Parasitoid host location: using volatiles on different spatial scales and in context of distorting elements
Ecology of Land Use
Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
Final Report Abstract
Insect natural enemies such as predators and parasitoids play an important role in biological pest control and their foraging efficiency may determine if herbivore-infestations in agricultural fields reach the threshold above which insecticides need to be applied. Parasitoid wasps such as our model species Cotesia glomerata lay their eggs in other insects. Their larvae develop inside this living insect host, which dies after the parasitoid has emerged. Most parasitoids can only use one or a few closely related insect species for their development and their ability to find suitable hosts determines their reproductive success. Their host searching behaviour has been extensively studied in laboratory setups, but we know very little about how these insect find their hosts under variable natural conditions and at larger spatial scales. Laboratory experiments have shown that parasitoids use herbivoreinduced plant volatiles (HIPVs) as foraging cues to detect plants that are infested with their host herbivores from a distance, but unfavourable weather conditions and volatiles emitted by neighbouring plants may impede HIPV-guided foraging in the field. In my project, field experiments revealed that high wind speed and low humidity impeded odourguided upwind flight of parasitoids over 0.5 and 5 m distance, thereby reducing their host-finding success. The effect of wind speed can be explained by the inability of parasitoids to fly upwind if the wind speed exceeds a wind force of two, but I was surprised to see that low humidity also decreased the host-finding success of parasitoids. Laboratory experiments suggest that this may be the result of a reduced emission of HIPVs during low humidity conditions which may decrease the detectability of host-infested plants in the field. My results suggest that adverse weather conditions limit the time during which parasitoids can forage effectively and indicate that weather conditions may explain differences in the effectiveness of biological pest control between different regions and years. Crops and natural plant communities are usually attacked by several herbivore species in parallel. My results show that the presence of neighbouring plants which are infested by non-host herbivores decrease the foraging efficiency of parasitoids. Non-host infested plants emit similar HIPVs as hostinfested plants and parasitoids did not prefer HIPVs from a host-infested plant over HIPVs from a nonhost infested plant. The parasitoids landed multiple times on a non-host infested plant and searched it thoroughly before they moved on to other plants. Confusion caused by the presence of non-host infested plants resulted in an eight-fold delay in host-finding when the plant patch was approached in upwind flight. Under field conditions, strong winds often impede upwind flight and confusion by nonhost infested plants was even higher when parasitoids had to approach the plant patch in downwind flight. When they entered the plant patch in downwind flight, they did not perceive HIPVs from the plants located downwind and spend 4.5 times longer on the first non-host infested plant they encountered, compared to parasitoids that entered the patch in upwind flight. Considering that adverse weather conditions limit the time available for foraging, a reduction in foraging efficiency by the presence of non-host infested plants may lower the number of hosts a parasitoid can parasitize during its short life of a few weeks,. The level of biological pest control by parasitoids may thus decrease when many non-host herbivores are present in the crop. This work highlights the complex ways in which environmental factors such as weather conditions and herbivore community interact in influencing the host finding success of parasitoids. More experiments at (semi-)natural conditions are needed in order to understand how parasitoids manage to find their hosts under these challenging natural conditions and to develop methods to increase their effectiveness as biocontrol agents.
Publications
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(2019) Getting confused: learning reduces parasitoid foraging efficiency in some environments with non-host-infested plants. Oecologia 189 (4) 919–930
Vosteen, I., van den Meiracker, N. & Poelman, E.H.
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(2019) Understanding insect foraging in complex habitats by comparing trophic levels: insights from specialist host-parasitoid-hyperparasitoid systems. Current Opinion in Insect Science 32: 54-60
Aartsma, Y., Cusumano, A., Fernández de Bobadilla, M., Rusman, Q., Vosteen, I., Poelman, E.H.
