Should I lay or should I wait? Egg-laying in the two-spotted spider mite Tetranychus urticae Koch

Optimality theory predicts that females tend to maximize their offspring survival by choosing the egg- laying site. In this context, the use of conspecific cues allows a more reliable assessment of the habitat quality. To test this hypothesis, Tetranychus urticae Koch is an appropriate biological model as it is a phytophagous mite living in group, protected against external aggression by a common web. Experiments were conducted to determine the respective influence of substrate (living substrate: bean leaf vs. non-living substrate: glass plate), silk and presence of conspecific eggs on the egg-laying behavior of T. urticae females. On both living and non-living substrates, the presence of silk positively influenced the probability of a female to lay an egg, but had no influence on the number of eggs deposited. The egg-laying behavior was mainly determined by the nature of the substrate with mites laying fewer eggs on a non-living substrate than on a living one. The presence of a conspecific egg had no impact on either the probability of laying an egg or on the oviposition rate. This study showed a high variability among females in their fecundity and egg-laying performance. The physiology of females (individual fecundity), the egg-laying substrate and to a lesser extent the presence of silk impacted on the decision of spider mites to lay eggs.