Bio-ecological studies of Tuta absoluta in Sudan

Abstract

Production of tomato crops in Sudan is threatened by the invasion of Tuta absoluta (Meyrick). This pest develops and disperses rapidly and it is also known to develop resistance to insecticides. Knowledge of its bio-ecology and genetic diversity in its newly invaded areas can be used to develop sustainable and effective IPM strategies. Results of field surveys conducted in the major tomato producing States of Sudan showed that the highest abundance of T. absoluta occurs in winter and early summer seasons (December-April). However, its population densities declined in the late summer season and remained very low during the fall season (July-October). In the field, T. absoluta was found infesting only tomato (Solanum lycopersicum ), eggplant (S. melongena ), black nightshade (S. nigrum ) and gubbain (S. dibium Fr.). The T. absoluta highest infestation level was on tomato, followed by eggplant, gubbain and black nightshade. Six hymenopteran parasitoids and two predatory species were found at the surveyed sites. The parasitoid species are Bracon nigircans Szepligeti (Braconidae), Bracon hebetor (Say) (Braconidae), Dolichogenidea appellator (Telenga) (Braconidae), Eupelmus sp. (Eupelmidae), and two species belonging to Pteromalidae and Ichneumonidae families. The predators found were Nesidiocoris tenuis (Nesibug) (Miridae) and Chrysoperla sp. (Chrysopidae). Infestation of T. absoluta appears approximately two months after seeding of tomato and eggplant during the winter seasons in Gezira State. Pest incidence and infestation rate on tomato were significantly higher than on eggplant. In no-choice tests under laboratory conditions, T. absoluta laid eggs on tomato, black nightshade, French bean (Phaseolus vulgaris) and pepper (Capsicum annuum) with a significantly higher number of eggs laid on tomato. In choice tests, females laid eggs only on tomato and black nightshade with a higher number of eggs laid on tomato. In the larval performance test, 88.5%, 68.5% and 3% of inoculated neonate larvae survived to the adult stage on tomato, black nightshade and French bean, respectively. Rearing host plants did not significantly affect female fecundity. Offspring (F1) of T. absoluta reared on tomato and French bean maintained a strong ovipositional preference towards tomato. However, offspring F1, F2 and F3 reared on nightshade showed a comparable preference to oviposit on both tomato and black nightshade. The gregarious ectoparasitoid B. nigricans strongly accepted fourth instar T. absoluta larvae for oviposition followed by the third instar larvae in laboratory experiments. Fourth instar larvae also yielded a higher number of parasitoid offspring compared to third instar larvae. The performance of D. appellator in terms of the total number of offspring produced and female progeny were similar for second and third instar larvae of T. absoluta. The preimaginal developmental time for both parasitoid species did not vary with either instar of the larval host or sex of the parasitoid. Bracon nigricans adult longevity was similar for both sexes, while the longevity of D. appellator females was longer than for males. Molecular analysis showed a high genetic homogeneity in T. absoluta populations collected from Sudan, Uganda, Senegal and Tanzania.