Common milkweed (Asclepias syriaca) has recently been a focus of conservation efforts because of its role in the Monarch lifecycle (Danaus plexippus). However, several other ecological partners may have large effects on common milkweed’s populations, including the longhorned milkweed beetle (Tetraopes tetrophthalmus). The beetle lives its entire lifecycle on common milkweed, feeding on the roots as a larva and on the leaves as an adult. Because of this close association, it has the potential to influence both asexual reproduction (root sprouting) and sexual reproduction. We address three questions 1) Because milkweed is a root sprouter, does feeding by longhorned milkweed beetle larva at low densities stimulate asexual reproduction? 2) Does herbivory on the leaves by the longhorned milkweed beetle adults alter nectar chemistry? 3) Are adult longhorned milkweed beetles pollinators themselves or do they alter the behavior of other pollinators? We conducted a greenhouse experiment where we added beetle larva onto the roots of common milkweed to address question 1. In the field, we manipulated herbivory by beetles on stems, collected nectar and conducted pollinator watches to address questions 2 and 3.
Results/Conclusions
We found that the while beetle density had a negative effect on above ground biomass (t = -2.27, P = 0.023), it did not influence the number of new stems produced (t = -1.237, P = 0.216). The initial number of buds predicted the number of final buds (t = 5.04, P = 0.000) but not the final number of stems (t = -0.158, P = 0.874). Longhorned milkweed beetles did not appear to be pollinators: longhorned beetles were recorded in the pollinator watches six times out of 749 insect visitations, they were also never collected with pollinia on their feet (n = 338). In contrast the solider beetle (Chauliognathus marginatus), was recorded in pollinator watches 52 times. Solider beetles were collected with an average of 1.8 pollinia and a maximum of eight pollinia on their feet (n = 148). However, plants that had more damage from the longhorned milkweed beetles had an increase in the number of pollinator visits (z = 2.131, P = 0.0331), but damage had no effect on the average amount of time per visit (t = 1.540, P = 0.129).