Assessment of damage and yield losses on maize by the lepidopterans Chilo partellus (Crambidae), Busseolafusca (Noctuidae) and Sesamia calamistis (Noctuidae)

Lepidopteran stem borers are among field insect pests responsible for the low maize production in sub-Saharan Africa. In Kenya, Chilo partellus Swinhoe (Pyralidae), Busseola fusca Fuller (Noctuidae) and Sesamia calamistis Hampson (Noctuidae) constitute the major proportion of stemborer community. Concerted efforts to reduce stemborer populations in maize fields have been emphasized without a good understanding of potential losses associated with damage by respective stemborer species. This study aimed at assessing larval effects of the three stemborers on maize growth and how these relate to yield losses. A field experiment was initiated with both .stemborer species (B. fusca, S. calamistis and C. partellus) and densities (0, 10, 15 and 30) as the treatment factors. H513 maize variety was planted and artificially infested with the first instar larvae 21 days after emergence (DAE). Plants were caged immediately after infestation to restrict movement of the introduced larvae within the cage and eliminate external infestations. Effects of the treatment factors on the growth parameters were taken until the maize was harvested. Results indicated that larval survival for each stemborer species decreased with increasing larval infestations for all the stemborers regardless of the initial infestations. Not more than four larvae of each stemborer managed to survive in each stem of maize. It was observed that, C. partellus and B. fusca had comparatively higher percentage survival than S. calamistis larvae. There were no significant differences between the low infestation density of 10 larvae/plant and medium infestation density of 15 for all the stem borers (p> 0.05). However at higher infestation density of 30 larvae/plant, low survival of larvae was realized which was statistically lower as compared to the densities of 10 and 15 for all the stemborers. Proportionate larval effect of each species was observed and it was reported that, C. partellus and B. fusca larval reductions did not produce any difference on the plant height (p> 0.05). However, S. calamistis larva produced significantly lower effect on the height of maize (p< 0.05). There was evidence of variation in the lengths of stem tunnelling associated with the different stemborer species (p< 0.05). Busseola fusca larva caused the highest length of stem tunnelling, followed by C. partellus and S. calamistis larva produced the least. Chilo partellus and B. fusca larva produced significantly higher reductions in cob weights in comparison to S. calamistis (p< 0.05). Busseola fusca larva produced higher effect on grain weight reduction as compared to C. partellus and S. calamistis. Relationships between stemborer larva and their effect on maize growth showed a strong link between stem tunnelling and yield reduction. C. partellus and B. fusca were observed to be major stem borers of maize while S. calamistis was least important.