Heat Effects on Monarch Butterfly

I recently was out looking for Monarch Butterfly larvae, and after searching roadsides and vacant lots in the area, I found only one in my own yard. Since I know temperatures and heating or growing degree days (GDD) affect the emergence of the adult butterflies, I was curious as to what affects this might also have on larvae. We know that a definite number of GDD is needed by each species and we can predict the emergence of adults once we know the number of GDD needed to break diapause. This past spring was a great example of this with much earlier than normal flights of our spring butterflies after the exceptionally warm March and April.

The hotter than normal temperatures and drought conditions have affected butterflies in general, by the drying up of both nectar and larval host plants. This translates to less food to sustain reproducing adults and fewer opportunities for the female to find suitable host plants to oviposit on. The lack of water and puddles also affects the male’s ability to acquire the needed minerals for reproduction as is done through puddling.

To avoid the effects of the heat of the day, Monarch larvae will move to the shaded under side of the leaf. Other practices to avoid the heat include moving down the plant stem to near the ground, or retreating to the ground litter for shelter and relief from the heat. Research has shown that Monarch larvae and chrysalises are affected by higher temperatures. With temperatures reaching into the mid 90’s or higher for hours as they have consistently done this summer, the mortality rate increases in both stages.

Higher than optimum temperatures in the 8o’s, as well as shorter periods of higher (mid 90’s and higher) temperatures, also affect larval development.  My own observation and other research show that the time in the chrysalis is less with higher temperatures. Research has also shown that larval development times are shortened. Higher temperatures also result in lower larval and adult weights. Darker winged adults are also a result of higher temperatures.