Phenology of Milkweed (Asclepiadaceae) Growth and Monarch (Lepidoptera: Nymphalidae) Reproduction in Kentucky and Ovipositional Preference between Common and Honeyvine Milkweed

We studied the phenology of monarch (Danaus plexippus L.) reproduction, as well as the early season phenology of its host plant species, in Kentucky. We also examined the ovipositional preference of D. plexippus for common milkweed (Asclepias syriaca L.) relative to honeyvine milkweed (Cynanchum laeve [Michaux]); the latter is much more abundant than the former in and around cultivated fields in Kentucky. We recorded the growth of A. syriaca and C. laeve weekly in two natural stands of each species, beginning when the plants first emerged (early May) and ending when the plants began flowering (late June). The earliest arriving adult monarchs were observed in Kentucky during April and May 2000. Milkweed plants were large enough to support monarch reproduction by mid May, but no eggs and only a single larva were seen on the milkweed plants at any of our research sites during May or June. Danaus plexippus eggs were first observed at those sites in mid July and were found until late August; larvae were present from late July until mid September. At our research site where A. syriaca and C. laeve plants were interspersed with one another, D. plexippus females laid virtually all eggs on A. syriaca plants. However, in single-species stands of A. syriaca or C. laeve plants at six other research sites, numbers of monarch eggs and larvae per plant did not differ between the two plant species. Finally, because of concerns that transgenic Bt corn pollen may harm monarch larvae, we compared our results on the monarch's reproductive phenology with the seasonal timing of corn anthesis (pollen shed) in Kentucky. Statewide weekly data on corn silking reported by the USDA Statistical Reporting Service (5-year average) suggest that more than 80% of Kentucky's corn crop typically completes anthesis by the end of July. Thus, while the monarch does use C. laeve, a host plant common in Kentucky corn fields, a large portion of the D. plexippus larval population is present in the state after most corn plants have shed pollen. This asynchrony should reduce potentially harmful effects of transgenic Bt corn pollen on monarchs in Kentucky.