Stage-specific intraguild interactions between the predatory stink bug Arma custos and ectoparasitic mite Pyemotes zhonghuajia

The predatory sting bug Arma custos and ectoparasitic mite Pyemotes zhonghuajia are important natural enemies which may co-occur in agroecosystems in China. Understanding interspecific interactions is essential for optimizing multi-agent biological control. This study presents the first systematic assessment of the compatibility and intraguild effects between these two species. Laboratory bioassays evaluated the impact of P. zhonghuajia density [0 (control), 1, 5, 10, 15, or 20 mites] on A. custos egg hatch rate and incubation period, and effect of mite density [0 (control), 1, 5, 10, 20, or 30 mites] on the paralysis and lethalityl of A. custos nymphs and adults. Results show that P. zhonghuajia did not significantly affect egg viability (82–86% hatch rate) and development (8.4–8.8 d), indicating baseline egg-stage compatibility. In contrast, nymphs and adults experienced strong mite density- and time-dependent paralysis and mortality. The young 1st–2nd-instar nymphs of A. custos were highly susceptible and completely paralyzed or died when they were exposed to 5–10 mites for 24 h; whereas older instars and adults exhibited higher tolerance, where high mortality occurred at higher mite densities of ≥ 20 individuals. This is the first study to estimate the critical density-dependent lethality: 5 mites for 1st–3rd instars, 10 mites for 4th–5th instars, and > 10 mites for adults. These findings reveal a parasitism-mediated intraguild interaction, in which P. zhonghuajia dominated nymphal and adult stages of A. custos, while eggs remained largely unaffected. The results provide essential guidance for the future design of coordinated release strategies under field conditions, as they emphasize that temporal or spatial niche separation would be necessary to reduce direct suppression of A. custos and maintain the complementary pest control potential of both agents in applied settings. This study advances understanding of intraguild dynamics among natural enemies, offering practical and theoretical insights for multi-agent biodiversity-based pest management that require further field validation.