Strategic mowing of roadside milkweeds increases monarch butterfly oviposition

Abstract

The eastern North American migratory population of monarch butterflies (Danaus plexippus) has declined precipitously due, in part, to the widespread decline of its obligate host plant, milkweed (Asclepias spp.). Linear right-of-ways (e.g. roadsides, power line corridors) are believed to be a significant source of milkweed and represent a valuable target for restoration efforts. Although many current mowing practices in these habitats are detrimental for monarchs because mowing occurs too frequently or is poorly timed, strategic mowing could be beneficial for monarch reproduction if it produces young milkweed at the right time of season. To address this, forty-nine paired experimental and control plots containing common milkweed (Asclepias syriaca) were established along habitat adjacent to a two-lane highway in southern Ontario. Experimental plots were mowed once per season and followed one of eight mowing treatments that occurred between mid-June and early August. Milkweed characteristics and the presence of monarchs at all life stages were monitored within plots from late June through September. Overall, mowed plots had higher egg abundance than unmowed controls. Within mowed plots, egg abundance/plant was highest in plots mowed between the 2nd and 3rd weeks of July. At this latitude (43°N), mowing past this window was less effective and would have resulted in high mortality of developing monarchs. Our results suggest that mowing common milkweed once before egg laying peaks could maximize monarch butterfly reproduction in managed landscapes.