Some ecological studies on the cabbage whitefly, Aleyrodes brassicae (Walker). Hemiptera - homoptera - aleyrodidae

Abstract

The development of known population cohorts, subjected to partial or complete predator and parasite exclusion, were studied during the summer period for the three years 1976, 1977 and 1978.

The data for 1977 and 1978 were compiled into life tables and subjected to key factor analysis.

The residual k₅ (adult mortality) contributed most to total mortality in both years, but the key factor varied between years and sometimes between generations. Of the actual field mortalities involving the developmental instars, most mortality occurred with young larvae and there is an indication that airborne predators were largely responsible.

When adult whiteflies were confined on leaves of Brussels sprout of different physiological ages, significantly more eggs were deposited on young leaves than on either mature or senescent ones. The rate of development and survival of subsequent larvae were also favoured when they were reared on young leaves.

The position of leaves viz, horizontal, vertical or inverted, did not significantly affect egg deposition or larval development. When whiteflies were caged on upper and lower leaf surfaces, the latter provided conditions that gave higher egg deposition and improved survival rate of larvae than the former. Adults showed a pronounced oviposition preference for lower surfaces when compared to upper ones, irrespective of leaf position.

Oviposition and feeding of whiteflies are so closely linked that nutritional factors are considered responsible for both types of discrimination.

When adults were confined to basal, middle and apical parts of leaves, egg deposition, fecundity, rate of larval development and adult longevity were all favoured by the basal part, differences between the other two parts were less consistent but, generally, performance was better on the middle part than on the apical part. There was a correlation between performance on leaf parts and differences in total nitrogen content.

The effects of water stress on egg deposition and larval development were not significant although fewer eggs were laid on leaves subjected to the most severe water stress regime.

Whitefly performance was also compared on plants given different levels of potassium and nitrogen. Most eggs were deposited on plants given high levels of potassium, but when such levels were combined with high nitrogen levels, larval development became more prolonged than under most other treatments. Whole leaf analysis revealed an inverse correlation between potassium levels in the nutrient solution and levels of soluble nitrogen in the leaves.