Lesser Peach Tree Borers Now Flying

OMAFRA staff are evaluating several types of mating disruption dispensers for managing clearwing borers in peach orchards and comparing these to conventional insecticide-based programs (Project runs from 2007-2009 in Niagara). As a result, we have pheromone traps up for both (greater) peach tree borer (PTB) and lesser peach tree borers (LPTB). Our first LPTB were caught on Friday, June 6th, 2008 (about two weeks behind 2006 and 2007). For those interested in managing these insects, read on!

Peach trees are subject to attack by two clearwing borers, the (greater) peach tree borer (Synanthedon exitiosa) (Figure 1) and the lesser peach tree borer (S. pictipes) (Figure 2). As their names suggest, moths from this group possess transparent wings with striking black markings along the margins and shiny black abdomens with faint yellow banding. These day-flying moths are often mistaken with wasps. While the moths are quite beautiful to behold, their larvae can cause considerable damage in orchards when left unmanaged.

Figure 1. Peach tree borer adult - note clear wings. Adult males have a wingspan of 25-28 mm. Males have faint yellow bands on 3rd, 4th 5th and 6th abdominal segments.

Figure 2. LPTB adult in trap. Adult males have a wingspan of 15-23 mm. Males have faint yellow bands on the 2nd and 4th abdominal segments. Banding patterns may be hard to see on trapped specimens. Examination of the scales below the antennae and in front of the eyes (under magnification) will reveal pale patches.

Lesser peach tree borer (LPTB) and (greater) peach tree borer (PTB) are site specific pests. While historical wisdom suggests that PTB is more common in Niagara and LPTB a problem in Essex/Kent, both species may occur in mixed populations. Recent work in Niagara indicates LPTB populations are causing economic damage in some peach orchards. Damage is caused by larval feeding on callus tissue, which is formed by the cambium around and over wounds. Consequently, their feeding interferes with the tree's natural defence system, exacerbating previous injuries and allowing entry by other pests or pathogens. If left uncontrolled, trunks and limbs can be girdled, resulting in the death of the affected limb or the tree.

Peach tree borers typically attack the roots and the trunk near the soil line (although I have detected them on the trunk and occasionally in scaffold cankers). They seem to be more common in young plantings, where infestation of whips by a single larva can result in tree death. Lesser peach tree borer larvae take advantage of previously damaged plant tissue (cankers, pruning scars, winter-injury, other insect damage) to establish themselves. They are found under the bark of injured trunks, scaffold limbs or branches, and are commonly associated with cankers. Signs of infestation include frass (insect excrement) at the soil level and in or around cankers. The frass appears as rusty brown granular material that is often mixed with ooze in cankers (Figure 3). Digging around the frass and ooze will often reveal a feeding larva (Figure 4). At this time of year, as moths are beginning to emerge, it is not uncommon to see pupae or pupal cases sticking out of damaged areas.

Figure 3. Frass and ooze in canker.

Figure 4. Clearwing borer larva.

The general biology of both pests is similar. Both species overwinter as larvae, completing development the following spring/summer before emerging as adults. However LPTB overwinters anywhere from a tiny second-instar larva to a large, mature sixth instar larva. Because of the variable stages of development of overwintering larvae, emergence of the LPTB adults is not synchronous and can occur over a very long period, from late May to late September, without any obvious peaks. Monitoring in Niagara indicates PTB adults emerge later than LPTB adults, usually by early to mid-July, and their flight continues into September. A peak is not always evident. Adults mate shortly after they emerge and females lay eggs soon thereafter. A single female is capable of laying hundreds of eggs. Eggs hatch within 7 to 10 days, depending on the weather, and larvae begin feeding under the protection of the bark.

Pheromone traps for both species are commercially available and should be placed in the orchard mid-May, before the anticipated onset of flight. If you're not certain which pest(s) dominate in your orchard, traps can also help you to determine where to direct efforts in terms of timing insecticide applications. Traps for LPTB should be hung in the tree at a height of approximately 1.5 m off the ground, at the location where most mating activity occurs. Traps for PTB should be placed lower on the tree, closer to the ground. The pheromone for LPTB is species-specific and should not trap other species of clearwing moths (in contrast, the pheromone for the GPTB will also trap dogwood borer and some other clearwing moths). With that being said, proper identification of captured insects is essential, as other moths and other insects may inadvertently end up in pheromone traps (Figure 5).

Figure 5. Relative size of PTB, LPTB and dogwood borer (pest of apples).

Insecticides are generally applied shortly after flight begins (either species), timed to coincide with first egg hatch (not peak activity), about 7-10 days after the first moths are caught in pheromone traps. Thiodan / Thionex (endosulfan) (up to 3 applications) or Sevin (carbaryl) are registered for both LPTB, and should be applied so that thorough coverage of the trunk and lower limbs is accomplished. Thiodan / Thionex are registered for PTB. Remember, the female moths will search for even very small wounds, and the larvae are protected once they enter the tree, so good coverage is critical. A hand gun is suitable for smaller plantings; an airblast sprayer can accomplish the task with the lower nozzles open, high water volume and slow driving speed. Two or three applications at 5-10 day intervals are needed in areas with a history of high pressure - remember that these moths are active pretty well all season. Keep preharvest intervals in mind when selecting insecticides (PHI is 15 days for endosulfan; PHI is 1 day for carbaryl on peach or 5 for apricots). Avoid spraying the fruiting area. All new trees should have an endosulfan dip before planting. Note that the residual activity of registered insecticides does not provide coverage for protecting trees against infestation when considering the length of pest activity.

Insecticides can help deal with borers if applied correctly, but don't forget about the importance of maintaining vigorous, healthy plants and preventing mechanical injury - these practices can help deny entry into peach trees. It is also helpful to prune out broken limbs and limbs with previous borer damage.

Isomate P is registered for use in managing PTB, but is not considered effective for managing LPTB (in contrast, Isomate LPTB, which is not yet registered for use in Canada, can manage PTB when applied at a higher rate). Mating disruption products can provide season-long management of clearwing borers. We'll keep you posted on the results of our demonstration trials with mating disruption products Isomate P (PTB), Isomate Dual (both species) and Isomate LPTB (added in 2008) and their performance relative to insecticides.