Tritrophic level interactions in Ethiopian tomato systems: effects of plants on potato tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera. Gelechiidae), and its parasitoids

The study was made mainly to assess the possible causes for the increased economic importance of the potato tuber moth (PTM) on tomatoes in Ethiopia. Different trials with specific objectives were done by comparing three tomato cultivars: chrry (small fruit) processing (medium sized fruit) and fresh market (large fruit), In the first trial, the natural infestation rates of PTM on the cultivars was studied in the absence and presence of a proximate potato crop. In the lone tomato plots there were no significant differences in adult PTM activity on the cultivars. However the larval population buildup and associated leaf damage were significantly higher on the cherry than fresh market tomato. In the presence of proximate potato neither the adult activity nor larval population buildup and associated leaf damage were significantly different among the cultivars. In the lone tomato plots there was significantly higher fruit infestation of the fresh market cultivar and a similar result was obtained in the presence of proximate potato. There was, therefore, variation in the susceptibilities of the cultivars and the presence of potato influenced the responses. The importance of leaf trichomes in oviposition preference and larval settling of PTM were also studied. The PTM showed significant preference for the cherry cultivar with the densest leaf trichomes but in an oviposition cage where an ovipositing female was allowed to choose among the tomato cultivars and the potato it oviposited significantly more eggs on the latter. Although the density of the type VI glandular head trichomes was significantly higher on the cherry cultivar leaves there were no significant differences in the settling of neonate larvae among the tomatoes at either plant stage. In addition, leaf- and fruit-feeding assays were made to study the effects of the cultivars on the moth's survival growth and development and to relate the effects to the amounts of a tomatine the major allelochemical in tomatoes. More larvae survived on the cherry cultivar at the preblossom stage. At the blossom state there was reduced larval survival on all tomato cultivars which was most pronounced on the fresh market cultivar. Larval and larva-pupal development times were significantly shorter on potato. At the preblossom state larval development time was significantly longer on the processing cultivar; whereas at the blossom stage it was significantly longer on the cherry and fresh market cultivar. Larval body weight was significantly high on the fresh market tomato at the preblossom stage but at the blossom stage the differences in weight were not significant. Foliage a-tomatine content determined using HPLC was found to be significantly dependent on plant stage and cultivar. The correlations of the performance parameters with a-tomatine contents in the foliages of the tomatoes were variable at both phenologies. Despite the reduction in a-tomatine content in the cherry foliage at the blossom stage larval development was dignificantly delayed and mortality increased. There was significant amount of a-tomatine in the processing cultivar foliage at the blossom stage and yet the development time for larvae was significantly shorter than on the other cultivars. The results of the fruit-feeding assays whowed significantly higher infestation on ripening fruits of the fresh market compared to the cherry and processing cultivars. However, survival of larvae was significantly and consistently higher on the fresh market cultivar at the three fruit stages. Larval body weight was significantly higher on the fruits of the fresh market cultivar than the others at the three fruit stages. The head capsule size of the larvae was also significantly larger on the fresh market cultivar than on the others. The amounts of a-tomatine present in the fruit loculi were significantly different among the cultivars. In the fresh market cultivar there were traceable amounts of a-tomatine only in the fruit loculi of immature green fruits but not in fruits at the other fruit stages. The a-tomatine contents were significantly and negatively correlated with larvae head capsule size in maturing green and ripening fruits. Head capsule size was significantly lower on the cherry and processing cultivars than on the fresh market fruits. the other trial investigated the significance of natural enemies as a factor in PTM morality. It was in general concluded that there is differential susceptibility of tomato cultivars to PTM and that natural enemies are not an important factor in PTM mortality on tomatoes in Ethiopia.