Can insecticide applications used to kill vector insects prevent pine wilt disease?

Area of Science:

• Forest Entomology
• Plant Pathology
• Insecticide Efficacy

Background:

• Pine wilt disease (PWD) is a significant threat to pine forests, primarily spread by Monochamus beetles carrying pine wood nematodes (PWNs).
• Aerial insecticide application is a key strategy to control Monochamus beetle populations and curb PWD transmission.
• Thiacloprid is an insecticide used to manage vector populations, but its systemic residues and long-term efficacy require investigation.

Purpose of the Study:

• To evaluate the mortality of Monochamus beetles after aerial thiacloprid application in Pinus densiflora.
• To assess the rate of PWN transmission in relation to systemic thiacloprid residues in pine trees.
• To determine the impact of thiacloprid treatment timing on vector mortality and tree survival.

Main Methods:

• Thiacloprid was sprayed on 5-year-old Pinus densiflora in controlled nursery conditions.
• Monochamus alternatus adults, known vectors of PWNs, were introduced at 1 and 15 days post-treatment (T1 and T15 groups).
• Insect mortality, PWN presence in beetles, thiacloprid residues in branches, and tree mortality were monitored.

Main Results:

• Higher thiacloprid residues were detected in trees treated 1 day prior to beetle introduction (T1) compared to 15 days prior (T15).
• Most Monochamus alternatus adults died in both T1 and T15 groups, with PWNs detected in 51.3% of recaptured beetles.
• Tree mortality was 0% in T1 and 16.7% in T15, significantly lower than the 50-60% mortality observed in non-sprayed control groups.

Conclusions:

• Aerial thiacloprid application effectively reduces Monochamus beetle populations.
• Despite beetle mortality, PWN transmission remains a concern, particularly when beetles encounter lower systemic thiacloprid residues.
• Current insecticide strategies may be limited in fully preventing PWN transmission due to residual insecticide levels.