Increasing host patch exploitation efficiency of parasitoids with temperature changes

Greenhouses offer the advantage of controlling environmental temperature. Although this has mostly been used to increase plant growth and the growth season, little work has been done on using temperature control to increase efficiency of biological control agents. Travel time between patches of hosts (pest insects) is known to affect the behaviour of biological control agents (e.g, parasitoids) (Charnov 1976; Barrette et al. 2010). Host patch exploitation has also been shown to be affected by temperature within the patch (Moiroux et al. 2016). Therefore, exploitation of host patches could potentially be modified if temperature during travel between patches affects time perception of parasitoids. In a proof of concept, we investigated the influence of temperature on the perceived duration of a travel between host patches of an egg parasitoid, Trichogramma euproctidis. We simulated travel between host patches by isolating wasps for a 24 h period (hereafter ‘travel’), following Barrette et al. (2010), at three different temperatures (14, 24 and 34°C) and then measured a number of behaviours associated with patch exploitation (number of hosts parasitized, patch residence time, active exploitation time).
Travel temperature did not influence the number of hosts parasitized per patch (14°C: 36.3 ± 1.3 hosts; 24°C: 38.5 ± 1.3 hosts; 34°C: 38.3 ± 1.3 hosts). However, both residence time in host patches and active exploitation time increased with an increase in temperature during travel.
These results demonstrate the thermal sensitivity of time perception in parasitoids and that actively modifying thermal variation in the greenhouse environment could affect the balance between thorough within pest patch exploitation and increased biological control agents dispersal to exploit more pest patches. In a crop susceptible to qualitative damage by an insect pest, the growers will want a biocontrol agent to leave no pests behind. According to our preliminary results, the air temperature above the plants should be kept cooler than around the plants when the objective is to obtain thorough host patch exploitation. In contrast, when quantitative damage is the issue, increasing the dispersal of a biological control agent would be beneficial to the growers. Our preliminary results suggest that the air temperature above the plants should be kept warmer than around the plants when the objective is to increase the number of host patches visited.
The potential benefits of the increased exploitation efficiency of biological control agents will need to be balanced with the effects of temperature on plant growth and productivity. Temperature stratification technologies might have to be developed inside greenhouses to obtain the desired contrasts, such as cooling systems for tables and varying ventilation speed. These preliminary results need to be validated in a more formal experiment in a greenhouse setting.