The Japanese beetle is a highly destructive invasive pest that is an annual problem for many Nebraskans. Its presence has grown over the last decade and is getting worse. Adults of this metallic beetle feed on over 300 different species of plants and larvae are often the bane of turfgrass. While treatment options do exist, they are often limited by several factors. However, different cultural practices can mitigate much of the damage done by these invaders.
How beetles affect trees
The attack on these trees usually takes place in the mid-summer with telltale “skeletonizing” of the leaves, where only the veins are left intact. Adult Japanese beetles can defoliate a tree quickly as they are attracted to feeding sites by both the smell of the plant and sex pheromones sent out by other beetles. While this may leave a tree looking brown and in danger, it will usually recover unless hit year after year. Additionally, beetle populations tend to fluctuate over time as the invasion front progresses.
Although treatment options are available for some trees there are a few important considerations to take into account.
- Most of the systemic insecticides used are extremely toxic to pollinators and can make their way into the tree’s pollen. For this reason, it is against the law to apply neonicotinoid class insecticides to linden trees.
- While trees sprays can work, they are most often applied only after the beetles have been attacking a tree and thus may not save the tree from being partially defoliated. It is also important to note that while individual trees may be treated (or lawns for grubs), the beetles are strong fliers and can come in from some distance away.
- Pheromone traps sold at many stores are also discouraged since research has shown they attract more beetles than they can actually kill, even if modified with larger containers.
In many cases, the best way to deal with Japanese beetles is to simply try to keep the tree as healthy as possible otherwise. Beetle populations will likely fluctuate after a few years and they are unlikely to hit the tree so often that they actually kill it. By the time the beetles get to the tree, it has already had about half of its growing season to store up energy, so it isn’t at a total loss. Diversifying future plantings will also help ensure fewer trees in the landscape are susceptible to the beetle as well. Remember, keeping a tree healthy through proper planting, watering, mulching, and trimming is the first line of defense against many pest insects and diseases.