Effects of Duration and Timing of Heat Stress on Monarch Butterfly (Danaus plexippus) (Lepidoptera: Nymphalidae) Development

Exposure to temperatures above 29?C has been shown to be detrimental to the de velopment of Monarch Butterfly (Danaus plexippus L.) (Lepidoptera: Nymphalidae) larvae, while it has been assumed that temperatures above 33? are lethal. However, the details of larval sensitiv ity to high temperatures are not clearly understood. We examined the effects of different lengths and timing of high-temperature exposure on larval and pupal mortality, development time, and adult mass. The experimental high temperature was 36?, while 27? was the moderate temperature at which individuals were kept when not exposed to the high temperature. In three experiments, larvae and pupae were subjected either to constant exposure to 36? initiated at various developmental stages or to fluctuating temperatures (daily high-temperature pulses of 6 or 12 hours) throughout devel opment. In general, we found that increasing lengths of constant exposure resulted in increasingly higher mortality, longer development time, and lighter adult mass, while daily temperature fluctua tions resulted in very little mortality and shorter development times. Mortality, development time, and adult size were affected differently by the initiation of high-temperature treatment at the vari ous stages of development. Because faster development is likely to be beneficial to Monarchs, cer tain high-temperature regimes that allow for shortened time to adulthood may not be as detrimen tal to larvae and pupae as previously was assumed. key words: Danaus plexippus, heat stress, larval development, Monarch Butterfly, thermoperiod