Timing trunk sprays for the borer complex in cherries
Trunk sprays in early June will be well-timed for American plum and lesser peachtree borers.
The complex of borer pests in cherries is comprised of three species: American plum borers, lesser peachtree borers and greater peachtree borers. All three species are moths in their adult stage, and the larvae are borers that feed under tree bark. These larvae, or borers, feed on cambium tissue, which can girdle the tree and cause tree decline or death. While these borers also attack other tree fruits including apples (American plum borers), peaches, pears, plums and nectarines, the mechanization trunk-shaking of cherries is linked with increased American plum borer and lesser peachtree borer infestation in cherry orchards.
Mechanical harvesting can cause damage to tree trunks, and over multiple seasons the cambium tissue that is exposed when a tree is damaged is a more attractive egglaying site for American plum borers and lesser peachtree borers. Lesser peachtree borers are also capable of causing tree mortality if populations reach damaging levels, but often American plum borers cause more damage than either lesser peachtree borers or greater peachtree borers, as there are usually more American plum borer larvae per wound and American plum borers feed horizontally, which girdles the trees more effectively than feeding horizontally and vertically. Greater peachtree borers are unique, as females do not need a damaged site to lay eggs, and these larvae feed on the cambium at or up to 6 inches below the soil line, which renders peachtree borer damage difficult to diagnose if it is underground.
American plum borers are the first of these borer species to begin flying in the spring around the popcorn stage in tart cherries, followed by lesser peachtree borers near petal fall and greater peachtree borers after shuck-split. After emergence, adults mate and females find sites to lay eggs such as in the crevices of injured bark or, in the case of greater peachtree borers, most eggs are laid at the base of trees. Females lay eggs singly or in clusters and larvae of each species can be found in groups on a tree.
While American plum borers larvae primarily feed on cambium tissue up to 2-3 feet on the trunk from the soil, lesser peachtree borers larvae can be found feeding on cambium tissue on any part of a tree that has been injured. Unlike American plum borers and lesser peachtree borers, greater peachtree borer larvae will bore into tree bark and attack healthy, undamaged trees. Greater peachtree borers typically feed on the lower portion of trunks from a few inches above to a few inches below the soil and will also feed on roots.
There is only one generation of lesser peachtree borers and greater peachtree borers per season, and a second generation of American plum borers emerges around cherry harvest through early fall. All three species overwinter as larvae; American plum borer larvae pass the winter in a silken cocoon called a hibernaculum. In the spring, these larvae will resume feeding and the cycle begins again.
Although their life cycles are similar, the emergence patterns of these borers are staggered and make timing management strategies difficult. Currently, the most reliable control tactic is a trunk spray of chlorpyrifos (Lorsban). This trunk spray primarily targets young larvae that hatch and begin feeding in the spring and can be timed to control multiple borer species. Michigan State University Extension suggests growers monitor for adult activity using pheromone-baited delta traps, and the insecticide application should be made when multiple adult borer species are active. A well-timed trunk spray is key for optimizing the value of the spray as well as management of borer larvae. Furthermore, taking special care to spray the base of the trunk will help with greater peachtree borer control.
Note: Lorsban has phytotoxic effects on sweet cherry and will cause foliar and fruit injury. In cherries, do not apply Lorsban as a trunk spray within 21 days before harvest; be sure to check pesticide labels for rates and additional use requirements.