Wildlife Repellent Studies
Dan and Joanne Campbell

Study area: Young Douglas-fir and Noble fir christmas tree plantations on the Heritage Tree Farms south of Molalla, Oregon, were examined in June 1998 for browsing damage by black-tailed deer and Roosevelt elk. Both winter and early summer browsing damage was extensive on Douglas-fir, but adjacent Noble fir were not browsed. To help prevent further damage to the Douglas-fir, and to evaluate Seadust and Seadust II wildlife repellents, blocks of approximately 300 Douglas-fir trees were marked in each of two plantations. Field 55C (upper) had small Douglas-fir, averaging near 20 inches tall, planted as 2-0 seedlings in 1996-97. Field 55-A had larger Douglas-fir, averaging near 40 inches tall, planted as 1-1 seedlings in 1994.

These plantations and adjacent forest land were used by deer and elk during most of the year. Prior winter damage, primarily by elk, was more extensive on the smaller Douglas-fir with 236 of 298 (79.2%) browsed, than on the larger Douglas-fir which had 84 of 287 (30.3%) browsed. Examination of spring growth on June 16, 1998, showed that deer (primarily) had already browsed 33.2% of the new terminals on the small Douglas-fir, and that 50% had new lateral growth already browsed. The large Douglas-fir had 27.2% of the new terminals browsed, and 45.3% had new laterals browsed at the time of treatment application. Some of the large Douglas-fir had been manually sheared for shaping during the previous August. Although the plantation with small Douglas-fir had been recently cultivated, higher levels of browsing damage were probably reduced by the availability of preferred flowering catsear or false dandelion in the plantation during mid-June. All trees were examined for any physical damage or discoloration before and after treatment applications, and for possible phytotoxicity or discoloration from treatments. Trees were also examined for damage by other animals such as pocket gophers, which were routinely trapped when evident in the plantations.

Methods: To evaluate reduction of further browsing damage, we installed 12 blocks of approximately 25 seedlings each in each plantation, and randomly assigned treatments of Seadust plus Bond adhesive, Seadust II plus Bond adhesive, and Bond adhesive (Control treatment) to each of 4 blocks. All tree heights and terminal growth were measured to the nearest 1/2 inch, on June 16 and 17, 1998.

On July 22, 1998, all trees were re-measured and examined for damage to treated and control terminals and laterals. Second bud-burst was recorded for many of the seedlings on this date, but only a few control trees had browsing on the second bud-burst.

Analyses were made of browsing damage and height growth which occurred from the period of treatment (June 16 and 17) until July 22, 1998. Differences in means for the percentages of trees damaged after treatments, and differences among height growth for all treatment means, and differences between browsed and unbrowsed trees, were analyzed using analysis of variance (ANOVA) and Duncan’s new multiple range test to determine differences at the p<05 and p<01 levels of significance.

Results:

Deer Browsing: Browsing by deer on the new growth of the small Douglas-fir was significantly reduced after treatment by Seadust and Seadust II repellents. The difference in means was highly significant (p<01) between Seadust and control trees and between Seadust II and control trees (Table 1). Browsing on the large Douglas-fir after treatment was negligible for Seadust treatments and for the control treatment. There was no significant difference (p<05) in browsing among Seadust treatments and the control treatment (Table 1).

Tree Height Growth: Although the height growth of the small Douglas-fir Christmas trees averaged more than the unbrowsed trees, analyses of growth among all 12 treatment blocks showed there were no significant differences. Individual trees grew as much as 8.5 inches, but differences in mean height growth of the small trees from mid-June until new bud-set in mid-July were not significant (p<05). Within each treated block of small Douglas-fir, the mean height growth (Table 2) for blocks of unbrowsed trees was more than for browsed trees, but differences in mean height growth were not significant (p<05). Trees in the 12 large Douglas-fir plantation treatment blocks grew rapidly from June 16 to July 22, 1998. Height growth increased up to 22 inches, but mean height growth from treatment until July 22 averaged about 11 inches. Height increase means for Seadust treated tree blocks were 10.2-10.8 inches, Seadust II treated blocks 11.4-12.6 inches, and control blocks 10.2-11.4 inches. The mean height growth among treatment blocks was not significantly different (p<05).

Tree Condition and Phytotoxicity: Treatments did not cause any adverse effects on tree growth and did not cause discoloration. The most obvious detrimental effect was the slow recovery of growth from previous winter damage by elk; some trees were deleted because of mortality or poor condition caused by the earlier damage, Most of the healthy trees had begun second bud-burst by July 22, 1998.

Conclusions: Seadust repellents significantly reduced further late spring deer browsing damage to small Douglas-fir Christmas trees, but was applied too long after new spring growth had started to prevent damage. Repellent treatments should be applied soon after bud-burst so that repellent is adhered to the new needles and transported with elongating stems and branches to protect them from browsing.

These appeared to be slow recovery on many small Christmas trees from winter browsing damage. Some mortality was caused by winter browsing damage. Where winter damage is anticipated, small trees should be treated at the time of planting or before damage occurs.

No phytotoxicity or discoloration of foliage was observed for repellent treatments or controls. The repellent was well attached to the foliage by the adhesive spray, and not affected by rain. The large Douglas-fir Christmas trees did not benefit from the late spring application of repellent. Unless browsing damage cannot be tolerated, repellent treatment of Douglas-fir Christmas trees over 3 feet tall will probably not be significant value under similar conditions, which includes the availability of forbs preferred as browse. Editor’s Note: This is the last of a three-part series by Dan Campbell. He is a retired USDA Wildlife Research Biologist and heads the private consulting firm of Wildlife Services, Inc.