 Scott Ludwig |
The term “borers” is a very generic term when used in regards to tree pests. There are a number of different pests capable of boring into a tree. These include longhorned beetles, flatheaded beetles, ambrosia beetles and even caterpillars. Not all of these pests can be controlled using the same management techniques. You need to know which borer group your trees are susceptible to and develop your management plans accordingly. Most insect borers are attracted to weakened, damaged, dying or dead plants. These are referred to as “secondary invaders” because they attack only after a plant has been weakened by another stress. Secondary invaders are a good indicator of other problems with the health of the tree, and their presence may contribute to the tree’s decline. Damage symptoms vary by borer group. By knowing the damage signs of each group, it is often possible to detect infestations as they occur. Unfortunately, with groups such as ambrosia beetles and longhorned beetles, it may be too late to kill the larvae after the beetle has entered the tree. Due to the unique nature of ambrosia beetles, I will not be covering them in this month’s column. Longhorned beetles or round-headed borers Adults are called longhorned beetles because their antennae are as long as or longer than their bodies. They lay their eggs on bark, and newly hatched larvae tunnel underneath bark and into the heartwood. The tunnels are oval to almost round in cross sections. While tunneling, larvae may pack their tunnels with excrement (frass) that looks like compressed wood fibers, or push frass out of the holes they produce. You can often detect an infestation by the frass that may be visible on the bark. This group is rarely a problem in nursery production. Since the larvae burrow into the heartwood of the tree, the larvae can’t be managed once they infest the tree. Preventive trunk sprays can be used to protect the trees from infestation. Metallic wood-boring beetles or flatheaded borers Adult beetles are flat and boat-shaped, and they have large eyes, short antennae and distinctive metallic colors. They can be green, blue, bronze, black and copper. Their larvae have widened, flattened body segments just behind their head. As the larvae tunnel beneath bark, they produce oval or flattened tunnels in cross sections. Metallic wood-boring beetle feeding usually girdles the trunk and branches. If the tree is small or if enough beetles are present, this girdling can result in disfigured trees or even death. Preventive trunk sprays are often effective. Systemic insecticides have also been used to prevent larvae development and can be used to kill larvae once they start developing. Neonicotinoids can effectively manage flatheaded borers. However, label rates are often appropriate for landscape trees and not nursery production. If you decide to treat trees at your nursery, check your application rate and volume to make sure you do not exceed the maximum application rates per acre. Clearwing moths There are a number of clearwing moths that can be pests of nursery crops. The larvae of the moths tunnel into the trees in the same manner as beetle borers. Depending on the species, different parts of the tree can be attacked. The caterpillars can be identified by their “false legs” on the undersides of some of the abdominal segments. These are not found on beetle larvae. Peach tree borer is probably one of the most important pests in this group.Larvae tunnel into the roots and lower trunks of peach, cherry, plum, nectarine and apricot. They feed on growing tissue and inner bark. Infested trees can be identified by masses of sap around damage sites at the base of the trunk. Other clearwing borers include lilac or ash borer, dogwood borer and lesser peach tree borer. Managing wood-boring insects Most wood-boring insects are secondary invaders. If your trees are being attacked, they are probably under stress. If you have more than the random plant being attacked, then you probably need to assess your production practices. Proper tree care discourages most borer pests and will help the plants survive an attack. Sap flow from healthy trees is their primary defense from damage by many borer pests. There are some practices that you want to avoid to prevent borer attack. Delayed potting of trees to a large container size can stress trees. But with the current financial situation, many growers are holding on to plants longer than they should before shifting them up. If you are trellising your trees, make sure the support wire is not damaging the tree. I have observed a strong relationship between trees damaged by trellises and borer attack. Under- and over-watering and too much or too little fertilization can also stress the trees. If you use slow-release fertilizer that releases based on temperature, try to avoid putting it out in the heat of the summer when you could get a large release of nitrogen into the pot. My recommendation is to top dress in the spring or after the heat of summer. Non-systemic trunk sprays There are a limited number of insecticides registered for borer management. Most products are applied as sprays to the trunks and branches and are non-systemic (e.g., bifenthrin, chlorpyrifos and permethrin). Trunk sprays will not kill larvae that have already penetrated beneath the bark. They target adult and larval stages tunneling through the treated bark. Effective treatment for borers requires that all surfaces be covered with the insecticide. There are a few cases where the treatment can be targeted to a specific location on the tree, such as the lower trunk for peach tree borers. If you are using an airblast sprayer, make sure the insecticide spray is getting to all sides of the tree. A spray that only covers half the trunk will not be 100- percent effective, as it is leaving part of the tree exposed. Systemic insecticides There are only a few systemic insecticides currently registered for borer control (e.g., acetamiprid, dinotefuran and imidacloprid). These products can be used for preventive and curative control of flatheaded borers. Due to the cost and label restriction associated with trying to treat all the trees on your nursery, you may want to consider using these insecticides on infested trees when you detect damage. Monitoring If you are using preventive trunk sprays, is it critical to time your treatments to coincide with the time adult beetles are laying eggs. The life cycles of some insect borers may be known in your area, but this is often not the case. If you are battling a species that occurs over a long time period or a number of different species, multiple insecticide applications may be needed for effective control. Depending on the type of beetle you are trying to monitor, you might need different traps. There are pheromone traps available for many of the clearwing borers. A few years ago Jason Oliver at Tennessee State University started working on developing a better trap for flatheaded borers. Shortly after he started his work, emerald ash borer entered the picture, and the importance of an effective monitoring tool for flatheaded borers increased. Oliver discovered this beetle group is attracted to the purple. Purple panel traps are now available to monitor for adult flatheaded borers. |
Explore the March 2010 Issue
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