The Effect of Plastic Tree Wrap on Borer Incidence in Dogwood

Materials and Methods

This study took place at Valley Forge National Historic Park in southeastern Pennsylvania during the summer of 1988. Trees were selected from a large, open area planted mainly in dogwood. Normally, several dozen dogwoods of 2.5 cm diameter are planted by park personnel in spring, and a ring around each tree is mulched to reduce mower damage. Plastic tree wrap is applied shortly after planting. However, for a variety of reasons some trees lack wrap. The wrap would fall off, be rubbed off by deer, be removed by visitors to the park, or the newly planted tree would be overlooked and never receive tree wrap. Ideally, the wrap was designed to spiral loosely around the tree trunk. This allowed the wrap to expand as the tree grew. Occasionally, the wrap would adhere to the trunk of the tree, and depending on the length of time that the wrap was in place, the wrap would eventually fit snugly against the bark. This variation in the tightness of the tree wrap created three classes of wrap, which were evaluated in this study: unwrapped trees, trees with loose fitting wrap, and snugly wrapped trees. In all cases, the trees were planted and wrapped from one to five years.

For each wrapped tree in the study, the circumference at the top and the bottom of the wrap and the length of the wrap from top to bottom was measured. To determine the tightness, the wrap was pulled from the trunk of the tree at the top and bottom of the wrap, and the gap between the wrap and the trunk was measured. If there was no gap, the wrap was noted as tight. The wrap was removed from the tree, and the trunk was closely observed for galleries or signs (frass, larvae, pupal cases) of borers. Galleries were excavated, and larvae, if present, were removed. For each unwrapped tree in the study, the circumference at the top and bottom and the length of the longest continuous section of trunk were measured. The tree was closely observed for signs of borers and galleries, which were excavated when observed as previously described.

Results and Discussion

A total of 56 trees were examined, and twenty three of these were found to be infested by the dogwood borer. Using the G test for equal frequencies (10), and assuming an equal probability of infestation among different categories of tree wrap, the number of infested trees was not significantly different than expected. However, the numbers of galleries observed and live borers excavated were significantly different from the expected occurrence (galleries: P 0.025, 2 df; borers: p 0.005, 2 df) (Figure 1). This suggests that while the incidence of infestation is not different between wrapped and unwrapped trees, the frequency and success of infestation was greater in wrapped trees.

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Figure 1.

Observed vs. expected incidence of attack by the dogwood borer, galleries observed, and live borers excavated from galleries in 56 trees.

The smallest tree that was infested had a diameter of 3 cm. The largest infested tree was 7.9 cm in diameter. The largest tree in the study was 12.5 cm in diameter. Potter and Timmons (7) found that trees less than 15 cm in diameter were more likely to be attacked by borers on the main trunk. Because all of the trees were in this vulnerable size category, studies were initiated to determine the relation between borer infestation and tree size. Dogwood trees were grouped into size categories and compared to the probability of borer attack among size classes. Borer infestation was not found to be related to a specific size category (G test, p 0.75, 8 df) (Figure 2).

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Figure 2.

Observed vs. expected occurrence of borer galleries by tree diameter.

These results confirm the findings of earlier studies suggesting that tree wraps may increase the likelihood of infestation by clearwing borers (5, 6, 9). The frequency of borer infestation was clearly higher in trees that were wrapped than in those unwrapped. Snugly wrapped trees were infested more frequently than loosely wrapped trees. The bark of the snugly wrapped trees was usually observed to be damp and soft, while the bark of the unwrapped and the loosely wrapped trees was dry. Holes in the wrap and spaces between the winds afforded areas of entry for the borer adult egg laying females. The snug wrap may have provided a favorable environment for larvae by protecting them from the elements and from predation and parasitism. Parasitism rates of up to 50% have been reported (11). King (5) used paper wrappers on peach trees to rear lesser peach tree borers, Synanthedon pictipes, for research purposes. He concluded that wrapping created conditions favorable for the larvae and should not be considered as a means of protection from borers.

At the present time we do not know the relative risks and benefits associated with tree wraps. Tree wrapping may reduce injury to the trunk from deer, rodents, lawnmowers, and sun. However, they may also increase the frequency and success of borer infestations. Species of trees that are known to have frequent borer pests may be better protected from them as well as vertebrate, lawnmower, and sun injury through the use of something other than close-fitting tree wraps. As a standard management practice, tree wraps should be checked and refitted periodically.

Summary

Damage from deer activity has prompted Valley Forge National Historic Park ţo apply plastic tree wraps to the trunks of young ornamental flowering dogwoods. These wraps have been found to contribute to the frequency and the success of infestation by the dogwood borer. Trees under 15 cm in diameter were found to be equally susceptible to infestation. These data suggest that an alternative means of deer protection is necessary in dogwood plantings.