water suspension with EPN applied encapsulated in alginate beads. Alginate beads were applied during sowing (early) or one week (late) before CF infestation. The late bead treatment significantly reduced visible infestation rates and mining by the CF larvae in harvested radishes, and both bead treatments (early and late) led to a lower number of externally visible mines compared to control and suspension treatments. Interestingly, the application of empty beads without nematodes also mitigated CF infestation. However, after prolonged storage of the harvested radishes almost all of them were found to be infested, independent of treatment. In conclusion, application of EPN in alginate beads can be more effective than in suspension, most likely because it increases EPN persistence. Yet, their effective application is contingent on improving and optimizing the encapsulation process, as well as on selecting an appropriate application scheme in terms of timing and EPN dose.

Plants under attack by caterpillars emit volatile compounds that attract the herbivore’s natural enemies. In maize, the caterpillar-induced production of volatiles involves the phytohormone jasmonic acid (JA). In contrast, pathogen attack usually up-regulates the salicylic acid (SA)-pathway and results in systemic acquired resistance (SAR) against plant diseases. Activation of the SA-pathway has often been found to repress JA-dependent direct defenses, but little is known about the effects of SAR induction on indirect defenses such as volatile emission and parasitoid attraction. We examined if induction of SAR in maize, by chemical elicitation with the SA-mimic benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), attenuates the emission of volatiles induced by Spodoptera littoralis or exogenously applied JA. In addition, we determined how these treatments affected the attractiveness of the plants to the parasitoid Microplitis rufiventris in a six-arm-olfactometer. BTH treatment alone resulted in significant systemic resistance of maize seedlings against the pathogen Setosphaeria turcica, but had no detectable effect on volatile emissions. Induction of SAR significantly reduced the emission rates of two compounds (indole and (E)-β-caryophyllene) in JA-treated plants, whereas no such negative cross-talk was found in caterpillar-damaged plants. Surprisingly, however, BTH treatment prior to caterpillar-feeding made the plants far more attractive to the parasitoid than plants that were only damaged by the herbivore. Control experiments showed that this response was due to plant-mediated effects rather than attractiveness of BTH itself. We conclude that in the studied system, plant protection by SAR activation is compatible with and can even enhance indirect defense against herbivores.

Field studies were carried out to evaluate whether inundative releases of Trichogramma brassicae Bezdenko (Hymenoptera: Trichogrammatidae) against the European corn borer, Ostrinia nubilalis Hubner (Lepidoptera: Crambidae), can have negative effects on the indigenous arthropod fauna in adjacent non-target habitats. Emigration of released T. brassicae from maize fields into non-target habitats was monitored, and its persistence in non-target habitats was assessed throughout the season. For this purpose, sticky traps were installed on metal sticks along transects from the inside of T. brassicae treated maize fields into sown wildflower strips and a natural common reed stand. In addition, cards with host eggs were used to monitor Trichogramma within potential overwintering sites. Although the highest numbers of released parasitoids were trapped within maize, a considerable amount of T. brassicae moved into non-target habitats. We found a significant transient increase of T. brassicae in both wildflower strips and reeds and no consistent decrease in numbers of trapped wasps up to 40 m from the maize field borders, when monitored directly following mass releases. Indigenous Trichogramma species were present both in the wildflower strip and reeds, and T. brassicae represented only a minor part of the overall Trichogramma population therein, except for a few days after release. However, a few T. brassicae persisted in non-target habitats throughout the season and were still present prior to releases in the subsequent year. These low numbers are not expected to seriously affect populations of native Trichogramma or non-target host species.

In order to find their prey, natural enemies of herbivores often make effective use of plant volatiles that are emitted by plants on which the herbivores have been feeding. The phenomenon of herbivore-induced emissions of attractants has been well investigated for mite–plant interactions and for interactions between leaf feeding caterpillars and plants. Herbivore-induced emissions of chemical signals appear to be common in plants, but little is known about induction by herbivores that have different feeding habits. We obtained more knowledge on this by comparing the volatile emissions induced in maize plants by a folivorous caterpillar ( Spodoptera littoralis ), a stemborer ( Ostrinia nubilalis ), and an aphid ( Rhopalosiphum maidis ). As controls we also measured the emissions of healthy, undamaged plants and plants that were mechanically damaged and then treated with caterpillar regurgitate. Volatiles were collected twice daily for 2 h over a 3-day period after initial infestation or mechanical damage. Quantitatively, the plants infested with S. littoralis emitted by far the most. Their emissions started several hours after initial damage, lasted for the 3 days, and were the highest on the third day. The volatile profile was the same for the regurgitate-treated plants, but here the emissions dropped rapidly after the first day. The plants infested by O. nubilalis emitted the same blend of volatiles, but in much lower quantities. In addition to the known induced maize volatiles, the Ostrinia -damaged plants emitted some highly volatile, still unidentified compounds, which may be specific for the frass of this insect or emitted from the damaged plant stem. The aphids induced no measurable emissions of volatiles in the maize, even after heavy infestation. This is perhaps because several aphids, including R. maidis, barely damage the plant cells, and may not trigger a plant response. These findings suggest that induction of volatiles is the result of cell tissue damage, particularly to the leaves of the plant. This should have consequences also for the search strategies employed by the natural enemies of the respective herbivores. It can be expected that enemies of stemborers use some highly volatile compounds in addition to the known induced compounds. Natural enemies of some aphids may have to resort to other foraging cues, as the plant appears to provide them with no or very little olfactory information.