Assessment of insecticide resistance in five insect pests attacking field and vegetable crops in Nicaragua

Field populations of Hypothenemus hampei (Ferrari), Plutella xylostella (L. ), Spodoptera exigua (Hubner), Helicoverpa zea (Boddie), and Bemisia tabaci (Gennadius) were tested for resistance to several insecticides commonly us ed in Nicaragua. Assays were conducted to estimate the LD(50)s or LC(50)s a nd the corresponding resistance ratios. A diagnostic concentration was used to discriminate between susceptible and resistant strains of H. hampei. Th e tests with >6,000 a hampei adults collected from six different sites indi cate the absence of resistance to endosulfan. Resistance to cypermethrin, d eltamethrin, chlorfluazuron, thiocyclam, and methamidophos was documented i n six field populations of P, xylostella. High levels of resistance to cype rmethrin and deltamethrin, but moderate levels of resistance to chlorpyriph os and methomyl, were also documented in two field populations of S, exigua . Moderate levels of resistance to cypermethrin, deltamethrin and chlorpyri phos were also documented in three field populations of H. tea. Moderate to high levels of resistance to bifenthrin, methamidophos and endosulfan were documented in four field populations of B. tabaci. The presence of signifi cant correlations between LD(50)s or LC(50)s suggests the occurrence of cro ss-resistance or simultaneous selection for resistance by different insecti cides with different modes of action. Our data could not differentiate betw een these two possibilities. Because insecticides will continue being used in Nicaragua, a resistance management program is urgently needed. The imple mentation of integrated pest management tactics must be accompanied by spec ific regulations for pesticide registration. In the future, pesticide regis tration regulations in Nicaragua should include periodic resistance monitor ing. The mechanisms to cover the costs of resistance monitoring and resista nce management should also be established.