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.

An invitation to measure insect cold tolerance: Methods, approaches, and workflow

“…Bale et al, 1989;Cloudsley-Thompson, 1973;Miller, 1978). In particular, directly plunging an insect into an extreme temperature can significantly decrease survivorship, compared to that achieved during a ramping regime (Nguyen et al, 2014). Conversely, slow cooling rates can make experiments impractically long, and may allow insects to mount a physiological response to cold, such as that observed during rapid cold-hardening (Kelty and Lee, 1999;Nguyen et al, 2014).…”

“…In particular, directly plunging an insect into an extreme temperature can significantly decrease survivorship, compared to that achieved during a ramping regime (Nguyen et al, 2014). Conversely, slow cooling rates can make experiments impractically long, and may allow insects to mount a physiological response to cold, such as that observed during rapid cold-hardening (Kelty and Lee, 1999;Nguyen et al, 2014). Most authors currently choose cooling rates that compromise between ecological relevance and time investment; generally 0.1 to 0.5 °C/min (e.g.…”

Diamondback Moth, Plutella xylostella (L.) in Southern Africa: Research Trends, Challenges and Insights on Sustainable Management Options

“…In SA, temperature is projected to increase by 1-3 • C by 2050 [45][46][47] and its effects are likely to be more pronounced in the drier tropics than the humid subtropics [8,9]. In laboratory experiments, DBM showed activity over a broad temperature range, measured as LLTs and ULTs [36,38]. This may mean that, under the currently projected climate change in SA, DBM pest status is likely to increase, exacerbating already failing management practices [18,25,48].…”

“…Insecticide resistance associated with high temperatures has been recorded in different species [55], including variations in P. xylostella susceptibility to some organophosphates [55,56]. Therefore, under current climatic projections in SA [9,38,57], it is highly unlikely that DBM populations will decline due to the physiological stress associated with high or low temperature scenarios [36]. Sub-Saharan Africa's majority of rural small-scale farmers remains at the core of food production, but their production ecosystems are the most prone to climate change effects [9,45,57,58].…”

“…On the other hand, African urban areas of the host DBM and evidence is presented that these traits may have coevolved, parasitoid efficacy in the face of climate change may be compromised [26]. Without coevolution of thermal tolerance, DBM challenge may likely intensify due to conducive climatic conditions [37][38][39][40][41] that may stimulate increased pest activity (feeding, breeding, and migration) [26,38]. Therefore, without efficient control mechanisms, the DBM problem could continue to increase despite the intensive pesticide use, which to-date may have been short-term, ineffective, unsustainable and expensive [18].…”