Population dynamics and integrated management of Prostephanus truncatus (Coleoptera : Bostrichidae) in Manica Province, Mozambique

Abstract

The Larger grain borer, Prostephanus truncatus (Coleoptera: Bostrichidae), was introduced from Southern and Central America into Africa during the late 1970s, and has since then established in 20 African countries. This pest reduces the storage period of maize grain and cassava chips in granaries of small scale farmers. This reduced storage period results from larvae and adult feeding with subsequent shortening of the period that these commodities are available as food. Depending on storage period, yield losses of up to 45% and 100% have been recorded for maize and cassava chips respectively in West Africa, while 62% yield losses of maize during storage have been reported in Mozambique. In Mozambique, P. truncatus was first recorded in 1980 in the Manica Province. Prostephanus truncatus occurs throughout Mozambique with higher population densities and maize grain damage in the Manica and Tete Provinces, where it is a severe pest of maize grain, cassava chips and other cereals. Chemical and biological control measures have been used for P. truncatus. However, the pest continuously disperses, colonizes new habitats and causes much injury to stored maize grain and cassava chips. This study aimed to review current literature regarding P. truncatus and to assess the effect of abiotic and biotic factors in spatial and temporal fluctuations of P. truncatus numbers. Host plant species, altitude and climatic factors were also taken in account in this study. Moreover, the study aimed at assessing the effect of mass trapping in P. truncatus numbers and assessing suitability of wild plants and firewood as hosts of P. truncatus in Mozambique with reference to the central and Southern regions. This project will provide this knowledge which is scarce or non-existent in Mozambique. Results will inform rural small farmers, government and non-government organizations, decision makers and stakeholders regarding improved pest management measures and could hence reduce population densities of the pest with particular reference to the Manica Province in Mozambique. Prostephanus truncatus pest status in Africa is high and the degree of infestation and damage vary between regions. This variation in pest status is due to climatic conditions, food sources, and differences in storage infra-structure development and efficacy of control methods. Its temporal and spatial dispersion is unpredictable and depends on ecological factors, maize and dry cassava trade routes, and availability of forest host plants. Development of sustainable integrated management strategies is a key to future successful management of this pest. Area-wide management strategies using the predator, Teretrius nigrescens, parasitoids, plant derived products and environmentally friendly insecticides are needed. Integrated management practices must be based on improved knowledge of P. truncatus population dynamics and its determining factors. Generalized linear models showed that abiotic and biotic factors affect P. truncatus numbers. However, the strength of the effect varied between villages and years. Mean numbers of host plant species, the maximum and minimum temperature, relative humidity and rainfall had an effect on P. truncatus number over years. Average temperature did not affect trap catches of P. truncatus. Models were not validated due to absence of previous similar data on P. truncatus in the studied villages and in Mozambique in general. Higher numbers of P. truncatus were caught in Massingir (231 individuals per trap per month) followed by Machipanda village (104). Moreover, the high numbers of beetles were present during both the dry and rainy seasons and in all land use types of the Massingir village compared to other villages. In general higher numbers of beetles were trapped during the rain season and in human settlements. Designed models and the analysis of P. truncatus flight activity per season and per land use type form a baseline for further studies toward predicting dispersal and potential areas of invasion by P. truncatus in Mozambique. Mass trapping of P. truncatus with the use of universal moth traps (Uni-traps) showed that the lowest mean number of beetles per trap captured over a month-period was 26 individuals at Mapai in August while the highest was at the village of Massingir in November (8089). The highest mean beetle density per maize ear (335 individuals) was found in control granaries. The maize kernel weight reduction was higher and increased over time in granaries without traps than in those with them. These findings indicate that mass trapping with the use of Uni-traps represents a potential method for effective control of P. truncatus in granaries of small scale farmers. Research on suitability of plant species sold and used as firewood recorded, P. truncatus in three (Brachystegia spiciformis, Colophospermum mopane and Strychnos spinosa) of the six plant species used for these purposes in Manica and Gaza Provinces. Moreover the pest survived and bred in 13 tree and 7 grass species. Dry maize stalks were also highly suitable for pest survival. Dry wood of the tree species B. spiciformis, Colophospermum mopane and Cassia abbreviata as well as the grasses Acroceras macrum and Hyparrhenia hirta were very good hosts for development of P. truncatus. The sale and transport of firewood that host P. truncatus may be an important driver of the spread of this species. Uninfested areas where plant hosts occur and are abundant, are likely to be infested by this pest in the future. This knowledge will contribute to the development of practical integrated measures for management of P. truncatus