Herbivores select host plants depending on plant quality and the presence of predators and competitors. Competing herbivores change host plant quantity through consumption, but they can also change plant quality through induction of plant defences, and this affects the performance of herbivores that arrive later on the plant. Some herbivores, such as the spider mite Tetranychus evansi, do not induce, but suppress plant defences, and later-arriving herbivores can profit from this suppression. It has been suggested that the dense web produced by this spider mite serves to prevent other herbivores to settle on the plant and benefit from the suppressed defences. Here, we confirmed this by studying the preference and performance of the whitefly Bemisia tabaci, a generalist herbivorous pest. To disentangle the effects through changes in plant defences from the effects of spider-mite web, we included treatments with a strain of the closely-related web-producing spider mite T. urticae, which induces plant defences. Whiteflies did perform worse on plants with defences induced by T. urticae, but, in contrast to other herbivores, did not perform better on plants with defences suppressed by T. evansi. Moreover, the web of both spider mites reduced the juvenile survival of whiteflies, and whiteflies avoided plants that were covered with web. Hence, whitefly performance was not only affected by plant quality and induced plant defences, but also through the web produced by spider mites, which thus serves to protect against potential competitors, especially when these could profit from the suppression of plant defences by the mites.

草食动物根据植物质量以及捕食者和竞争者的存在来选择寄主植物。竞争的食草动物通过消耗改变宿主植物的数量，但它们也可以通过诱导植物防御来改变植物质量，这会影响后来到达植物的食草动物的性能。一些食草动物，例如叶螨Tetranychus evansi，不会诱导，但会抑制植物防御，后来到达的食草动物可以从这种抑制中受益。有人建议，这种蜘蛛螨产生的致密网可以防止其他食草动物在植物上定居并从抑制的防御中受益。在这里，我们通过研究粉虱Bemisia tabaci的偏好和性能来证实这一点，一种多面性食草害虫。为了通过植物防御变化从蜘蛛螨网的影响中解开这些影响，我们使用了一种密切相关的产生蜘蛛网的蜘蛛螨T. urticae菌株进行处理，该菌株诱导植物防御。粉虱在防御由T. urticae诱导的植物上表现较差，但与其他食草动物相比，在防御由T. evansi抑制的植物上表现不佳. 此外，两种蜘蛛螨的网都降低了粉虱的幼年存活率，粉虱避开了被网覆盖的植物。因此，粉虱的表现不仅受到植物质量和诱​​导的植物防御的影响，而且还受到红蜘蛛产生的网的影响，从而防止潜在的竞争对手，特别是当这些竞争对手可以从螨对植物防御的抑制中获益时。