The showy milkweed, Asclepias speciosa, is a long-lived herbaceous perennial distributed across much of the western part of North America. Asclepias speciosa has a number of insect herbivores and in turn has developed a number of anti-predation defenses including latex. In our early observations, we found that Milkweed herbivores have higher movement both on and between plants at higher temperatures and the phenology of species changed over a summer temperature gradient. This undergraduate-led project investigated abundance of milkweed symbionts over a summer temperature gradient. We predicted higher densities of leaf and longhorn beetles would be found on milkweed at higher temperatures. We sampled 22 sites in the Yakima Valley of Central Washington for the following variables (1) Date, Time, and Location (2) Local Climate (3) Patch size and density (4) Insects present and their location on plant (5) Ground temp at bottom of milkweed stalk (6) Air temp at top of milkweed stalk (7) Phenology, height, and % damage of a subsample of plants.
We found that the activity level and abundance of insect herbivores increased with ground temperature (P<0.01), but leaf damage to A. speciosa is positively impacted only by air temperature (P <0.01). We also found a trend toward decreased abundance of long-horned beetles as temperature increased while aphid abundance increased with increasing temperatures. This project will continue for the 2017 summer field season, after a marked increase in winter precipitation, with the hope of finding more detail about their phenology. The effects of temperature on milkweed herbivores could impact such behaviors as feeding habits, reproductive efficiency, and activity levels. As we learn more about the relationship between temperature and milkweed herbivory, we can better understand the impact these communities have on our world.