A 4-yr study determined the impact of methyl parathion, as commonly used against the alfalfa weevil, on the alfalfa weevil parasiteBathyplectes curculionisin Pennsylvania. The 1st generation ofB. curculioniseach spring coincided with the peak alfalfa weevil larval population and produced the greatest number of parasitized larvae. Each year the insecticide treatment significantly reduced the number of immature and adult parasites. The parasite-host ratio also declined relative to that in the unsprayed area. One week after spraying, each year, the numbers of immature parasites were reduced 93.2–100%. This reduction over the total period between spray and harvest ranged between 84.6 and 95.3%. In addition, the number of nonparasitized host weevil larvae also declined to near undetectable levels, consequently, reducing the oviposition potential for any adult parasites surviving in the sprayed area.The number of parasite larvae in the methyl parathion area declined significantly during the 3-yr spray program. During the 4th yr, when no spray was applied,B. curculionislarval populations increased in both the sprayed and unsprayed areas. However, the population in the sprayed area remained significantly lower than that in the unsprayed area indicating thatB. curculionispopulations did not fully recover following 3 yr of insecticide stress. The reduction in parasite numbers during this study was accompanied by a 9x increase in the weevil larval population. It is apparent that alfalfa weevil management schemes which do not judiciously integrate insecticide programs withB. curculionisbiology can greatly reduce this parasite's potential.