Factors Influencing the Sensitivity of Binary Preference Assays for Determining Cereal Cultivar Resistance to Grasshoppers

Abstract

The feeding behaviour of herbivorous insects has become the focus of renewed interest in recognition of its critical importance in screening cultivated plants for resistant germplasm. Determination of biotic potential was viewed as the ultimate test for resistance to grasshoppers in agro-ecosystems dominated by cereal monoculture, because of the direct relationship between it and population dynamics. Such studies, however, are labour-intensive and may extend over several months. As an alternative to more timeconsuming assay procedures, in this study we examined a behavioural assay and some of the factors that influence the results. In the majority of assays in the present study, the total number of grasshoppers visiting the plants were lower on the second day than the first day using second instar grasshoppers. In contrast, the total numbers of grasshoppers recorded on the plants was higher on the second day than the first day for the majority of assays using neonates. The extent of defoliation also was noticeably greater on the second day, except when both plants available to the grasshoppers were non-preferred. This difference which was reflected in greater sensitivity of the assays using neonates serves to highlight one of the problems when grasshoppers at later stages of development are used in binary preference assays. The observations of feeding patterns are discussed in terms of some recent concepts