Figure 1. An adult round-headed apple tree borer. Copyright 2005: Tom Murray (Bugguide.net)
Dear readers, I hope your spring and summer seasons are off to a great start! I haven’t heard much in the last couple of weeks about common fruit insect pests, so I thought I would highlight an interesting, but probably uncommon insect issue for most readers of Facts for Fancy Fruit: the round-headed apple tree borer!
The round-headed apple tree borer is a ¾-inch long-horned beetle (Figure 1) that attacks mainly apple trees, but other wild and cultivated fruit trees can also host this beetle. The adults are active from June to September outside the tree, while the larvae (called grubs) and pupae (pre-adult stage) remain protected inside the tree. This long-horned beetle isn’t a frequent pest of active, managed orchards; rather it’s seen most often in backyard trees or old, neglected orchard trees. The adult beetle is quite eye-catching, you have to admit! I had never heard of this insect, or seen it myself until an email request came through with pictures of injury to a backyard apple tree, along with pictures of the pupa.
Figure 2. Larva (grub) of the round-headed apple tree borer. Photo credit: https://bugwoodcloud.org/images/768×512/3066002.jpg
https://bugwoodcloud.org/images/768×512/3066002.jpg
As with many beetles, especially the borers, it is the larval stage that is most damaging to the tree (Figure 2). The young larvae tunnel and feed beneath the tree bark during the first year, remaining inside the trunk to overwinter. The following year, the larvae bore into the wood and continue to develop and feed for a second (and sometimes third or fourth!) summer before emerging as adults. You can imagine the injury the tree sustains while the larvae spend all that time inside the trunk, feeding and continuing to develop.
Infested trees are compromised in their ability to transport water and nutrients, so as time passes, trees grow more slowly, may have little foliage, and young trees may break off in the wind. Signs of boring by the larvae include reddish-brown, sawdust-like frass (insect solid waste) that is pushed out of the tunnels onto the surface of the trunk, dark-colored bark that may be sunken, and even oozing sap.
Thankfully, this beetle can be managed by 1) removing wild host trees in the vicinity, including mountain ash, crabapple, hawthorn, and cotoneaster (especially if you suspect any are infested), 2) removing weedy vegetation or guards from the base of trees (these can make the habitat more attractive for the beetle), 3) applying insecticides or a non-chemical whitewash (50:50 mixture of white latex (not acrylic) paint and water) to the lower trunk to kill adults and deter egg-laying, respectively, and 4) using wire mosquito screen or ¼-inch mesh hardware cloth around the lower 2 feet of the trunk to exclude female beetles as they try to lay eggs. If you are really hard core, you can pull back the bark and physically remove shallow larvae from decaying tissue with a knife, or use a stiff wire inserted into suspected tunnels to pierce the larvae, which are soft bodied. Eep, this feels like an early Halloween article suddenly, sorry!
If you are curious about insects you see in your orchard or backyard, please reach out to me at long132@purdue.edu! I’m happy to write a newsletter about insects and insect topics you’re interested in learning more about.