La perception du temps et sa modulation par la température chez les guêpes parasitoïdes

Optimality models assume that animals foraging for resources use the rate of fitness gain to optimize several behaviours such as patch time allocation and progeny investment. Although the duration of multiple behaviors needs to be regulated, few evidence of time perception are currently available for insects and none for parasitic wasps, despite their importance as ecological models. This capacity to measure time could be affected by temperature, since parasitic wasps are poikilotherms. We hypothesized that parasitic wasps should have the capacity to perceive time, both prospectively (measure of elapsed time) and retrospectively (duration of a past event), in order to optimize foraging decisions related to host patch exploitation and reproduction. We also hypothesized that temperature would affect the time perception of parasitic wasps.   For the prospective measure of time, we used the associative learning abilities of Microplitis croceipes (Hymenoptera: Braconidae). We trained them to associate an odour to the duration of an interval between hosts. Following their training, they were tested in a wind tunnel. The wasps chose significantly more the odour associated with the test interval they were trained to recognize. We also looked into the role of energy expenditure in the measure of time. When wasps were prevented from moving freely during the interval between hosts, they chose randomly in the wind tunnel. The lack of energy expenditure thus rendered them unable to measure time. Energy expenditure is therefore an essential proxy to measure time.   For the retrospective measure of time, we used the host evaluation process of Trichogramma euproctidis (Hymenoptera: Trichogrammatidae). Trichogramma wasps use the duration of the initial transit on the host egg to evaluate its size and decide how many eggs to lay. We artificially increased the initial transit duration of T. euproctidis by suspending the host egg to make it appear larger than an unsuspended egg of the same size. An increase in initial transit duration increased clutch size in the host. This demonstrates the capacity of T. euproctidis to measure the duration of the initial transit duration, and therefore a retrospective measure of time.   To ascertain if temperature modified the measure of time in poikilotherms species, we measured the patch exploitation behaviour of T. euproctidis following intervals at different temperatures. Wasps are expected to stay longer and leave at a lower rate of fitness gain following a long travel duration between host patches than for a short travel. We tested the impact of a 24 h travel duration at different temperatures on patch exploitation. An increase in temperature during travel time increased patch residence time and decreased the rate of fitness gain at departure, both of which are associated to an increase in travel time. The reverse was observed when travel occurred at a lower temperature. The increase or decrease in temperature affected the measure of time by speeding or slowing the clock respectively and making the travel appear longer or shorter than it actually was.   Our results clearly show that parasitic wasps are able to measure time, both retrospectively and prospectively. There is now solid evidence that at least two species of parasitic wasps possess the capacity to measure time, an essential component of optimality models. We now also have evidence that energy expenditure is one of the proxies used to measure time.   Our results are also the first evidence of the impact of temperature on time perception in insects. The use of energy expenditure as a proxy to measure time could explain some of its thermal-sensitivity, since parasitic wasps are poikilotherms. This temperature sensitive measure of time could affect biological control strategies. In augmentative biological control, releasing parasitic wasps during the day could lead to a higher level of patch exploitation than early in the day because of the temperature gradient between the air and the plants.

Keyphrases: Trichogramma, Measure of time, Temperature, Animal behaviour, Optimality model