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Simulating Alfalfa Weevil Effects by Defoliation¹

Yield loss assessment and the economic evaluation of pest control practices require an understanding of how pests affect crops. Hand removal of alfalfa (Medicago saliva L.) leaflets to simulate insect defoliation was used (a) to compare simulated and natural defoliation by the alfalfa weevil (Hypera postica Gyllenhal), (b) to relate yield reduction to the amount of leaf mass removed, and (c) to determine the effect of defoliation on subsequent shoot growth. Leaflets were removed during a period of 11 days to simulate the pattern of alfalfa weevil defoliation. In two experiments, greenhouse-grown plants were defoliated starting at the first-bud and first-flower stages of development, respectively. Defoliation treatments removed all leaflets within 5, 15, or 30 cm of the growing tip by the end of the defoliation period, simulating feeding of about 3, 6, and 11 larvae/stem, respectively. Recovery growth was followed for up to 5 weeks after the end of defoliation.

Simulated insect defoliation reduced yield, delayed maturation, reduced stem lengths, and increased axillary branching (for the 5-cm treatment only) similar to effects observed in the field. The concentrations of crude protein and digestible nutrients were only slightly influenced by simulated defoliation. At the end of defoliation, the yield of residual alfalfa herbage was inversely proportional to the extent of defoliation (P 0.01). The mass of the removed leaflets did not fully account for the yield reduction, but the relationship between extent of defoliation and the sum of the residual herbage and the removed leaflets was not statistically significant. Growth rates following the end of defoliation were not influenced by defoliation treatment. The data indicate that hand removal of alfalfa leaflets simulates defoliation by the alfalfa weevil if defoliation is extended over a period of days. Such treatment neither inhibited nor stimulated growth following defoliation.

Key Words: Medicago sativa L. • Hypera poltica Gyllenhal • Chronological leaf removal • Insect damage • Recovery growth

² Associate professor of agronomy, Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853.

Received for publication December 11, 1981.

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