High temperature disinfestation of false codling moth larvae

The factors affecting thermal limits of insects are of central importance to predicting the influence of changing environmental conditions on their distribution and abundance, which has significant implications for pest management strategies. For holometabolous insects, it is of particular importance to establish the life-stage-related variation in acute critical thermal limits to activity and survival.

We aimed to fill the crucial knowledge gap of thermal tolerance limits in adult and larval false codling moth (FCM), Thaumatotibia leucotreta by examining thermal tolerance (upper and lower thermal limits) using a variety of different tolerance measures. We investigated the effect of different rates of heating and cooling on critical thermal limits as well as rapid heat hardening (acute plasticity) and survival assays across the different life stages. We specifically tested the prediction, generated from dynamic ramping assays, that a life-stage with a strong positive association between thermal tolerance estimates and ramping rate show less pronounced hardening responses.

Using a suite of standard lab thermal tolerance assays we tested the ability of FCM larvae and adults to survive and remain active across a diverse range of thermal conditions.

• Final-instar larvae are generally more tolerant to a broader range of thermal conditions than adults (lethal temperature range: larvae 61.2°C vs. adults 49.2°C following 2 h exposure). Larvae show a stronger positive effect of ramping rate on critical thermal estimates than adults, but adults were more thermally plastic, supporting the aforementioned hypothesis.
• The difference in basal and plastic thermal tolerance between life-stages suggests there are trade-offs between these in leucotreta, indicating that hardening effects and their variation among life-stages could play a role in predicting the impact of ramping rate variation under natural conditions.

Larvae of FCM are able to survive a much broader thermal range of conditions than adults. This has significant implications for population dynamics modelling and post-harvest fruit disinfestation. Although larvae are more tolerant to a wider range of acute thermal conditions, adults are more plastic (able to adjust to novel conditions). It is critical that any attempt to forecast or predict mortality owing to thermal stress considers life-stage related variation, and is mindful of both basal and inducible forms of tolerance depending on the time-scales under consideration.