Effects of continuing exposures to allelochemicals on southern armyworm larvae

The effects of the ingestion of two allelochemicals on the histology and autecological fitness on the larvae of the southern armyworm, Spodoptera eridania, were studied. Final (sixth) instar larvae were fed ad libitum on a semi-synthetic diet containing: (1) no additional chemicals (control), (2) the monoterpene pulegone in increasing concentrations or (3) KCN in increasing concentration.

Pulegone reduced the ingestion rate delaying the completion of the final instar. Adults formed normally but appeared to lay fewer eggs than normal; at the 0.2% dosage the eggs were non-viable. Lesions were absent in the fat body, midgut and testes of larvae fed these diets.

Midguts of larvae fed KCN demonstrated extensive goblet cell necrosis and hypertrophy of the columnar cells. There was also a proliferation of basophilic granules in the interperitrophic space; histochemical analysis revealed these to be bacteria. KCN acted as a phagostimulant, increasing the final pupal weight but having no other effect on autecological fitness.

The pathophysiological and ecological significance of these results are discussed in reference to plant-insect herbivore interactions.