Factors Affecting Stem Borer Parasitoid Species Diversity and Parasitism in Cultivated and Natural Habitats

Abstract

The effects of biotic and abiotic factors on stem borer parasitoid diversity, abundance, and parasitism were studied in cultivated and natural habitats in four agroecological zones in Kenya. Comparing habitat types, we found partial support for the "natural enemy" hypothesis, whereby, across all localities, parasitoid diversity was higher in more diverse host plant communities in natural habitats, whereas parasitoid abundance was higher in cultivated habitats. For both habitats, parasitoid richness was mainly influenced by stem borer density and/or its interaction with stem borer richness, whereas parasitoid abundance was mainly affected by stem borer abundance. Parasitoid richness was higher in localities (with bimodal rainfall distribution) with increased spatial and temporal availability of host plants that harbored the borers. Across seasons, parasitoid richness was lower in both cultivated and natural habitats in the driest locality, Mtito Andei. Overall, parasitoid diversity was low in Suam and Mtito Andei, where maize cultivation was practiced on a commercial scale and intense grazing activities persist across seasons, respectively. Across localities, habitats, and seasons, stem borer parasitism was positively correlated with parasitoid richness and abundance. Furthermore, the interaction of rainfall and altitude influenced the presence and absence of parasitoids, and consequently, stem borer parasitism. Parasitism was positively and negatively correlated with temperature in cultivated and natural habitats, respectively. Overall, natural habitats seem to serve as important refugia for sustaining parasitoid diversity, which in turn can affect stem borer parasitism in the cereal cropping system.

Parasitism of Lepidopterous Stem Borers in Cultivated and Natural Habitats

“…Irrespective of habitat type, stem borer parasitism is positively correlated with both parasitoid richness and abundance. Furthermore, being negatively correlated with temperature in natural habitats, parasitoids seem sensitive to harsh/extreme temperatures in this habitat, thereby either lowering their performance or decimating some of their populations (Mailafiya et al 2010). Also, heavy rainfall, especially at higher altitudes, is detrimental to both stem borers (parastized or not) and parasitoids as well as their activities (Schulthess et al 2001; Ndemah et al 2003; Mailafiya et al 2010)…”

“…Natural habitats have high stem borer diversity (Le Ru et al 2006a, b), and thus, serve as refugia for sustaining parasitoid diversity within the ecosystem (Ndemah et al 2002; Mailafiya et al 2009, 2010). Also, across different ecological regions and seasons, stem borer parasitism is generally positively correlated with parasitoid richness and abundance (Mailafiya et al 2010). Herbivore parasitism, however, can vary with habitat type depending on the prevailing conditions in a given ecosystem (Landis et al 2000; Altieri and Nicholls 2004).…”

Assessing the impact of arthropod natural enemies on crop pests at the field scale

“…Despite successful examples of integration of parasitoids into IPM programs difficulties arise when attempting to determine the actual impact of parasitoids on pest population growth. The most common approach is to estimate parasitoid abundance using parasitism rate, assuming high apparent parasitism rate equals high parasitoid abundance and equates to high levels of possible control (e.g., Yu & Byers, ; Mailafiya et al ., ). However, additional factors such as host and parasitoid immigration and emigration, rates of reproduction and other sources of mortality will impact parasitoid abundance.…”

Importance of wild host plants for stem borer parasitoid diversity and control of parasitism in cereal cropping ecosystems in Kenya

“…Altogether, our results do not support the suggestion of Nye (1960) and Seshu Reddy (1985), encouraging the removal and destruction of wild host plants of lepidopteran stem borers in the vicinity of cultivated cereals as a measure for checking crop infestation, their suggestion clearly did not consider the impact of natural vegetation or habitat destruction on natural enemies of stem borers. Natural habitats, as a component of the ecosystem, provide refuges requisite to parasitoid survival (Godfray 1994;Thies et al 2005;Wajnberg et al 2008), and thereby serve as reservoirs for maintaining stem borer parasitoid diversity (Mailafiya et al 2009(Mailafiya et al , 2010a. Also, by attacking several herbivore host species in both cultivated and natural habitats, various stem borer parasitoid species can change between habitats so as to parasitise hosts, when host availability alternates either across seasons or habitat types (Mailafiya et al 2009(Mailafiya et al , 2010a.…”

“…Natural habitats, as a component of the ecosystem, provide refuges requisite to parasitoid survival (Godfray 1994;Thies et al 2005;Wajnberg et al 2008), and thereby serve as reservoirs for maintaining stem borer parasitoid diversity (Mailafiya et al 2009(Mailafiya et al , 2010a. Also, by attacking several herbivore host species in both cultivated and natural habitats, various stem borer parasitoid species can change between habitats so as to parasitise hosts, when host availability alternates either across seasons or habitat types (Mailafiya et al 2009(Mailafiya et al , 2010a. For instance, in the forest zone of Cameroon in Central Africa, parasitoids of cereal stem borers were found attacking four non-economically important stem borer species in natural habitats (Ndemah et al 2001(Ndemah et al , 2007.…”