Relationship between cereal leaf beetle (Coleoptera : Chrysomelidae) egg and fourth-instar populations and impact of fourth-instar defoliation of winter wheat yields in North Carolina and Virginia

Cereal leaf beetle, Oulema melanopus (L.), has become a serious pest of sma ll grains in the mid-Atlantic legion of the United States. Existing thresho lds for implementing control measures allowed too much leaf damage aid cons equent, yield loss to occur before rc commending treatment. Information on beetle biology and crop response to injury. both prerequisites for developi ng new management strategies, was lacking for this region. A 3-yr project w as initiated to generate an area wide cereal leaf beetle biological and yie ld impact database for winter wheat, and to evaluate the injury and yield l oss potential of different population densities. Over the study period. bee tle populations were evaluated at 26 winter wheat field locations in Virgin ia and North Carolina. Eggs and larvae, classified to instar, were collecte d twice each week from February to June. Replicated insecticide versus noni nsecticide treatments were conducted at each location where leaf defoliatio n and field were documented. Results showed that the relationship between 5 0th percentile egg and fourth-instar population estimates were in strong ag reement (y = 0.36x - 0.01: r(2) = 0.79). Potentially detrimental larval inf estations were forecast before appearance of foliage injury from egg popula tions present during the stem elongation to flag leaf emergence development al stages. A significant positive linens relationship between total fourth instar per stem population estimates and percent flag leaf defoliation was detected (y = 20.29x + 1.34:r(2) = 0.60). A weaker but still significant re lationship between the total fourth-instar population estimates and percent yield loss was found (y = 11.74x + 6.51;r(2) = 0.26), indicating that fact ors in addition to flag leaf injury, primarily by fourth instars, also cont ributed to reduced yields.