In females of haplodiploid animals, female production requires fertilisation, whereas male production does not. Therefore, haplodiploid species can display extraordinary sex ratios. Constrained sex allocation occurs when a female cannot produce daughters. This can be due to virginity but may also occur after mating due to insemination failure, selfish genetic elements or physiological constraints. Here, we investigated the mechanism underlying constrained sex allocation in Pezothrips kellyanus. In this species some mated females produce highly female-biased broods, yet, for unknown reasons, others produce extremely male-biased broods. Using crossing experiments controlled for maternally inherited endosymbionts we confirmed that constrained females were successfully inseminated. Furthermore, male-biased offspring production was not paternally inherited, ruling out paternal sex ratio elements previously identified as sex ratio distorters in some parasitoid wasps. Next, we excluded mating time and paternal fitness effects (male size) on sex allocation. However, we found that constrained sex allocation only occurred in small females producing smaller eggs than large females producing larger eggs and female-biased broods. Consequently, the bimodal sex allocation pattern correlates with maternal condition, and may have arisen (adaptively or non-adaptively) in association with an egg size-mediated fertilization mechanism recently detected in some haplodiploids, with egg size positively affecting fertilization success. 

The data set describes the post-mating constrained sex allocation in Kelly’s citrus thrips, Pezothrips kellyanus (Thysanoptera: Thripidae). It demonstrates the results of three separate crossing experiments: experiment 1) analysis of mating effectiveness by dissection of spermathecae of mated females; experiment 2) test for the presence of a paternal sex ratio (PSR) element by assessing the sex ratio patterns in two successive generations, as well as the data for mating time and male size (forewing length) measurements; and experiment 3) analysis of effect of maternal fitness [female body size (forewing length) and size of eggs ovipositioned before mating] on sex ratio after mating. Mother types are categorised according to their mating status and their offspring sex ratio: virgins [V], constrained mated females producing male-biased broods (M-broods) [M(♂)] and unconstrained mated females, producing female biased broods (F-broods) [M(♀)]. The brood types of mated females are categorized as F and M denoting female-biased and male-biased brood types, respectively. According to the data, all P.kellyanus mated females were successfully inseminated irrespective of their brood-types produced (experiment 1 – spreadsheet 1). M-brood production is not paternally inheritable: the prevalence of M-brood production was not different between matings that involved males produced by virgin and unconstrained mothers (experiment 2 – spreadsheets 2 and 3). Moreover, brood-type was not affected by mating duration and male size (experiment 2 – spreadsheets 4 and 5). Finally, the data reveals mother size and egg size (maternal effects) influence brood-type: smaller females (producing smaller eggs in virginity) are more likely to produce M-broods after mating. These results indicate that the constrained sex allocation in P. kellyanusis due to the maternal condition and the resource investment towards egg size.