Pesticide Residues Reshape Predator-Eats-Predator Dynamics on Coconut Palms

On the leaflets of coconut palms, two predatory mites hunt side by side. Amblyseius largoensis and Euseius alatus both prey on the coconut mite Aceria guerreronis, one of the most economically damaging pests of coconut production, and both are considered promising biological control agents. But they are also potential rivals, and under the wrong conditions, rivals become meals. A new laboratory study published in the journal Ecotoxicology shows that when residues of the widely used acaricide abamectin linger on leaf surfaces, the delicate balance between these two predators shifts dramatically, with consequences that ripple outward to the very pest management programs the chemical is meant to support.

The research, led by Bruna R. M. Campelo and colleagues at the Universidade Federal de Pernambuco in Brazil, focused on a phenomenon ecologists call intraguild predation, or IGP. This interaction blends predation with competition: it occurs when one species consumes another that relies on the same food resources. Intraguild predation is widespread across the animal kingdom, spanning multiple trophic levels, and it can shape community structure, species abundance, and even the evolutionary trajectories of the species involved. In agricultural systems, where multiple natural enemies are often released or conserved to suppress a shared pest, IGP can quietly undermine biological control by turning allies into adversaries.

The two mite species at the center of the study occupy distinctly different ecological niches. Euseius alatus belongs to a group of phytoseiid mites classified as Type IV predators, meaning it feeds preferentially on pollen. Amblyseius largoensis, by contrast, is a Type III generalist predator with a stronger dependence on animal prey. These trophic differences matter. When pollen is abundant, a pollen-loving predator has less reason to attack its rival, whereas a prey-dependent predator may continue hunting regardless. The researchers hypothesized that pollen availability would dampen intraguild predation, especially in E. alatus, and that sublethal abamectin exposure would further distort predation and reproduction in ways that could determine which species ultimately prevails.

To test these ideas, the team ran a series of controlled experiments using jack bean leaf discs as experimental arenas. Adult females of each predator species, starved for three hours to standardize hunger, were placed singly into arenas containing five eggs and five larvae of the other species. Some arenas carried residues of abamectin at 3.375 milligrams of active ingredient per liter, a concentration representing 25 percent of the recommended field dose and chosen because preliminary tests showed it allowed partial survival, enabling the assessment of sublethal effects. The researchers note that this level plausibly reflects environmentally relevant residues expected on coconut leaflets after field spraying. Some arenas also received a small quantity of coconut palm pollen, a naturally available alternative food in the field. After 24 hours, the team counted consumed prey and eggs laid.

The results revealed a striking asymmetry between the two predators. For A. largoensis, pollen availability significantly reduced intraguild prey consumption, but pesticide residues had no detectable effect on feeding. For E. alatus, however, both pesticide residues and pollen availability, as well as their interaction, strongly shaped consumption. E. alatus ate the most intraguild prey when neither pesticide nor pollen was present, and the least when both were present together. In other words, the pollen-feeding specialist was far more sensitive to environmental conditions than its generalist counterpart, which continued to hunt at relatively high levels even when pollen was on offer.

Oviposition told a similar story of differential vulnerability. For A. largoensis, egg production declined in the presence of abamectin residues and rose when pollen was available, with no significant interaction between the two factors. For E. alatus, all three effects were significant: oviposition peaked only when pollen was available and pesticide residues were absent, and every other combination produced significantly lower egg output. These findings align with a growing body of evidence that sublethal pesticide exposure impairs foraging efficiency, handling time, and reproduction in phytoseiid mites, but they add a crucial twist by showing that these impairments propagate into interactions between predator species, not merely individual performance.

The population-level experiments drove the point home with almost brutal clarity. When ten adult females of each species were released together into arenas treated with abamectin and supplied with pollen, both populations declined steadily over time, but at different speeds. Euseius alatus went extinct by the fifth day of the experiment, while A. largoensis persisted until the eighth day. Statistical analysis using linear mixed-effects models confirmed a significant treatment-by-time interaction, indicating that the decline trajectories of the two species diverged under pesticide pressure, with E. alatus consistently more sensitive. Even without the pesticide, the outcome was one-sided: A. largoensis maintained consistently higher densities from the fourth day onward, and E. alatus was extinct by day 34.

Why does A. largoensis dominate? The authors point to a combination of intrinsic traits. A. largoensis is generally the larger of the two species, an advantage in aggressive encounters that likely allows it to overpower its smaller rival. Smaller predators often face higher handling costs or simply cannot attack heterospecific adults effectively, which could explain why E. alatus, despite being capable of consuming eggs and larvae of A. largoensis, could not hold its ground. Its heavier reliance on pollen may also leave it at a disadvantage when animal prey and safe alternative foods are scarce or contaminated. Field surveys of coconut acarofauna have consistently reported higher densities of A. largoensis than E. alatus, and the new experiments suggest that intraguild predation, body size, and feeding strategy together help explain that pattern.

The study’s broader message is that pesticides are not just toxins; they are ecological stressors capable of restructuring food webs. Abamectin, which belongs to the avermectin-milbemycin group and acts by activating glutamate-gated chloride channels in nerve cells, is highly effective against eriophyid mites like the coconut mite. But the new work demonstrates that residues too low to kill predatory mites outright can still reshape the direction and strength of their interactions, altering which species coexists and which is excluded. In sprayed coconut groves, the persistence of both predators may be compromised, potentially leading to the loss of one or both natural enemies. The authors also note a mitigating possibility: the spiral architecture of the coconut palm canopy, with leaves and fruit bunches arranged roughly 120 degrees apart and partially overlapping, may hinder uniform spray deposition, creating heterogeneous exposure that could help some mites survive even in treated areas, though this remains to be confirmed in the field.

For pest management, the implications are significant. Pesticide selectivity, the authors argue, should be evaluated not only by whether beneficial arthropods survive exposure but by how exposure alters the ecological interactions among them. Preserving those interactions is essential for the long-term stability of biological control within integrated pest management strategies. From an ecotoxicological standpoint, the findings highlight that conventional risk assessments, which typically focus on individual survival, may miss indirect effects that cascade from behavioral impairments to population-level shifts and, ultimately, to ecosystem services. Incorporating interaction-based endpoints into risk assessment frameworks, the researchers suggest, would offer a more realistic picture of the ecological risks pesticides pose in agroecosystems and better support the conservation of the beneficial arthropods on which sustainable agriculture depends. Further field studies, they caution, are needed to confirm how these laboratory dynamics play out under natural conditions on working coconut farms.

Subject of Research: Effects of sublethal abamectin exposure on intraguild predation and population dynamics between the predatory mites Amblyseius largoensis and Euseius alatus on coconut palms

Article Title: Sublethal abamectin exposure alters intraguild predation and population structure of predatory mites

Article References: Campelo, B. R. M., Monteiro, A. J. D., da S. Melo, J. W., & Lima, E. D. B. (2026). Sublethal abamectin exposure alters intraguild predation and population structure of predatory mites. Ecotoxicology, 35(8), Article 172. https://doi.org/10.1007/s10646-026-03159-9

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03159-9

Keywords: abamectin, intraguild predation, predatory mites, Phytoseiidae, ecotoxicology, biological control, coconut palm, Aceria guerreronis, sublethal effects, pollen, integrated pest management, population dynamics