Abstract:

Objective: Paracarophenax alternatus is a newly identified parasitic mite exhibiting parasitic efficacy against Monochamus alternatus eggs. This study aims to investigate the effects of environmental factors on the parasitism efficiency of P. alternatus, to evaluate its potential in biological control programs, to optimize artificial rearing systems, and to refine field application strategies. Method: An indoor feeding experiment under controlled laboratory conditions was conducted to assess the impacts of M. alternatus egg age (0–4 days post-oviposition), mite-to-egg ratios (1∶1–8∶1), temperature (17–33 ℃), relative humidity (RH: 60%–95%), and photoperiod (24L, 12L∶12D, 24D) on the parasitism rates of P. alternatus. A predictive model was developed using random forest regression analysis to quantify parasitism rates under varying environmental conditions. Result: 1） Under the conditions (mite-to-egg ratio 5∶1, 25°C, 85% RH, 12L∶12D), the parasitism rate of P. alternatus on newly laid eggs of M. alternatus was the highest, reaching 74.44%, significantly exceeded the parasitism rate of 44.44% on 4-day-old eggs of M. alternatus P<0.05), demonstrating that as the age of M. alternatus eggs increases, the parasitism rate significantly decreases. 2） Under the conditions (5∶1 mite-to-egg ratio, 25 ℃, and 12L∶12D), the parasitism rates of P. alternatus reached 93.33% and 88.00% at 95% and 90% RH, respectively, and were significantly higher than that of 30.00% at 60% RH (P<0.05). 3） Under the conditions (4∶1 mite-to-egg ratio and 85% RH), the parasitism rates of P. alternatus on the eggs of M. alternatu were 75%, 78.8%, and 83.3% at temperatures of 21 ℃, 25 ℃, and 29 ℃, respectively, and significantly (P<0.05) higher than those of 10% and 21.7% at 17 ℃ and 33 ℃. 4） Under the conditions (29°C, 85% RH, 12L∶12D, mite-to-egg ratios of 4∶1–8∶1), the parasitism rates of P. alternatus were 88.3%, 83.30%, and 86.7% at the mite egg ratios of 4:1, 6:1, and 8:1, respectively, and significantly higher than those of 41.70% and 36.67% at the mite egg ratios of 1∶1 and 2∶1 (P<0.05). 5） Under the conditions (mite egg ratio of 5:1, 25 ℃, and RH 85%, 24 D, 12 L: 12 D, and 24 L), the parasitism rates of P. alternatus at continuous darkness (24D) achieved the highest parasitism rate (92.50%), outperforming 12L∶12D (73.75%) and 24L (68.75%, P<0.05). 6） The random forest regression model exhibited robust accuracy (MSE = 100.68, R² = 0.83), and predicted the maximum parasitism rates (>90%) under conditions of freshly laid eggs, 5∶1 mite-to-egg ratio, 20–31 ℃, RH >93%, and 24D photoperiod. Conclusion: Egg age, temperature, humidity, photoperiod, and mite-to-egg ratio significantly influence the parasitism efficiency of P. alternatus on the eggs of M. alternatu. For optimal biological control or mass rearing, conditions should include mite-to-egg ratios ≥5∶1, temperatures of 21–29 ℃, RH ≥90%, and complete darkness. These findings provide critical insights for optimizing mass-rearing protocols and field deployment strategies to enhance the efficacy of P. alternatus in suppressing M. alternatus populations.

Key words: Paracarophenax alternatus, Monochamus alternatus, Bursaphelenchus xylophilus, parasitism rate, biological control