Carry-Over Niches for Lepidopteran Maize Stemborers and Associated Parasitoids during Non-Cropping Season

Abstract

Sources of infestation are the key elements to be considered in the development of habitat management techniques for the control of maize stemborers. Several wild plants, grasses mostly, have been identified that serve as hosts for stemborers and their parasitoids during the off-season when maize is not present in the field. However, their abundance is much lower in wild plants compared to cultivated fields. Thus, the role of wild plants as a reservoir for cereal stemborers and their parasitoids is still controversial, particularly in agro-ecosystems with reduced wild habitat. We studied the occurrence of different maize stemborers and associated parasitoids in maize stem residues and wild grasses during non-cropping seasons as potential carry-over populations to subsequent early season maize plants. Surveys were conducted in the central region of Kenya during long and short dry seasons in maize residues and wild grasses as well as during the two rainy seasons in maize plants at earlier and late whorl stages during the years of 2017 and 2018. Wild habitat had a higher species diversity than maize residues habitat, but maize residues had a higher abundance of maize stemborer species, such as Busseola fusca, Sesamia calamistis, and Chilo partellus, and of associated parasitoid species (i.e., Cotesia flavipes and Cotesia sesamiae) than wild plants. Our surveys, complemented by field parasitoid releases of C. flavipes and C. sesamiae, indicated that maize residues constitute a better refugia reservoir not only of the maize stemborers but also of C. flavipes and C. sesamiae during non-cropping seasons as compared to wild plants and, thus, might constitute in this region the main source of both stemborers and C. flavipes/C. sesamiae carry-over in maize plants during the subsequent cropping season. Thus, systematic destruction of maize residues would not help the biological control of lepidopteran stemborers. This is particularly true in areas with reduced wild habitat.

Keywords: biological control; habitat and pest management; maize residues; wild plants.

Figure 1

Number of stemborers of Busseola fusca, Sesamia calamistis, and Chilo partellus found in maize residues and wild plants per 100 maize residues or wild plants sampled in two different localities in the central region of Kenya (Makutano and Murang’a) during non-cropping seasons in 2017 and 2018. Non-significant differences between maize residues versus wild plants are shown by identical letters determined using Tukey’s multiple comparisons tests with the R package “lsmeans”, following a generalized linear model (GLM) with a negative binomial error distribution.

Figure 2

The principal component biplot showing the relation between abundance of stemborer species in maize plants of cultivated fields during cropping season and those in either maize residues or wild plants during non-cropping season.

Figure 3

A correlogram highlighting the direction and intensity of the correlation between the abundance of each stemborer species in maize plants of cultivated fields during cropping season and those in either maize residues or wild plants during non-cropping season. The blue color denotes a positive correlation and the red color a negative correlation. Higher intensity of the color indicates a strong correlation. Bf_MR, B. fusca in maize residues; Bf_MP, B. fusca in maize plants; Bf_WP, B. fusca in wild plants; Sc_MR, S. calamistis in maize residues; Sc_MP, S. calamistis in maize plants; Sc_WP, S. calamistis in wild plants; Cp_MR, C. partellus in maize residues; Cp_MP, C. partellus in maize plants; Cp_WP, C. partellus in wild plants; Os_MR, other species in maize residues; Os_MP, other species in maize plants; Os_WP, other species in wild plants. For other species (Os), see Table 2.

Figure 4

Percentages of females of Busseola fusca, Sesamia calamistis, and Chilo partellus found in the maize residue habitat during the non-cropping seasons and on maize plants in the cultivated habitat during the subsequent cropping seasons. The proportion of males and females was set to 100% to calculate the percentage of females. The proportions of males and females were compared between habitats for each species using a two-tailed Fisher’s exact probability 2 × 2 test (**: p < 0.01).

Figure 5

The mean number (±SE) of parasitoid species recovered per field in maize residues or wild plants during non-cropping seasons as well as in maize plants of cultivated fields during the subsequent cropping seasons in Makutano and Murang’a before (pre-release) and after or during (post and during release) Cotesia flavipes and Cotesia sesamiae releases.