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Oviposition preference, larval feeding preference and larval food quality of Heliothis armigera : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
Relationships between tomato fruitworm(Heliothis armigera conferta Walker) and four host plants(lucerne, tomato, aster, sweetcorn) were studied in field, greenhouse and controlled laboratory conditions at Palmerston North, New Zealand during 1984-1986. The objective of the field trials was to investigate seasonal development, population growth and feeding behaviour of larvae on the four host plants. In the laboratory, oviposition preference within and between plant species, effects of larval foods on development, parameters of larval food quality, feeding preference and induction of feeding preference by different foods were investigated. In the field very few (male) moths were caught by pheromone traps before January, numbers reached a peak in March and had declined to zero by late April. Individual meteorological parameters (minimum night temperature, maximum day temperature and rainfall) showed no significant correlation with moth catches. On mature stages of the four plants larvae fed preferentially on tomato fruits, sweetcorn cobs and aster flowers over plant leaves but were hardly observed at all on lucerne. Glasshouse experiments showed that female moths preferred to oviposit on the upper half of plants, leaves were preferred over other plant parts and upper leaf surfaces were preferred over lower leaf surfaces. Oviposition preference was however affected by the flowering stage of the plants. At the pre-flowering stage lucerne was the most preferred but at flowering aster was the most preferred. Odour played a significant role in plant selection for oviposition. Different larval foods gave significant differences in growth and development of the insect as measured by several biological parameters. When larval period, mortality, percent pupation, pupal weight, adult fecundity and life span were combined into an overall fitness index reproductive parts of all plants(flowers, fruits and cobs) gave better performance than leaves. Dry matter content (and its reciprocal water content) and nitrogen content of foods, considered alone or combination, did not provide an adequate measure of food quality for larvae. Larval growth rate on a particular food was clearly influenced by rate of ingestion of that food and larvae tended to consume less of those foods that were more readily digested and assimilated. The nitrogen requirement of H.armigera larvae for adequate growth and development appeared to be low at about 1.9% of dry matter which was approximately the nitrogen level found in the reproductive parts of the plants. Newly hatched larvae expressed clear preferences for particular plant species in the order lucerne leaves > tomato leaves > aster leaves > sweetcorn leaves. However, these preferences could be modified in later larvae by early feeding experience but artificial diet (based on kidney bean) had little effect on food preference. Feeding preference for reproductive parts of plants was more strongly expressed than for leaves and reproductive parts evoked greater induction of preference.