Mating probability

Within mating swarms, sex ratios typically tend to be male-biased, offering females increased mating opportunities. In our study, female mating probability was increased in both D. platura lines in response to increased male-to-female ratios rather than group density, a result congruent with trends observed in other arthropod species (Karlsson et al. Reference Karlsson, Eroukhmanoff and Svensson2010; Vahl et al. Reference Vahl, Boiteau, de Heij, MacKinley and Kokko2013). These trends also corroborate field observations of D. platura forming mating swarms (Miller and McClanahan Reference Miller and McClanahan1960; A. Bush-Beaupré, unpublished data).

An increase in female mating probability under male-biased sex ratios could be due to an increase in male mating–related activity, such as locomotion (Bahrndorff et al. Reference Bahrndorff, Kjaersgaard, Pertoldi, Loeschcke, Schou, Skovgård and Hald2012) and courtship (Leftwich et al. Reference Leftwich, Edward, Alphey, Gage and Chapman2012; Marie-Orleach et al. Reference Marie-Orleach, Bailey and Ritchie2019). Although male–male interactions, and thus aggressions, are more likely to occur under male-biased sex ratios, thereby reducing male mating success (Enders Reference Enders1993), male–male competition in our group treatments for both genetic lines could have switched from interference (aggression) to scramble competition as the group sex ratio became increasingly male-biased (Weir et al. Reference Weir, Grant and Hutchings2011).

Although male-biased sex ratios may cause an increase in female resistance behaviour (Carrillo et al. Reference Carrillo, Danielson-François, Siemann and Meffert2012), a decrease in such behaviour may also be observed as male insistence increases (Lauer et al. Reference Lauer, Sih and Krupa1996). If female resistance had increased to the point of overcoming male insistence in male-biased mating groups, mating probability would have decreased. This may be particularly true for the N-line because mating probability plateaued near 100%. However, mating probability plateaued at around 60% for H-line females, possibly indicating an increase in female resistance proportional to the overall increase in male insistence, assuming that male–male competition did not hinder their mating behaviour.

It is, however, possible that male behaviour may not have been the defining factor in the mating interactions observed. The increase in female mating probability could simply be due to an increase in the number of available males to choose from, a main characteristic of swarming behaviour (Downes Reference Downes1969). Female mate choice can be affected by multiple factors. For example, males in mating pairs tended to be larger than males sampled at random in swarms of the mayfly, Epeorus longimanus (Eaton) (Ephemeroptera: Heptageniidae) (Flecker et al. Reference Flecker, Allan and McClintock1988), and in the yellow dung fly, Scathophaga stercoraria Linnaeus (Diptera: Scathophagidae), larger males copulated and mate-guarded more under male-biased sex ratios (Otronen Reference Otronen1996). Even though we sieved pupae with 1.7-mm mesh for our experimental stocks, this did not entirely eliminate size variability. Females of both D. platura lines could therefore have chosen the larger males to mate with if these latter were more successful in winning male–male competitions, as Benelli et al. (Reference Benelli, Donati, Romano, Ragni, Bonsignori, Stefanini and Canale2016) observed in the olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae).

The low proportions of mated females in our smallest group composition (1 male and 1 female) were unexpected, especially for the H-line, in which no mating occurred within 42 days in all 10 replicates compared to 4 of 10 females for the N-line. This result is surprising because females remaining unmated even in the presence of a male would appear to be disadvantageous. However, lifelong female virginity is not uncommon in insects. According to a review by Rhainds (Reference Rhainds2019), a higher rate of lifelong virginity is observed in females that are flightless, short-lived, choosy, small, and have a long prereproductive maturation period (when coupled with a high rate of mate encounters leading to an increased rate of mating rejection). Lifetime female virginity is also affected by reproductive asynchrony, female-biased sex ratios, low population density at a large scale (Allee effect), and high population density at a fine scale (signal jamming). In the present study, D. platura females could fly, were relatively large, and were long-lived (over 42 days), and some had a short preoviposition period (an average of 10 days out of the 42-day trial period). Furthermore, the long duration of the experimental trials (up to 42 days) mitigated the potential effects of reproductive asynchrony, the sex ratio in the low-density treatment (1:1) was not female-biased, the relatively small size of the experimental arenas likely negated a putative Allee effect, and fine-scale population density was low. These facts suggest that H-line females are choosy towards the male with which they mate. The long duration of the trial additionally enforces this supposition because females are predicted to reduce their choosiness as they age as so to ensure fertilisation during their lifetime (Kokko and Mappes Reference Kokko and Mappes2005). With 6 of 10 N-line females remaining unmated in the 1:1 group composition, females of this line may also be choosy but to a lesser extent than H-line females are. Although the results obtained here suggest high mate selectivity in H-line females, this choosiness was not explicitly evaluated. As such, laboratory experiments in which females are offered a choice between males with different trait values (such as size or wing length) are needed to support or refute this hypothesis.

It is pertinent to note, however, that dense swarms (exceeding the volume of the arenas used in our experimental treatments) were frequently observed in the main colony cage of the H-line and were rarely witnessed for the N-line (A. Bush-Beaupré, unpublished data). It is therefore possible that the arena size was too small to allow H-line males to form a swarm, thus leading to a low mating probability. In addition, it is possible that the single male in the 1:1 male:female group treatment was not stimulated into courtship due to a lack of other males with which to form a swarm. A similar study conducted on D. platura by Hough-Goldstein et al. (Reference Hough-Goldstein, Hess and Cates1987) investigated the effect of group composition on mating dynamics and reported a mating probability of around 70% in a 15:15 male:female group composition, as well as some successful mating ranging from 0 to 25% in their 1:1 treatments, depending on the experiment. These results are highly congruent with those reported in the present study for the N-line and, although Hough-Goldstein et al. (Reference Hough-Goldstein, Hess and Cates1987) did not deposit voucher specimens or specify the capture locality of their founding stocks, we suspect that they worked with that genetic line. Considering that the two lines of D. platura differ in their mating systems (and possibly in other biological traits), there are clear limitations to the extrapolation and application of results obtained in studies where the line(s) identity was not determined and voucher specimens are unavailable for a posteriori determination. We therefore recommend that future work on D. platura involves the determination of the genetic line(s) under study.

Preoviposition period

In both D. platura lines, the fastest fertile oviposition (highest hazard ratio; Fig. 1B) was observed in the 16:16 group composition. A decrease in the time to first fertile oviposition was observed as the number of females within a group increased at high densities for both lines, whereas in the low-density treatment, the delay to first fertile egg seemed to decrease slightly as the number of males per female increased for the N-line. Although a higher number of females could have increased the probability that females with an intrinsically faster rate of oviposition were present in the group, the difference in the preoviposition period between the two lines suggests that this was not the main factor affecting the speed at which females lay their first fertile eggs. Additionally, if the preoviposition period was fully explained by the number of females within a group, we would expect a much shorter preoviposition period in the group with 16 females compared to the group with only five females. This was not the case for either line in our study. As such, the total number of females within a group does not fully explain the rate at which they lay their first fertile eggs.

The presence and oviposition of conspecific females can have different effects on oviposition behaviour, depending on the group. In some species, female crowding can increase the preoviposition period (Ambrose et al. Reference Ambrose, Sahaya Rani and Vennison1988). In the present study, the group with the most females (16:16) had the shortest preoviposition period in both lines. In the viburnum leaf beetle, Pyrrhalta viburni (Paykull) (Coleoptera: Chrysomelidae), females spent less time searching and selecting for an oviposition site when conspecific egg masses were present, thus reducing the preoviposition period (Desurmont et al. Reference Desurmont, Weston and Agrawal2014). Because we included a single oviposition site per experimental arena regardless of treatment, the presence of conspecific eggs may have been one of the factors explaining the shorter preoviposition period that we observed at high densities in the two lines of D. platura.

Because neither of the D. platura lines laid fertile eggs before the interval of day 4–6, we can infer that ovipositional maturity was reached no sooner than the first 4–6 days of their lifespan. This result is comparable to the average preoviposition period measured in the closely related Delia florilega (6.5 days; Kim and Eckenrode Reference Kim and Eckenrode1987). The average preoviposition period for both lines of D. platura was approximately 10 days, concurring with results obtained by McClanahan and Miller (Reference McClanahan and Miller1958), which ranged between 10 and 20 days. The minimum length of the preoviposition period observed in the present study (between 4 and 6 days) suggests that the absence of oviposition reported by Mlynarek et al. (Reference Mlynarek, Macdonald, Sim, Hiltz, McDonald and Blatt2020) for D. platura females aged 2–4 days exposed to different developmental stages of onion (Allium cepa) for 48 hours in a no-choice experiment may not have been due to a rejection of the oviposition substrate but caused instead by the fact that females had not yet reached ovipositional maturity.

Our results are congruent with results obtained by a companion experiment conducted on the reproductive compatibility of the two D. platura lines (Bush-Beaupré et al. Reference Bush-Beaupré, Bélisle, Fortier, Fournier, MacDonald and Savage2023). In the aforementioned study, the mating probability of intraline crosses (H-line males with H-line females and N-line males with N-line females) was compared to that of interline crosses (H-line males with N-line females and vice-versa) using a group composition of 30 males with two females. The mating probability of both lines in Bush-Beaupré et al. (Reference Bush-Beaupré, Bélisle, Fortier, Fournier, MacDonald and Savage2023) was equivalent to the mating probability obtained in the 30 males to two females group composition of the present study (approximately 80 and 70% for the N- and H-lines, respectively). Additionally, females of both lines in Bush-Beaupré et al. (Reference Bush-Beaupré, Bélisle, Fortier, Fournier, MacDonald and Savage2023) also had a mean preoviposition period of approximately 10 days. The congruence between the results reported in both studies emphasises their replicability and adds robustness to our estimate of the conditions that maximise mating probability in D. platura.

Applications

The results presented here highlight the need for further investigation into differences in biological and behavioural traits of the two lines of D. platura. In addition, the results emphasise the importance of considering the effects of sex ratio and density on the insect’s mating dynamics when designing lab and field experiments. Such trait differences may influence the methods required for efficient control of either line in crop productions. Considering that mating is optimal in male-biased groups for both lines of D. platura, the development of control methods such as the sterile insect technique or the release of individuals carrying pathogens would need to account for the high degree of sexual selection present in such group contexts (Cator et al. Reference Cator, Wyer and Harrington2020), especially considering the apparent high degree of choosiness of H-line females. As such, special care must be taken to ensure that released males will join and compete within swarms (Hendrichs et al. Reference Hendrichs, Robinson, Cayol and Enkerlin2002; Hassan et al. Reference Hassan, Zain, Basheer, Elhaj and El-Sayed2014) and be attractive to females (Shelly et al. Reference Shelly, Whittier and Kaneshiro1994). To further develop the potential for the sterile insect technique as a control method for the seedcorn maggot, we recommend that future studies investigate what landscape markers and environmental conditions influence swarm formation in each D. platura line, as well as the ability of released sterile males to join and compete within swarms of each target line. The results obtained in the present study also highlight the need for additional research into the mating dynamics of the two D. platura lines, including mate choice, competition for mates, and the effect of conspecifics on egg-laying dynamics.