Ovipositing females of a short-lived gall midge take time to assess grass seed heads

Abstract

The short-lived adult wheat midge Sitodiplosis mosellana deposits eggs on the seed head of various grasses close to the developing seeds on which larvae feed. The time taken to make egg-laying decisions is investigated using three types of wheat Triticum aestivum seed heads. Young Roblin, Old Roblin and Young Key differ in their effects on ovipositing females (72%, 22% and 6% of eggs in choice tests, respectively) and effects on feeding larvae (75%, 25% and 5% larval survival, respectively). Within seconds of arriving, the female is able to distinguish Young Roblin from the two lower-ranked types. However, the lower-ranked types are not rejected at this time. Instead, all head types are examined before the female eventually flies away. On Young Roblin, probing with the ovipositor is the first behaviour that occurs. Thereafter probing and insertion of the ovipositor occupy most of the female’s time and behavioural transitions tend to be ‘progressive’, signalling a shift from low to high intensity examining. Differences between females visiting Old Roblin and Young Key are significant but take longer to emerge. On both, sitting is the first behaviour but, over the next 5–10 min, the female on Young Key exhibits more sitting, walking and ‘regressive’ transitions than the female on Old Roblin. It is suggested that, when the ovipositing female is short-lived and incapable of controlled flight in all but essentially windless conditions, her behaviour is designed to thoroughly, rather than rapidly, examine a suboptimal host before abandoning it for the uncertain future of finding a better host.