PEST STATUS OF THE LARGER GRAIN BORER, PROSTEPHANUS TRUNCATUS (HORN) (COLEOPTERA: BOSTRICHIDAE) IN ZIMBABWE AND AN ASSESSMENT OF INHERENT SUSCEPTIBILITY OF SELECTED MAIZE VARIETIES TO THE PEST

This study determined the occurrence and importance of Prostephanus truncatus (Horn), the larger grain borer (LGB) (Coleoptera: Bostrichidae) in relation to other storage insect pests in farm-stored maize grain samples taken from various communal households located in eight districts of Mashonaland Central and Mashonaland East provinces in Zimbabwe. The study also evaluated inherent resistance of selected maize varieties to LGB and the susceptibility of the pest to currently registered grain protectants. Surveys showed that LGB was present in all the eight districts within the two provinces, and was recovered from 22.6 % of the total samples collected. LGB was found to be most abundant in Murewa district where it represented about 90 % of all the insect pests that were recovered from stored maize grain samples. Sitophilus zeamais was the most frequently occurring insect pest, and was recovered from almost all (92%) the grain samples collected. Laboratory evaluation of 25 maize varieties and breeding lines showed varying levels of susceptibility. Significant differences (P < 0.05) were observed in apparent grain weight loss, dust production and progeny counts. There were no significant differences (P > 0.05) among the varieties in LGB parent and progeny mortality. The relative susceptibility of the varieties was compared using the selection index and showed that six hybrids and one open pollinated variety were resistant whilst the remaining 18 varieties were susceptible to LGB. The seven resistant varieties (CKPH09002, CKPH09004, CKPH08010, CKPH08043, ZM309, CKPH09001 and CZH0913) showed considerably low apparent weight loss and frass production after a 95-day exposure to P. truncatus, suggesting that they contain traits that confer resistance to the pest. The study showed that host plant resistance can be used as an important component of an integrated pest management strategy against LGB. In laboratory evaluations of five currently registered grain protectants for efficacy against LGB, significant differences (P < 0.05) in LGB mortality among the treatments were noted after two days of exposure to insecticide-treated grain. Actellic Gold Chirindamatura Dust® (1.6 % pirimiphos methyl + 0.36 % thiamethoxam) was the fastest killer, achieving 83 % mortality after two days of exposure whilst Actellic Super Chirindamatura Dust® (1.6 % pirimiphos methyl + 0.3 % permethrin) gave the lowest insect kill (9.5 %). No significant differences (P > 0.05) were, however, observed after seven days of exposure. The results of the laboratory evaluation suggest that all the test formulations are efficacious against LGB when treatments are still very fresh. Completely different results may, however, be obtained when the treatments age. The results and implications of these studies are discussed in relation to storage pest management.