Three plant introductions (PI 171451, PI 227687, and PI 229358) of soybean [Glycine max (L.) Merr.] having resistance to the Mexican bean beetle [Epilachna variveslis Mulsant] were evaluated in the laboratory for leaffeeding resistance (antibiosis) to the bollworm [Heliothis zea (Boddie)] and tobacco budworm [H. virescens (Fabricius)]. Five commercial cultivars were used for comparison. Criteria for antibiosis were larval mortality, larval and pupal weights, and duration of larval stage. All three plant introductions were resistant to both species. Among larvae fed on the plant introductions, mortality was greater than among larvae fed on the commercial cultivars, weight gains of larvae were reduced and time to pupation was greater. On the basis of percentage of larval mortality, PI 171451 and PI 229358 were more resistant to the bollworm than to the tobacco budworm; PI 227687 was equally resistant to both species. None of the larvae of either species survived on PI 227687, which suggests that this plant introduction may have a different genetic basis for resistance.
A search for insect growth inhibitors in methanol extracts of soybean leaves resulted in isolation of pinitol. Pinitol caused a 50% reduction in weight gain (ED50) of Heliothis zea larvae at about 0.7% when added to a synthetic diet. Although myo-inositol is a normal component of the insect diet, it also caused growth inhibition at higher concentrations; ED50 4%.
Leaves of 6‐ to 8‐week old Melilotus infesta plants contained progressively higher levels of nitrate as the leaves developed from the tightly pinched to the fully expanded stage. In weevil feeding tests on water extracts of these leaves a close relationship was found between actual feeding and the feeding predicted on the basis of nitrate content. At a given level of nitrate, extracts of young M. infesta leaves were less attractive to the weevil than were extracts of young M. officinalis leaves. Leaves of mature M. infesta plants contained very little nitrate, but water extracts of such leaves remained unattractive to the weevil in feeding tests. These extracts were lower in Stimulant A activity and higher in Deterrent A activity than extracts of mature M. officinalis leaves. Swectclover weevil resistance is influenced by the balance between Stimulant A, Deterrent A, and Deterrent B (nitrate), and the relative contributions of the different factors vary with stage of development of the leaves and the plant. Nitrate appears to be the predominant water‐soluble factor in young leaves, and Deterrent A and Stimulant A assume increased importance in leaves from mature plants.
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