Two-spotted Spider Mites Respond to Chemical Cues Associated with Conspecifics’ Silk When Choosing a Microhabitat

Tetranychus urticae (the two-spotted spider mite) is a phytophagous agricultural pest that affects many economically important crops. Two-spotted spider mites are group-living arachnids that produce silk webbing which is used for protection and dispersal, and also influences settlement behavior of conspecifics. We investigated the effects of conspecific silk and frass on microhabitat selection of these mites and whether these materials are substrates for semiochemicals. Two-choice behavioral assays gave mites the option of settling in the presence or absence of cues produced by conspecifics. Both sexes of mites preferred to settle in the presence of silk produced by conspecific females, and males also responded positively to silk produced by other males. Frass alone had no effect on mite settlement choices, but mites tended to prefer frass in combination with silk over silk alone. Female mites did not respond to conspecific silk after it was washed with methanol, and a crude methanol extract of the silk was sufficient to elicit settlement behavior, confirming that a semiochemical is responsible for the effect. We conclude that silk-bound semiochemicals provide cues used by both male and female mites when selecting a microhabitat, and that frass may also provide relevant social cues.