Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2005
Publication Date: 3/1/2006
Citation: Nansen, C., Meikle, W.G., Phillips, T.W. 2006. Ovipositional response of the indianmeal moth, Plodia interpuctella (Hübner) (Lepidoptera: Pyralidae) to size, quality, and number of food patches. Annals of the Entomological Society of America 99/(2): 253-260. Interpretive Summary: 1. Problem. Prostephanus truncatus, an important, introduced pest of stored maize and cassava in Africa, are often caught with pheromone traps in forests. A previous paper by the same author showed that trap catches in forests peaked at different times compared to trap catches from agricultural areas, suggesting that the hosts in the forest, rather than weather, probably play a role. Little is known, however, about the hosts that they attack in the forest. Previous workers had found P. truncatus associated with branches of a tree, Lannea nigritana, that had been girdled by cerambycid beetles. Previous work also showed that not only are those sites very rich in insect species that compete with and predate on P. truncatus, but they are comparatively rare and small. So what, these authors ask, is the host of P. truncatus in the forest? 2. Approach. The approach had several distinct parts: 1. branches (both fresh, recently dead and girdled), roots and seeds of forest trees were collected, their moisture content was determined, and they were exposed to P. truncatus individuals in no-choice tests to determine whether P. truncatus could reproduce on the host; 2. the behavior (i.e. boring into the wood, reproducing) was noted and the potential reproductive rate for each host was calculated; 3. P. truncatus individuals were placed on soil in jars to observe whether they can burrow (and access potential hosts such as roots or buried seeds); and 4. the density of the potential hosts in the forest (determined in this paper and in previous work by the same author) and the reproductive potential on those hosts was used to evaluate, qualitatively, the possible importance of each kind of substrate: girdled branch, fresh branch, root and seed. 3. Results. Judging from the density of girdled branches of L. nigritana in the forest, the role of these branches in explaining the P. truncatus trap catches was believed to be minor. The number of P. truncatus that such girdled branches was estimated to produce, based on tree density data and assumptions about the number of branches that would be girdled per tree per year, was less than 400 per hectare per year, yet the number of beetles trapped in the forest exceeded 3100 beetles per hectare per year. No non-girdled branches were found to be as rich a substrate as girdled branches, and whether P. truncatus could overcome tree defenses to feed on non-girdled branches would be another question. Freshly-killed branches would, in any case, also be rich in competitors and predators. Roots of some trees proved sufficiently rich as a food source, but impossible to verify in the field and in any case the tree species with acceptable roots were not found to be associated with high P. truncatus trap catches. The final category, tree seeds, included one particular resource, teak seeds, that may be important hosts. Some P. truncatus were reared from teak seeds in no-choice tests. Although no P. truncatus emerged from seeds collected in the forest, given the extent of teak plantations in the forest, even very low emergence rates could produce enough individuals to explain the high trap catches. The implications on P. truncatus management, in particular on the lack of successful biocontrol by the introduced predator Teretrius nigrescens, are discussed. Further work is needed to collect seeds in the forest and confirm or refute this hypothesis.
Technical Abstract: Prostephanus truncatus is an important insect pest on stored maize and cassava in Africa, but it is also believed to be well established in savanna and forest habitats. However, fairly little is known about potential hosts of P. truncatus in non-agricultural habitats. In no-choice experiments, we evaluated the ability of P. truncatus to attack and reproduce on the following potential forest hosts: (1) fresh branches from 26 tree species, (2) dry branches from 13 tree species, (3) fresh roots from 18 tree species, (4) dry roots from two tree species, and (5) seeds from four tree species. Heavy attacks on fresh branches were recorded in 18% of all infestations, and they occurred on 11 different tree species. Heavy attacks also occurred on fresh roots from four tree species. High reproductive rate was found on branches from four species (Delonix regia, Ceiba pentandra, Manihot esculenta, and Ficus exasperata) and on roots from F. capensis and F. exasperata. In behavioral experiments, a substantial proportion of P. truncatus individuals chose to burrow into the soil when free flight was allowed. Seeds from four tree species Afzelia africana, Vitellaria paradoxa, Elaeis guineensis (oil palm), and Tectona grandis (teak) were also tested as potential forest hosts, and P. truncatus reproduction was obtained on teak seeds. Based on estimates of P. truncatus production in the Lama Forest and surrounding teak plantations from branches and seeds of tree species shown in laboratory tests to be susceptible to P. truncatus, we estimated that even a very low rate of successful reproduction by P. truncatus on these seeds would yield several magnitudes more P. truncatus than would be expected from branches, including the only previously identified host, branches of Lannea nigritana previously girdled by cerambycids. We therefore suggest that seeds and roots may be important non-agricultural hosts for P. truncatus in West African savanna areas.