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

Authors: Moran, Patrick; Miskella, John; MORGAN, CHRISTY - Former ARS Employee; Madsen, John

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2023
Publication Date: 5/5/2023
Citation: Moran, P.J., Miskella Jr., J.J., Morgan, C.M., Madsen, J.D. 2023. Toxicity of herbicides used for control of waterhyacinth in the California Delta towards the planthopper Megamelus scutellaris released for biological control. Biocontrol Science and Technology. 33(5): 448-466. https://doi.org/10.1080/09583157.2023.2196707.
DOI: https://doi.org/10.1080/09583157.2023.2196707

Interpretive Summary: Water hyacinth, native to South America, is a major invasive floating aquatic weed of rivers, lakes, and reservoirs throughout tropical, sub-tropical and warm temperate areas of the world. This floating weed hinders water flow to pumping facilities, reduces recreational water use and reduces light and oxygen for submerged plants and fish. In the Sacramento-San Joaquin Delta ("the Delta") of northern California, which is the critical junction in California's supply system for irrigated agriculture, chemical herbicides are applied to 2,000 to 5,000 acres of water hyacinth each year. In the 1960s-1980s, the USDA-Agricultural Research Service (ARS) discovered, characterized, and released several insects as biological control agents as an alternative to herbicides, only one of which, a leaf-chewing and stem-boring weevil (Neochetina bruchi), became established in the Delta. Since 2010, ARS and state cooperators have released a new agent, a planthopper (Megamelus scutellaris) that feeds on plant juices inside leaves. This planthopper has also been released in Florida and in South Africa and has established large populations. Some impact is apparent, but chemical herbicides must still be used in many cases. This study examined the direct toxicity of five herbicides-2,4-D, diquat, glyphosate, imazamox, and penoxsulam-and two products used as 'surfactants', which help herbicides penetrate waxy leaf surfaces-one derived from paraffin oil, and another derived from vegetable oils. In laboratory tests, planthoppers were allowed to feed for 24 hours on leaves that had been dipped in herbicide or surfactant solutions, or leaves that were just dipped in water. In field tests, small caged tanks of water hyacinth were infested with planthoppers and sprayed the next day with herbicide or surfactant solutions, or just water. An insecticide treatment was used to verify that the planthoppers were successfully exposed to the other treeatments. Among the herbicides, only diquat showed toxicity to the planthoppers, killing as many as the insecticide in the lab (over 90%) and killing more planthoppers (up to 64% more than when sprayed with water) in the field tests than any other herbicide, although still less than the insecticide (up to 98% more killed than when sprayed with water). The paraffin oil-based surfactant was less toxic but did kill up to 49% of the planthoppers in field tests. These results indicate that 2,4-D, glyphosate, imazamox and penoxsulam are not toxic to the water hyacinth planthopper, while diquat is toxic, and a surfactant used with many of these herbicides is moderately toxic. For most herbicides, integrated biological and chemical control of water hyacinth can be carried out without concerns about direct toxicity to the planthopper.

Technical Abstract: The water hyacinth planthopper Megamelus scutellaris has been released since 2010 for biological control of water hyacinth (Eichhornia crassipes or Pontederia crassipes) in Florida and is widely established there, with additional, less successful releases in Louisiana and California, and this planthopper is established and having impact in South Africa. Management of water hyacinth, especially in subtropical and temperate climates continues to require the use of herbicides. Knowledge of the direct toxicity of herbicides to M. scutellaris would aid in control planning. We tested the contact toxicity of 2,4-D, diquat dibromide, glyphosate, imazamox and penoxsulam as well as a paraffinic oil-based and a vegetable oil-based surfactant, by exposing planthoppers to dipped leaves in the laboratory and by applying chemicals to caged, infested mesocosms in the field. Exposure to residue of the herbicides 2,4-D, glyphosate, imazamox and penoxsulam did not cause toxicity to M. scutellaris in in statistical comparisons to exposure to water-dipped leaves in the laboratory, or 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 in two trials, and exposure to leaves dipped in paraffinic-oil based surfactant also increased mortality (40–50%). In field mesocosms, application of diquat reduced recovery of live M. scutellaris after 72 hours by 64% in one of two trials in which it was tested, and the surfactant reduced recovery by 36%–49% in two trials, while positive control (insecticide) treatment reduced recovery by 57%–98%. The results suggest that, for all of the herbicides tested except diquat, biological and chemical control of water hyacinth can be safely integrated in terms of direct exposure of M. scutellaris to herbicide residues or sprays.