Thermal tolerance limits of diamondback moth in ramping and plunging assays

Abstract

Thermal sensitivity is a crucial determinant of insect abundance and distribution. The way it is measured can have a critical influence on the conclusions made. Diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an important insect pest of cruciferous crops around the world and the thermal responses of polyphagous species are critical to understand the influences of a rapidly changing climate on their distribution and abundance. Experiments were carried out to the lethal temperature limits (ULT₀ and LLT₀: temperatures where there is no survival) as well as Upper and Lower Lethal Temperature (ULT₂₅ and LLT₂₅) (temperature where 25% DBM survived) of lab-reared adult DBM population to extreme temperatures attained by either two-way ramping (ramping temperatures from baseline to LT₂₅ and ramping back again) or sudden plunging method. In this study the ULT0 for DBM was recorded as 42.6°C and LLT₀ was recorded as -16.5°C. DBM had an ULT₂₅ of 41.8°C and LLT25 of -15.2°C. The duration of exposure to extreme temperatures had significant impacts on survival of DBM, with extreme temperatures and/or longer durations contributing to higher lethality. Comparing the two-way ramping temperature treatment to that of direct plunging temperature treatment, our study clearly demonstrated that DBM was more tolerant to temperature in the two-way ramping assay than that of the plunging assay for cold temperatures, but at warmer temperatures survival exhibited no differences between ramping and plunging. These results suggest that DBM will not be put under physiological stress from a rapidly changing climate, rather access to host plants in marginal habitats has enabled them to expand their distribution. Two-way temperature ramping enhances survival of DBM at cold temperatures, and this needs to be examined across a range of taxa and life stages to determine if enhanced survival is widespread incorporating a ramping recovery method.

Figure 1. Schematic diagram of experimental protocols used for: (a) two-way ramping temperature assay, (b) plunging temperature assay.

Figure 2. Contour plot of the relationship between the survivorship (%) of DBM (P. xylostella) at (a) low temperatures with duration of exposure (10 minutes, 30 minutes, 60 minutes and 120 minutes) and (b) high temperatures with duration of exposure.

Survival is indicated by different colour shades (blue  = 0% survival, red  = 100% survival).

Figure 3. Mean survival (%) of DBM (P. xylostella) exposed to plunging and two-way ramping (a) cold temperatures and (b) hot temperatures.