Chemicals affecting the feeding preference of the Solanaceae-feeding lady beetle Henosepilachna vigintioctomaculata (Coleoptera: Coccinellidae)

The phytophagous lady beetle, Henosepilachna vigintioctomaculata, feeds mainly on potato, tomato, and eggplant leaves. The methanol extracts of tomato and eggplant leaves stimulated feeding activity in the adult beetles. The feeding stimulants from the lipid-soluble fractions of tomato and eggplant leaves were found to be same compounds, identified as methyl linoleate and methyl linolenate. The feeding stimulants in the aqueous fractions of tomato leaves were identified as three sugars - fructose, glucose, and sucrose - and in eggplant leaves, the feeding stimulant was one sugar, sucrose. Although methyl linoleate and methyl linolenate were inactive without sugars, they acted synergistically with sugars, and the amounts of methyl linoleate, methyl linolenate, and sugars contained in tomato and eggplant leaves were adequate to stimulate feeding activity in the beetles. It is suggested that the synergisms of methyl esters of unsaturated fatty acid and sugars play important roles in host selection of this insect. Solanaceae species are often rich in alkaloids, which act as feeding deterrents for many insect species. Leaves of potato, Chinese wolfberry, and black nightshade, which are suitable hosts of H. vigintioctomaculata, contain alpha-solanine and alpha-chaconine as the main alkaloids. A mixture of alpha-solanine and alpha-chaconine showed neither feeding stimulant nor inhibitory activity at a concentration of 1 g leaf equivalents of the above plants. alpha-Solanine and tomatine contained in tomato did not inhibit the beetle's feeding at a concentration of 0.5%. On the contrary, nicotine and capsaicin contained in non-hosts (tobacco and red pepper, respectively) showed feeding deterrent activities at a concentration of 0.1%. It is thought that adaptation of the beetles to alkaloids contained in solanaceous hosts also plays an important role in their host selection.