RELATIVE ABUNDANCE AND DISTRIBUTION OF STEM BORERS IN INTERCROPS OF Zea mays L., Sorghum bicolor L. AND THREE GRAMINAE REFUGIA IN TRANS-NZOIA COUNTY.

Abstract

This study aimed at determining the relative abundance and distribution of stem borers in intercrops of Zea mays L. and Sorghum bicolor L. and three graminae refugia due to the great economic damages caused by the stem borers to these hosts. It involved two gramineous crops: maize Zea mays L., sorghum Sorghum bicolor L. and three gramineous forages: Napier grass Pennisetum purpureum Schumach, Sudan grass and giant Setaria grass. These were planted both in pure and mixed stands and sampling for the borer infestation done throughout the phenology of crops. Field and laboratory bioassays were conducted to determine biophysical efficacies of the stem borers. Three stem borers were recorded: Busseola fusca (Fuller), Chilo partellus (Swinhoe) and Sesamia calamistis (Hampson). Busseola fusca was more abundant in the highlands (1.5+ 0.6 larvae/plant) at 9 weeks after emergence (WAE). The C. partellus predominantly featured in lowlands with (2.2 + 0.8 larvae/ plant) at 7 WAE. This was punctuated with the S. calamistis in the lowlands. Overally, B. fusca was the most prevalent with a mean of 6.4 while S. calamistis had the least prevalence with a mean of 2.0.The type of the gramineous refugia had a significant (p < 0.05) effect to the magnitude of damage caused by the stem borers. Busseola fusca was the most devastating species with a mean of 4.9, 3.9, 2.2 and 1.4 borers while S. calamistis was the least devastating with a mean of 1.1, 0.9, 0.5 and 0.6 borers in maize, sorghum, Napier and Sudan grass. However, C. partellus was the most devastating species in giant Setaria grass with a mean of 3.3 and B. fusca being the least abundant with a mean of 0.7 borer. Maize was the most damaged host indicating that it provided the best geographical requisites and nutritional attributes that were more attractive to B. fusca.. The type of diet fed on by the stem borers had a significant difference (p < 0.05) on the fecundity of the stem borers. Stem borers fed on the artificial diet had a mean egg production of 90.4 relative to 53.7 of stem borers fed on the natural diet due to balanced disaproportionality of the nutrients, typical of wild crop regimes. The number of entry and exit holes (r = 0.059**) positively correlated with the number of larvae recovered from dissected stems. Also, the number of larvae recovered from attacked stems positively ( r = 0.074**) correlated with damage of stem borers on the leaves implying that the more the moths enter the stems, the more larvae emerge causing severe damage on the morphology of the gramineous refugia, leaves inclusive. The stem diameter had a positively correlated (r = 0.062**) with the number of larvae recovered. The type of gramineous forage had a significant (F = 46.3*; p < 0.05) effect on the damage caused by stem borers to maize and sorghum. Pennisetum purpureum was the most effective gramineous forage refugia with the potency of being utilized in the push – pull management strategy of the stem borers. It reduced damage caused by stem borers to 2.0% and 5.7% in maize and sorghum respectively. This implies that it had desirable traits attractive to the stem borers especially the great devastating B. fusca due to chemical and biophysical morphology and stem diameter. B.fusca was the most abundant and devastating both in the laboratory bioassays and field. Maize was the most damaged host and Napier grass was the most preferred forage refugia.