Comparative studies on the adult dispersal behaviour of busseola fusca (fuller) (Lepidoptera:Noctuidae) and chilo partellus swinhoe (lepidoptera:pyralidae)

Abstract

Two oil soluble dyes, Calco Red N-1700 and Sudan Blau-670 were added to the artificial diet of Chilo partellus and Busseola fusca. The two dyes effectively marked the larvae, adults, eggs and first stage F1 larvae. The dye treatments did not have any effects on mean fecundity, longevity, pupal weights and flight duration of mated female moths of the two stem borer species. Further experiments showed that, B fusca reared on a normal diet for 18 days and transferred to the Calco Red dyed could still lay marked eggs. The dye was found to be more appropriate for further marking experiments because marked moths laid red eggs which can easily be detected on maize plants during dispersal studies. Busseola fusca C.partellus reared on different host plants showed moths laid red eggs, which can easily be detected on maize plants during dispersal studies. Busseola fusca and C. partellus reared on different host plants showed better growth and development on cultivated sorghum, maize and the wild sorghum species (Sorghum versicolor and S. arundinaceum) while the poorest performance was on napier grass (Pennisetum purpureum). The poor performance observed on P. purpureum may be attributed to presence of antibiotic factors or to poor nutrients contents. The two wild sorghum species may therefore act as important alternate hosts or trap crops. Two-choice oviposition preference experiments where insects reared on a given host (''rearing host'') were tested for oviposition alternately with each of the other hosts (''test host'') revealed that for both species, moths which were reared on a given host preferred to lay more eggs on that host. For example, B. fusca reared on cultivated sorghum and tested for oviposition alternately with either maize or Napier glass preferred to lay more eggs on sorghum. Similary C. partellus reared on maize and tested for oviposition alternately with cultivated sorghum and wild sorghum (S.versicolor) preferred to lay more eggs on maize. These findings reveal that there can be a variation in an insect's oviposition response to a plant as a result of its previous larval food experience. This altered oviposition preference in favour of the plant species upon which the larvae were reared on is a phenomenon referred to as Hopkins' host selection principle (Hopkins, 1917). However, this principle was not evident in multiple choice oviposition preference experiments indicating that its validity as a general principle operating in adult oviposition preferences of B. fusca and C. partellus moths still remains to be proved. Results of the experiment investigating changes in insect density from a central release point using oviposition by internally marked female moths as a measure of insect density showed that, for C. partellus there was a profound falling off in the mean number of eggs and egg batches with increasing distance. Hence, the maize plants which were near the release point received the highest number of eggs and egg batches than those that were far away. This phenomenon referred to as ''edge effect'' could have been brought about by a situation whereby a large proportion of C. partellus female moths preferred to disperse horizontally as opposed to a vertical dispersal which could have taken them high up and possibly even outside the sampling area. As for B. fusca, the mean egg batches and egg numbers showed no significant differences and were more or less uniformly distributed throughout the dispersal distance; hence ''edge effect'' was not evident. Results also showed that at each concentric circle, more eggs of C. partellus were recovered than those of B. fusca. The higher recovery rate for C. partellus eggs is usually associated with very low dispersal rates whereas on the other hand, the lower recovery rate for B. fusca eggs is associated with high dispersal rates. Results of the experiment involving sampling to recover eggs and larvae from the wild populations of B. fusca and C. partellus showed that, the maize plants at the edge of the experimetnal plot (bordering the wild host plants) received the highest number of C. partellus eggs and egg batches, hence ''edge effect'' was evident. In contrast, the mean number of eggs, egg batches and larvae of B. fusca showed a more or less uniform distribution and therefore ''edge effect'' was not evident. Therefore B. fusca and C. partellus behave quite differently and the former distributes its eggs more or less uniformly whereas the latter lays more eggs on the nearest host plants. An important implication of ''edge effect'' is the C. partellus can be controlled by use of trap crops or by applying insecticides only on the margins of the field where infestation is severe. Most eggs of C. partellus will be laid on the trap crop, which can be destroyed or cut as fodder. However, trap crops unless very attractive may have very little value for the protection of crops against the invasion of crops by B. fusca because the latter is capable of dispersing its eggs in a more or less uniform manner and even more widely.