Beetle pests of honeylocust
Order: Coleoptera
Family: Meloidae, Buprestidae
Description and life history
Several beetle species can attack honeylocust (Gleditsia triacanthos) in Colorado. Two species of particular importance are the ashgray blister beetle (Epicauta fabricii) and Agrilus difficilis, commonly referred to as the “honeylocust borer”. While heavy infestations can defoliate trees or reduce tree growth rate and vigor, feeding injuries are usually cosmetic and do not affect the health of the tree.
Ashgray blister beetle
Infestations of the ashgray blister beetle (Meloidae) can end abruptly and are most likely to occur on honeylocust trees in late June or early July. Adults of this species are oblong beetles with large heads, long constricted necks, and gray bodies that measure about 12.7 mm (1/2 inch) in length. Blister beetles lay masses of eggs in soil near the eggs of certain grasshopper species. Upon hatching, beetle larvae begin feeding within the nearby grasshopper egg pod. Mature larvae overwinter within the grasshopper egg pod until the following spring, at which point they exit the egg pod and pupate in soil. For this reason, infestations tend to be associated with grasshopper outbreaks. These beetles are active and tend to congregate in swarms when feeding. These beetles will readily drop off plants to escape when disturbed. One generation of ashgray blister beetle is likely produced each year. In addition to attacking honeylocust, infestations can occur in alfalfa, beans, peas, sweet clover, and sometimes potato.
Agrilus difficilis
Agrilus difficilis is a buprestid beetle in Colorado that commonly attacks honeylocust trees stressed from drought, root pruning, or disease. Borer activity tends to be most concentrated near existing canker wounds. Adults are about 7-13 mm (~1/4-1/2 inch) long with short antennae, large prominent eyes, and yellow spots on the upper abdomen beneath the elytra (wing covers). The larvae have an enlarged head region and elongated, flat, white bodies. This species overwinters as larvae under the bark of a host tree, and adults begin emerging the following spring in late-May. After feeding for a couple of weeks, females lay eggs in bark crevices. One generation is produced each year. Honeylocust is the main host of A. difficilis.
Quick Facts
- Two species of beetle, the ashgray blister beetle and Agrilus difficilis, are considered pests of honeylocust trees in Colorado. While they do not typically cause serious injury, heavy infestations can contribute to the declining health of trees.
- The ashgray blister beetle is relatively large and adults can aggregate on host plants in large numbers. Infestations of Agrilus difficilis tend to occur on trees that are in declining health and are less noticeable since larvae feed and develop under the bark.
- While the exact type of feeding injury depends on the specific pest, some common feeding injuries include defoliation, notches in leaf margins, and the presence of oozing wounds in tree bark.
- Insecticides can be applied to foliage to help control heavy infestations of the ashgray blister beetle. Systemic insecticides can help protect susceptible trees from A. difficilis.
Ashgray blister beetle. Note the constricted region behind the head and gray body. Image credit: Ward Upham, Kansas State University, Bugwood.org
Adult of A. difficilis. Note the metallic green coloration and yellow spots (arrow) on the upper abdomen. Image credit: Hanna Royals, Screening Aids, USDA APHIS PPQ, Bugwood.org
Injury
High densities of ashgray blister beetles can severely defoliate trees. All blister beetles secrete a compound called cantharidin, which is toxic to mammals if ingested and can cause blistering after contact with human skin.
Injuries produced by A. difficilis can appear as wet areas on the bark surface with large quantities of oozing sap around holes. Exposure to air causes the sap to harden. Larvae excavate serpentine galleries in the wood, which can disrupt nutrient exchange of the host tree, leading to progressive canopy thinning. Adults leave small D-shaped exit holes in bark after emerging and produce notches in the leaf margins when feeding.
Monitoring
Ashgray blister beetles are large and often aggregate in high densities on the host plant, making them easy to spot. Agrilus difficilis activity produces wounds and oozing secretions in bark. Removing portions of bark will reveal frass-filled galleries in infested trees. The presence of D-shaped exit holes in honeylocust indicate that the tree has been attacked by A. difficilis.
Management
Taking steps to control the resident grasshopper population will help reduce the density of ashgray blister beetles in the area. Chemical management of the ashgray blister beetle can be accomplished through spot treatments with any foliar insecticide commonly used for insect control on trees and shrubs. Insecticides should be applied before serious injuries are inflicted on the honeylocust tree.
Since A. difficilis attacks stressed or injured trees, maintaining tree health can be effective in preventing infestations. Examples include proper trimming, drainage, and the avoidance of mechanical or environmental injury. Promptly removing dead limbs, trees, or the bark from felled trees is also recommended. Supplemental insecticides can be applied to the tree trunk during egg-laying and egg-hatch periods in early summer. Soil drenches with systemic insecticides applied in the spring can help protect trees that are not already severely injured by A. difficilis.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
ICA. (n.d.). Blister Beetles. Illinois College of Aces – Extension & Outreach. Available http://extension.cropsciences.illinois.edu/fieldcrops/insects/blister_beetles/#:~:text=The%20ash%2Dgray%20blister%20beetle,edge%20of%20each%20wing%20cover.
Kerzicnik, L. 2021. Blister Beetles in the Yard and Garden. University of Montana – Extension. Available https://www.montana.edu/extension/ipm/documents/entomology/ent-fact-sheets-2020-2021/Blister%20beetles_2021_urban%20alert_ADA%20compliant.pdf
Zeleznik, J. et al. 2005. Insect and Disease Management Guide for Woody Plants in North Dakota. North Dakota State University – Extension. Available https://library.ndsu.edu/ir/bitstream/handle/10365/5261/f1192.pdf?sequence=1