The abundance and microhabitat use of rodents were investigated in four different habitats: two rain-fed crop fields with differing stone bund density, an irrigated field and bushland. A total of 444 individual small mammals belonging to six rodent and one shrew species were recorded in trapping grids and line transects. Of these, 230 individuals (52%) belonged to three pest rodent species of crop fields in northern Ethiopia: Stenocephalemys albipes (65%), Mastomys awashensis (25%) and Arvicanthis niloticus (10%). Population abundance of the three species was higher in the early dry season compared to the rainy season. While the bushland was significantly (p < 0.05) favoured by S. albipes and M. awashensis in both seasons, the irrigated field was preferred by Arvicanthis niloticus in the early dry season. In the early dry season, the microhabitat use of A. niloticus was strongly associated with the type of ground cover (herb) (R2adj = 0.152, P < 0.01). While M. awashensis was associated with vegetation density (R2adj = 0.13, P < 0.01), S. albipes was associated with vegetation cover (R2adj = 0.102, P < 0.001). The findings indicate that co-occurring pest rodent species prefer different microhabitats. Understanding their co-occurrence particularly in crop fields is vital for crop protection as they are known serious agricultural pests in northern Ethiopia.
In the sorghum/aphid/ladybeetle ecosystem found in the Texas High Plains Region of the United States, we found that the corn leaf aphid, Rhopalosiphum maidis (Fitch), is a key aphid species that provides a critical early-season food source for native coccinellids. From 1988 to 2000 data on the seasonal abundance of sorghum-infesting aphids and ladybeetles were collected from a total of 21 irrigated and 12 rain-fed grain sorghum fields. The data indicated that successful biological control of the greenbug by coccinellids is normally dependent on early-season colonization of the sorghum field by corn leaf aphids. When corn leaf aphids exceeded 100/plant before sorghum boot stage greenbugs never exceeded 125 aphids/plant. In all cases where greenbugs were found in densities that would cause economic damage to sorghum (>250/plant), corn leaf aphids reached a density of 100 or more per plant after sorghum reached the boot stage. In irrigated fields, the first record of coccinellid eggs and peak coccinellid abundance were positively and significantly (p = 0.05) correlated to the day of the year when corn leaf aphids reached or exceeded a minimum of 100/plant and corn leaf aphid peak abundance in both irrigated and rainfed fields. On the other hand, greenbug peak abundance was significantly correlated only to coccinellid peak abundance in irrigated fields. Regression analyses indicated that in paired analyses of irrigated and rain-fed sorghum fields, an increase of one aphid at time t, resulted in an increase in coccinellid peak abundance from 0.024 to 0.025 per 15 m of row at time t + 2 depending on aphid species, if corn leaf aphids reached a level of 100 or more per plant by sorghum boot stage and irrigation parameters. We concluded that corn leaf aphids are an important early-season food source for predaceous coccinellids, drawing these predators into the fields where they feed on the aphids and deposit eggs, engendering a captive larval population that is present when greenbug first begin to enter the field later in the season.
Chemical cues involved in both host location and oviposition by Harmonia axyridis Pallas (Coleoptera: Coccinellidae) were investigated in laboratory and field experiments. Among the five volatiles tested in a four-arm olfactometer ((E)-β-farnesene, β-pinene, β-caryophyllene, cis-3-hexen-1-ol, and limonene), gravid H. axyridis females were significantly attracted to limonene and β-caryophyllene. These two chemicals also increased H. axyridis oviposition on plants. Attraction of H. axyridis towards limonene was further tested under field conditions using controlled-release dispensers. (E)-β-farnesene, a major aphid alarm pheromone and a potential predator kaironome, was also tested in this study. Field data confirmed those from laboratory studies concerning the attraction of H. axyridis towards limonene and a lack of response to (E)-β-farnesene. In contrast, the aphid predator Episyrphus balteatus DeGeer (Diptera: Syrphidae) was attracted to (E)-β-farnesene but not to limonene. These findings indicate that H. axyridis uses chemicals, such as limonene, for prey location and oviposition, and support the potential use of volatiles in the management of H. axyridis.