Phagostimulant activities of sucrose, sitosterol, stigmasterol, campesterol, and soybean leaf extractables were evaluated towards the third-instar larvae of the cabbage looper, Trichoplusia ni Hubner (Lepidoptera: Noctuidae) on elderberry-pith discs under laboratory conditions. There was a linear relationship between sucrose dosage and larval feeding on the pith discs. Water fraction from PI 227687 soybean leaf extractables at dosages > 400 μg/disc inhibited feeding by T. ni larvae. Ethyl acetate fraction showed phagostimulant properties comparable to sucrose. Sterols (5 μg/disc) and soybean leaf extractables (40 μg/disc) in combination with sucrose (400 μg/disc) showed synergistic effect as phagostimulants. Implications of genotypic differences in sterol content in relation to host plant resistance are discussed.

Investigations were carried out to establish the phagostimulatory effects of cheaply available materials (bag sugar, sucrose, molasses) on Chilo partellus larvae. Sugar and analytical grade sucrose (1% w/v solutions) were the best phagostimulants tested. Molasses was the least effective but a 10% w/v solution induced a significant feeding response. These stimulants, in combination with a Bacillus thuringiensis treatment, were tested to determine their effects on increasing the efficacy of the pathogen against this pest species. Larval mortality levels were significantly higher in the presence of a feeding stimulant than when B. thuringiensis alone was administered; inclusion of such adjuvants in a B. thuringiensis preparation could significantly enhance the field efficacy of the pathogen against C. partellus.

The feeding behaviour of third-instar larvae of Chilo partellus appear to be mediated by a complex profile of chemicals present in the whorls of sorghum. These chemicals were extracted sequentially into hexane, ethyl acetate and methanol. Methanolic extracts showed greater feeding stimulation than ethyl acetate extracts and these in turn showed greater feeding stimulation than hexane extracts. Extracts derived from a more susceptible cultivar IS 18363 showed greater feeding stimulation than extracts from a less susceptible cultivar IS 2205.

Phagostimulatory compounds present in ethyl acetate and methanolic extracts include phenolics and sugars, respectively. Combinations of these sets of phagostimulants synergize to give enhanced feeding activity of third-instar larvae. Chromatographic examination of extracts of IS 18363 and IS 2205 showed quantitative rather than qualitative differences: the more susceptible cultivar IS 18363 had higher phenolic and sugar contents than the less susceptible cultivar IS 2205. These results suggest that Chromatographic quantification of the different sets of phagostimulants might constitute a basis for resistance screening of different cultivars of sorghum.