Parasitoid wasps are extensively studied for their ability to reduce insect pests in cultures and storage, but also for their reproductive system. In these insects, sex determination is based on haplodiploidy, whereby males are haploid and females, diploid. As only female offspring require the use of sperm at oviposition, sex ratio is determined by the amount of sperm available to egg-laying females (Quicke, 1997, Chevrier and Bressac, 2002). Virgin females only lay male offspring, which are unable to control pests, and compromise biological control programmes. Therefore, sex ratio plays a major role for the success of parasitoid wasps in pest management. In most species, the sex ratio is female biased (Quicke, 1997), and whereas females usually mate only once (monoandry), males can mate multiple times (polygyny). The sex ratio results from an evolutionary trade-off, as an excess of males reduces egg laying and the control of targeted pests, and as a deficit in males results in unmated or badly inseminated females that lay male-biased clutches, compromising the efficiency of parasitism (Bressac et al., 2009, Chevrier et al., 2019). However, even when the proportion of males appears optimal, a decreased quality of reproduction may occur due to the suboptimal fertility of individual males. In many parasitoid wasps, so-called subfertile males produce a reduced amount of sperm (Lacoume et al., 2007, Chevrier et al., 2019, Chirault et al., 2015; El Sabrout et al., 2021).Table 1.

Insects are vulnerable to temperature variations because of their small size and ectothermic physiology (Ma et al., 2021). Heat waves at the pupal stage can reduce sperm production and thus induce sex ratio biases in the following generations. This thermal sensitivity was found in diverse parasitoid wasps belonging to the families Pteromalidae (Anisopteromalus calandrae –Nguyen et al., 2003; Dinarmus basalis – Lacoume et al., 2007; Nasonia vitripennis – Chirault et al., 2015), and Braconidae (Microplitis rufiventris – El Sabrout et al., 2021).

In the field, insects experience temperature variations that go beyond standard ambient temperature variation and they use many strategies to control the temperature actually felt, such as individual behaviour (Ma et al., 2021) or specific morphology (Bota-Sierra et al., 2022). However, most of these adaptations are not effective for immobile developmental stages, as are the pupae of holometabolous insects.

Males are generally more sensitive to heat than females (Chen et al., 2018), and the pupa is a critical stage because this is when spermatogenesis is most intensive (Chirault et al., 2016). Most insect males survive a limited heat shock and may not become sterile. However, single heat periods induce subfertility, as most males reproduce with lower performance than control males (Chevrier al., 2019). Moreover, in some parasitoid wasps, cold temperatures associated with short photoperiod induced a reproductive diapause of females (Chen et al., 2012). However, there is a lack of empirical evidence of the effect of lower temperatures on males.

Cotesia typhae (Hymenoptera, Braconidae) is a promising insect for the biocontrol of the maize pest Sesamia nonagrioides (Lepidoptera, Noctuidae) in Europe. C. typhae has been described as a specialized clade of Cotesia sesamiae in eastern Africa where it develops exclusively on S. nonagrioides on two associated plants (Kaiser et al., 2017; Gauthier et al 2018). In its native range, C. typhae is exposed to average temperatures between 20°C and 30°C with extremes varying between 15°C and 35°C (West Kenya, Kobodo, NASA Langley Research Center (LaRC), http://power.larc.nasa.gov). C. typhae is a gregarious species, and females lay between 80 and 100 eggs in their host (Benoist et al., 2020). After endoparasitic ovo-larval development, parasitoid larvae emerge from the host and spin their cocoon. The life cycle lasts 3 weeks at 27°C and adults live a few days in these laboratory conditions. The thermal resistance (CT min and CT max) of this insect remains to be investigated in the perspective of its introduction in Europe, where we expect heat shocks in summer amplified by climate change, and cold periods during winter.

We conducted laboratory experiments to characterize male fertility and to simulate summer and winter thermal variations on males at the pupal stage. The aim was to study a possible sensitivity to heat waves that are increasing in frequency due to climate change, and winter survival. This thermal sensitivity is also of major importance for storage and transport, in the perspective of production and commercialisation of C. typhae as a biological control agent. We focused on the main components of male fertility: sperm production by males, post-copulatory storage by females, and offspring sex ratio.