Parasitism of Lepidopterous Stem Borers in Cultivated and Natural Habitats

Plant infestation, stem borer density, parasitism, and parasitoid abundance were assessed during two years in two host plants, Zea mays (L.) (Cyperales: Poaceae) and Sorghum bicolor (L.) (Cyperales: Poaceae), in cultivated habitats. The four major host plants (Cyperus spp., Panicum spp., Pennisetum spp., and Sorghum spp.) found in natural habitats were also assessed, and both the cultivated and natural habitat species occurred in four agroecological zones in Kenya. Across habitats, plant infestation (23.2%), stem borer density (2.2 per plant), and larval parasitism (15.0%) were highest in maize in cultivated habitats. Pupal parasitism was not higher than 4.7% in both habitats, and did not vary with locality during each season or with host plant between each season. Cotesia sesamiae (Cameron) and C. flavipes Cameron (Hymenoptera: Braconidae) were the key parasitoids in cultivated habitats (both species accounted for 76.4% of parasitized stem borers in cereal crops), but not in natural habitats (the two Cotesia species accounted for 14.5% of parasitized stem borers in wild host plants). No single parasitoid species exerted high parasitism rates on stem borer populations in wild host plants. Low stem borer densities across seasons in natural habitats indicate that cereal stem borer pests do not necessarily survive the non-cropping season feeding actively in wild host plants. Although natural habitats provided refuges for some parasitoid species, stem borer parasitism was generally low in wild host plants. Overall, because parasitoids contribute little in reducing cereal stem borer pest populations in cultivated habitats, there is need to further enhance their effectiveness in the field to regulate these pests

Efficacy of cultural control measures against the banana weevil, Cosmopolites sordidus (Germar), in South Africa

The banana weevil, Cosmopolites sordidus, is the most important insect pest of banana and plantain in the world. Cultural control methods were investigated over 2 years in southern KwaZulu-Natal, South Africa. Harvesting at ground level and dissection of remnants (treatment 1), and covering the base of the mat (entire plant consisting of several meristems) with soil and moving debris to the inter-row (treatment 2), were compared to a positive control that involved treatment of plants with a registered pesticide (treatment 3), and a negative control that involved harvesting at 150 cm from the collar with no soil or sanitation amendments (treatment 4). Yield, weevil damage and pseudostem girth of plants were measured from August to November annually, while adult beetle densities were assessed over 4 weeks in October/November and April. Nematode samples were taken and analysed in October/November every year. Damage parameters included the coefficient of infestation, the percentage coefficient of infestation (PCI) at two intervals, the summed PCI value, the percentage cross-sectional damage of the central cylinder and cortex, and the mean cross-sectional damage percentage. A randomized block design with three replicates was used in the trial. The parameters were similar before the onset of the trial. Fruit yield and plant girth, corrected by nematode densities, were not significantly different in any treatment, nor were the nematodes controlled. Soil cover and recession of remnants was the only effective treatment, significantly reducing the CI, but not the adult density or the other damage parameters. Soil cover showed promise as a cultural control method because it only needs to be applied seasonally and reduced the percentage cross-sectional damage of the central cylinder, the damage parameter most closely related to yield, by 14%

Components of resistance to sorghum shoot fly, Atherigona soccata

Sorghum shoot fly, Atherigona soccata is one of the major constraints in sorghum production, and host plant resistance is one of the components to control sorghum shoot fly. Thirty sorghum genotypes were evaluated for different mechanisms of resistance and morphological and agronomic traits during the rainy and postrainy seasons. The sorghum genotypes, Maulee, Phule Anuradha, M 35-1, CSV 18R, IS 2312, Giddi Maldandi, and RVRT 3 suffered lower shoot fly damage, and also exhibited high grain yield potential during the postrainy season. ICSB 433, ICSV 700, ICSV 25019, ICSV 25022, ICSV 25026, ICSV 25039, PS 35805, Akola Kranti, and IS 18551 exhibited antixenosis for oviposition and antibiosis against sorghum shoot fly, A. soccata. Leaf glossiness, plant vigor, leafsheath pigmentation and trichomes were associated with resistance/susceptibility to shoot fly. Path coefficient analysis indicated that direct effects and correlation coefficients of leaf glossiness, plant vigor, plant height, plant color and trichomes were in the same direction, suggesting that these traits can be used to select sorghum genotypes for resistance to shoot fly. Principal co-ordinate analysis based on shoot fly resistance traits and morphological traits placed the test genotypes into different groups. The genotypes placed in different groups can be used to increase the levels and broaden the genetic base of resistance to shoot fly. The environmental coefficient of variation and phenotypic coefficient of variation for shoot fly resistance and morphological traits were quite high, indicating season specific expression of resistance to sorghum shoot fly. High broadsense heritability, genetic advance and genotypic coefficient of variation suggested the predominance of additive nature of genes controlling shoot fly resistance, suggesting that pedigree breeding can be used to transfer shoot fly resistance into high yielding cultivars. This information will be useful for developing shoot fly-resistant high yielding cultivars for sustainable crop production