The potential of mass rearing of Monoksa dorsiplana (Pteromalidae) a native gregarious ectoparasitoid of Pseudopachymeria spinipes (Bruchidae)in South America

In Chile and Uruguay,the gregarious Pteromalidae (Monoksa dorsiplana) has been discovered emerging from seeds of the persistent pods of Acacia caven attacked by the univoltin bruchid Pseudopachymeria spinipes. We investigated the potential for mass rearing of this gregarious ectoparasitoid on an alternative bruchid host, Callosobruchus maculatus, to use it against the bruchidae of native and cultured species of Leguminosea seeds in South America. The mass rearing of M.dorsiplana was carried out in a population cage where the density of egg-laying females per infested seed was increased from 1:1 on the first day to 5:1 on the last (fifth) day. Under these experimental conditions egg-clutch size per host increased, and at the same time the mortality of eggs laid also increased. The density of egg-laying females influenced the sex ratio which tended towards a balance of sons and daughters,in contrast to the sex ratio of a single egg-laying female per host (1 son to 7 daughters). The mean weight of adults emerging from a parasitized host was negatively correlated with the egg-clutch size, i.e., as egg-clutch size increased, adult weight decreased. All these results show that mass rearing of the gregarious ectoparasitoid M.dorsiplana was possible under laboratory conditions on an alternative bruchid host C.maculatus. As M.dorsiplana is a natural enemy of larval and pupal stages of bruchidae, the next step was to investigate whether the biological control of bruchid C.maculatus was possible in an experimental structure of stored beans.

💡 Research Summary

The study investigates the feasibility of mass‑rearing the native South‑American gregarious ectoparasitoid Monoksa dorsiplana as a biological control agent against bruchid pests that damage legume seeds. In its natural range (Chile and Uruguay) M. dorsiplana parasitizes the larvae and pupae of Pseudopachymeria spinipes inside persistent Acacia caven pods. Because the natural host is difficult to maintain in large numbers, the authors evaluated an alternative host, the widely distributed stored‑bean pest Callosobruchus maculatus.

A five‑day rearing experiment was conducted in a population cage. The density of egg‑laying females per infested seed was gradually increased: on day 1 a 1:1 ratio (one female per host) was used, and by day 5 the ratio reached 5:1 (five females per host). For each day the researchers recorded (i) the size of the egg clutch deposited on each host, (ii) the proportion of eggs that failed to develop, (iii) the sex ratio of emerging adults, and (iv) the average adult weight.

Key findings:

1. Clutch size increased markedly with female density. More females laying on the same host resulted in larger egg deposits, indicating that M. dorsiplana exploits the opportunity to maximize offspring numbers when hosts are abundant.

2. Egg mortality rose in parallel with clutch size. The higher number of eggs per host exceeded the nutritional and spatial capacity of the host, leading to competition among developing larvae and a higher proportion of dead eggs.

3. Sex ratio was strongly density‑dependent. With a single female per host the sex ratio was highly female‑biased (approximately 1 male to 7 females). When five females were present, the ratio approached parity (≈1:1). This suggests that M. dorsiplana adjusts the allocation of male versus female offspring in response to intra‑host competition, a strategy that can balance the reproductive output of the population under crowded conditions.

4. Adult weight showed a negative correlation with clutch size. Larger clutches produced lighter adults, reflecting reduced per‑larva resource availability. Since adult weight is often linked to fecundity, longevity, and overall fitness, this trade‑off highlights the importance of optimizing female density for quality as well as quantity of parasitoids.

Overall, the experiment demonstrated that M. dorsiplana can be successfully reared on C. maculatus under laboratory conditions, and that manipulation of female density allows control over critical biological parameters. The authors propose that, because M. dorsiplana attacks both larval and pupal stages of bruchids, the next logical step is to test its efficacy in a simulated storage environment containing beans infested with C. maculatus.

The paper concludes that mass‑rearing of this gregarious ectoparasitoid is feasible, and that the species holds promise as a native, environmentally friendly biocontrol agent for stored‑bean pests in South America. Further work should focus on large‑scale field trials, assessment of release rates, timing of applications, and integration with existing pest‑management programs to determine the practical utility and economic viability of M. dorsiplana in commercial storage facilities.