The diets supplied for C. lactis significantly affected the final population densities of A. swirskii [likelihood ratio (LR) χ22 = 130.51, P < 0.0001] and C. lactis (LR χ22 = 65.37, P < 0.0001) (Fig. 2). Final densities of A. swirskii were significantly higher when prey was supplied by the SY and Y diets, and prey density was significantly higher on the SY diet than on the S and Y diets alone.
In contrast, the initial predator:prey ratio did not affect the final population densities of A. swirskii (LR χ21 = 0.34, P = 0.56) or C. lactis (LR χ21 = 0.02, P = 0.90). There was also no significant interaction effect of diet and initial predator:prey ratio on the final population densities of A. swirskii (LR χ22 = 0.22, P = 0.89) or C. lactis (LR χ22 = 1.09, P = 0.58). The final predator:prey ratio varied among the different diets (LR χ22 = 6030.68, P = 0.0001) but was highest on the Y diet (Fig. 3). The final predator:prey ratio increased with the initial ratio (LR χ21 = 6.06, P = 0.01). There was a significant interaction between diet and initial predator:prey ratio (LR χ22 = 131.38, P = 0.0001).
This experiment showed that higher numbers of A. swirskii were produced when prey was provided by the SY and Y diets. However, the highest prey densities were observed on the SY diet, so the final predator:prey ratio was highest on the Y diet. Therefore, the Y diet was the most suitable medium for high rates of A. swirskii population growth. Because moisture increased in the SY diet after 2 weeks, this may have disturbed the foraging and reproduction of A. swirskii (even though C. lactis was still able to reproduce in these conditions). High prey densities might interfere with A. swirskii on the SY diet (Chant 1961); additionally, a saturating type II functional response as well as mutual interference by A. swirskii itself (Fathipour et al. 2020) may limit oviposition rates and fecundity (Nakamichi et al. 2020). In this study, A. swirskii populations (both males and females) increased 40.5 times after 30 days without additional prey at an initial predator:prey ratio of 1:20, on the baker’s yeast diet. In a previous study, with the same initial number of females and predator:prey ratio, the female-only population increased 21.5 times after 24 + 4 days when provided with an additional supply of prey (Ji et al. 2016). While it is difficult to compare these 2 studies because of differences in methodology and unreported sex ratios, this combination of dietary resources and prey appears to support sufficiently high rates of reproduction and survival for the predatory mite A. swirskii.
C. lactis exhibited a higher population growth on the S diet than on a diet of white sugar because of the higher water and nitrogen contents of the S diet (Iimuro 1956). The relatively low densities of both mites on the S diet during this study may be due to the nutrient content of the S diet, which was too poor for C. lactis (so the predator could not find enough prey), or because the S medium was too sticky for both mites to successfully forage and reproduce. While C. lactis can be reared on the Y diet alone (baker’s yeast, Chmielewski 1971), an SY mixture was optimal (4:6 ratio of white sugar and brewer’s yeast, Matsumoto 1964 and Okamoto 1984). Baker’s yeast is available in several different forms; coarse, oblong granules of dry yeast was used in this study. This form served as a porous medium and allowed free movement of both mites and provided sufficient nutrition for C. lactis. Therefore, the Y diet (baker’s yeast) was a suitable diet for C. lactis when rearing A. swirskii.
Rearing programs that involve factitious hosts can reduce the complexity and cost of producing biological control agents. Even though the predatory mite A. swirskii can reproduce on pollen, the collection of pollen is a laborious process and A. swirskii reared on cattail pollen do not reproduce as well as those provided with C. lactis (Nguyen et al. 2013). Moreover, when the rearing hosts themselves are crop pests, there can be a risk of spreading them along with the control agent when they are used in the mass culture of natural enemies. For instance, this became a problem in Spain when rearing Eretmocerus mundus (Mercet) on the Q strain of the whitefly Bemisia tabaci (Gennadius) (Van Driesche et al. 2008). However, the dried fruit mite C. lactis, which was used as an alternative prey in this study, did not damage crops.