Three hypotheses predict how insect herbivores perform on stressed host plants. The plant stress hypothesis (PSH) predicts improved insect performance on stressed hosts. The plant vigour hypothesis (PVH) predicts that insects closely associated with their host, such as gall-formers, will perform better on vigorously growing non-stressed hosts. The third hypothesis, the Insect Performance Hypothesis (IPH) predicts that wood-feeders, sap-feeders and miners will perform better on stressed hosts, while
leaf-feeders and gall-formers will perform better on non-stressed hosts. These hypotheses were developed, however, without separating different types of plant stress. In this review we tested these hypotheses
across five insect feeding-guilds and twelve host-plant stress types, from more than 200 published studies on insect performance. When all host-plant stress types were pooled, the results suggested wood, sap and leaf-feeders performed better on stressed host plants, while miners and gall-formers
performed better on non-stressed host plants, thus supporting the PVH. However, when all insect feeding-guilds were pooled, it was found that host-plant-stress type also influenced insect performance, which was generally higher when host plants were growing under reduced moisture, light or CO₂, increased soil nitrogen or on younger plants. When host-plant-stress type and insect feeding-guild were separated, it was found that insect performance across feeding guilds varied with the type of host-plant stress encountered suggesting that insects in different feeding guilds may respond to different physiological and morphological changes in the plant. This review highlights the fact that insect performance is often significantly affected by host-plant stress, but that the direction of the response is variable. Although this review did not fully support any of the three theoretical hypotheses tested, there were consistent relationships between some insect-feeding guilds and host-plant-stress types that would allow the prediction on whether a specific biological control agent might perform better under a specific host-plant stress.[Show full abstract]
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