Toxicity of herbicides used for control of waterhyacinth in the California Delta towards the planthopper Megamelus scutellaris released for biological control

The water hyacinth planthopper Megamelus scutellaris has been released for biological control of waterhyacinth (Eichhornia crassipes or Pontederia crassipes) in Florida and is widely established there, with additional releases in Louisiana and California, USA, and in South Africa. Management still often requires the use of herbicides. Knowledge of their direct toxicity to M. scutellaris would aid in integrated management. We tested the contact toxicity of 2,4-D, diquat, glyphosate, imazamox and penoxsulam as well as a paraffinic oil-based and a vegetable oil-based adjuvant, by exposing planthoppers to dipped leaves in the laboratory, and by applying chemicals to infested mesocosms in the field. Exposure to 2,4-D, glyphosate, imazamox and penoxsulam did not cause significant mortality to M. scutellaris compared to exposure to water-dipped leaves in the laboratory, and did not consistently reduce final counts of M. scutellaris compared to counts in water-sprayed mesocosms in field trials. Laboratory exposure to diquat-dipped leaves caused significantly (45%–69%) higher mortality than did exposure to water-dipped leaves, and exposure to leaves dipped in paraffinic-oil based adjuvant also increased mortality (40–50%). In field mesocosms, application of diquat reduced recovery of live M. scutellaris by 64% in one trial, and the same adjuvant reduced recovery by 36%–49% in two trials. The results suggest that biological and chemical control of water hyacinth using M. scutellaris can be safely integrated using most of the herbicides applied for control of this weed in California.