Insect communities changed by broadleaf herbicide application in central Oklahoma hayfields

In farming communities in Oklahoma, the use of broad-leaf herbicides is a common practice in hayfields in order to obtain more palatable hay for livestock.  These herbicides are intended to change the plant community in favor of grasses and may also influence insects, either due to direct exposure to the chemicals, or via plant composition.  Plant and insect communities were characterized in two hayfields, one treated with an herbicide, Weedmaster & AmTide MSM 60 DF,  in Noble County, Oklahoma.  Percent cover of common plant species was estimated and the treated field was dominated by grasses while the untreated field had greater diversity and a higher percent cover of forbs.  Insects were sampled with pitfall traps and sweep nets (N=10) and identified to Order.  Total arthropod density was higher (238 per sweep net sample, SD=176, t=6.2, p<0.001) in the herbicide treatment field than in the control field (143 , SD=85).  This trend was driven primarily by Collembola (110 vs. 22 in herbicide treated vs. control fields).  Coleoptera, Diptera, Thysanoptera, and Hymenoptera were also more abundant in the herbicide treated field.  In contrast, Hemiptera, primarily small Auchenorrhyncha (hoppers), were more abundant (t=5.8, p<0.001) in the untreated field (62, SD=45) than the treated field (22, SD=11). Higher arthropod density in the treated field suggests direct exposure to the herbicides did not negatively affect arthropods overall, which leads us to conclude that the differing insect communities are a result of altered plant composition.