Host plant evaluation behaviour of Bemisia tabaci and its modification by external or internal uptake of imidacloprid

The behaviour of adult Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) wa s observed on leaves of cotton, Gossypium hirsutum L., to study the process of host plant evaluation. Seven distinct behavioural elements were identif ied and recorded for a period of 20 min after initial contact with leaves. The behaviour of the whiteflies was observed on untreated leaves or on leav es treated with either foliar or systemically applied imidacloprid. Thus, c ontact with imidacloprid by whiteflies was either via the external cuticle or by ingestion, allowing determination of the effect of uptake route on ho st evaluation behaviour. Analysis of behavioural sequences and comparison o f latencies, frequencies and durations of elements of behaviour between tre atments revealed that host evaluation behaviour was similar on untreated an d foliar treated leaves, but markedly different on those treated systemical ly. After their first probe, whiteflies on systemically treated leaves were much more active, spending significantly greater time walking and dabbing the labium tip onto the leaf surface. There was also a 50% reduction in the total time spent probing into systemically treated leaves, compared to the other treatments. This effect was the result of a shorter mean probe durat ion, and no effect of treatment was found on the frequency of probing. Pre- oviposition behaviour was significantly more common on the systemically tre ated leaves, although the number of eggs laid was similar on all treatments . These results also show that the host evaluation phase of adult female B. tabaci was dominated by probing. The behavioural sequence that leads to pr obing was affected significantly by imidacloprid when ingested, but not by direct contact with this compound on the leaf surface. The wider implicatio ns for the mechanisms of whitefly-host plant interactions are discussed.