A new study found that brood parasitism exists between two predatory mite species that have a common prey and egg predator, and sometimes prey on each other’s younger stages. Gynaeseius liturivorus is a mite species that guards its eggs, whereas Neoseiulus californicus uses this protection to safeguard its own eggs, thereby engaging in brood parasitism. However, it was observed that this brood parasitism was subject to the presence of an egg predator—western flower thrips (Frankliniella occidentalis). In the absence of this egg predator, the parasite mite did not prefer to add its eggs to those of the host mites. This suggests that brood parasitism is dependent on the threat to the parasite offspring.
Newswise — Numerous animal types exhibit offspring nurturing, a phenomenon where attentive parents offer nourishment and defense against predators to their young. Nonetheless, many others opt against this practice and instead surreptitiously deposit their eggs alongside those of caring species to exploit free offspring care. This conduct, known as offspring parasitism, can be observed in fish, frogs, and birds, with the most renowned instance being the cuckoo bird. Currently, such offspring parasitism has also been documented in the insect realm, specifically in minuscule creatures referred to as mites. Generally, offspring parasitism burdens the host while benefiting the parasite. However, under specific circumstances, it can become costly for the parasite, resulting in voluntary or "optional" offspring parasitism.
Going into greater detail, Associate Prof. Yasuyuki Choh of Chiba University, Japan, and Arne Janssen from the University of Amsterdam and the Federal University of Viçosa provide further insight into the factors influencing facultative brood parasitism. "In our research, we examined whether the occurrence of brood parasitism in predatory mites is influenced by the potential risk of egg predation. We focused on two predatory mite species that share the same food source and are exposed to an egg predator," explains Dr. Choh. Their findings were published online in Functional Ecology on April 13, 2023.
For their investigation, the researchers specifically selected Neoseiulus californicus (N. californicus) and Gynaeseius liturivorus (G. liturivorus), two species of predatory mites that feed on western flower thrips (Frankliniella occidentalis). These thrips are significant agricultural pests, primarily consuming plant tissue and pollen. However, occasionally, they also pose a threat to the eggs of these predatory mites, acting as egg predators. Adding to the complexity, the adult stages of these two mite species also prey on the younger stages of each other. While G. liturivorus exhibits protective behavior towards its eggs by attacking and feeding on egg predators, N. californicus does not engage in such brood care activities.
Therefore, the researchers aimed to comprehend the dynamics of brood parasitism between N. californicus (parasite) and G. liturivorus (host) by investigating its impact on the host, parasite, and their offspring. To achieve this, they conducted observations on the interactions between the two mite species under two conditions: 1) in the presence of thrips (an egg predator) and 2) in the presence of two-spotted spider mites (a different source of prey but not an egg predator). By examining these scenarios, they sought to uncover how the presence or absence of these factors influenced the behaviors and outcomes of the host and parasite mites, as well as their respective offspring.
The study revealed that the adult female parasite displayed a strong inclination to deposit its eggs near the host eggs, even in the absence of adult female hosts. However, this preference was observed only in the presence of thrips, highlighting that brood parasitism might be contingent upon the risk of egg predation. Interestingly, when spider mites were introduced as an alternative prey source that does not attack the eggs of the predatory mites, the parasite did not exhibit a significant preference for placing its eggs with the host eggs. This suggests that brood parasitism is dependent on the presence of egg predators and is not a generalized behavior.
Furthermore, the researchers noted that brood parasitism did not occur when the host eggs were replaced with eggs of another predatory mite species, Phytoseiulus persimilis, which does not guard its eggs. This finding suggests that the parasite specifically chooses to add its eggs to the guarded host eggs when there is a risk of egg predation, thereby increasing the survival chances of its offspring. However, this behavior comes at a significant cost to the host, as it faces a higher risk of its eggs being preyed upon by the adult female parasite.
Overall, these findings shed light on the conditional nature of brood parasitism in relation to the presence of egg predators and highlight the trade-off between the survival benefits for the parasite offspring and the increased risk faced by the host's eggs.
The researchers additionally observed that the absence of egg predators results in a reversal of the risk of egg predation. Once the thrips were eliminated from the area by the scientists, the adult female host lost their ability to feed on thrips and transitioned to attacking and consuming the offspring of the parasite. This shift in behavior rendered brood parasitism disadvantageous for the parasite.
In elucidating the ramifications of this study, Dr. Choh states, "The optional aspect of brood parasitism not only minimizes expenses for the parasite when there are no egg predators but also hampers the development of host responses to evade brood parasitism, thereby instigating an evolutionary contest between the parasite and the host."
These discoveries underscore the importance of the peril connected to egg predation as a prominent catalyst for facultative brood parasitism and contribute to a enhanced comprehension of the progression of this risk-dependent form of brood parasitism.
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