Effect of postemergence fomesafen application on common milkweed (Asclepias syriaca) growth and utilization by monarchs (Danaus plexippus)

Field experiments were conducted to investigate impacts of herbicide injury to common milkweed (Asclepias syriaca L.) on oviposition preference of monarch butterflies (Danaus plexippus Linnaeus). Common milkweed seedlings were transplanted in patches containing five plants spaced 25 cm apart in a no-till soybean field shortly after soybean planting in 2016. Herbicide treatments included an untreated control and 0.14 kg ha(-1) fomesafen plus 0.5% crop oil concentrate applied postemergence. The experiment was repeated in the same area in 2017 by planting soybean no-till prior to common milkweed emergence. Common milkweed leaves displayed chlorosis and necrotic lesions five days after fomesafen application during both years. Two weeks following application many leaves damaged by fomesafen dehisced, and plants averaged an injury rating of 3.4 (visual scale of 1 = healthy and 5 = dead). Leaves emerging from the apical meristem following fomesafen application appeared normal, and four weeks after application injury ratings decreased to an average of 2.6. Dry weight of common milkweed ten weeks after application was not affected by fomesafen in either year. Common milkweed plants were examined for monarch eggs and larval instars weekly from May to August. In 2016, patches averaged 0.8 eggs, whereas in 2017 patches averaged 45.2 eggs. Fomesafen did not affect oviposition preference by monarch butterflies in either year. Larval instars on the milkweed patches were recorded throughout 2017. Presence and survival of instars were not affected by fomesafen, with patches averaging 7.5 and 0.1 first and fifth instars, respectively. In greenhouse experiments, common milkweed survived fomesafen rates up to 0.28 kg ha(-1), twice the rate typically used in soybean, and plants showed signs of recovery within two weeks of application.