394 Frank et al.: Imidacloprid Reduces Defoliation by Japanese Beetles 2 15.25). The average leaf area consumed on treated leaves was 0.003 ± 0.002 cm2 and the leaf area eaten on untreated leaves was 49.1 ± 0.75 cm2. In addition to dramatic reductions in feeding, half of the beetles that consumed leaves from trees treated with imidacloprid died or exhibited signs of intoxication such as trembling, lack of coordination, and, finally, cessation of movement and death. These symp- toms are common for insects intoxicated with imidacloprid (Suchail et al. 2001). No beetles that ate leaves from un- treated trees died during the course of the assay. Defoliation in the Field As a result of dramatically different levels of Japanese beetle abundance and associated damage during all 3 years, we ana- lyzed data from each year independently. The application of imidacloprid significantly reduced the proportion of severely damaged leaves in 2004 and 2005 (2004, P < 0.0002, F1,9 37.16; 2005, P < 0.0001, F1,940.09). In 2004, the propor- tions of severely damaged, as measured by leaves with 50% or more of their area missing, leaves on treated trees and untreated leaves were 0.07 ± 0.01 and 0.23 ± 0.03, respec- tively. In 2005, the proportion of severely damaged leaves on treated and untreated trees were 0.28 ± 0.03 and 0.74 ± 0.03, respectively. Populations of Japanese beetles during 2006 were very low and the resultant damage was negligible. Most leaves had less than 5% leaf surface area damaged by Japa- nese beetles on treated and control trees. The proportion of severely damaged leaves did not differ significantly between those treated with imidacloprid and those untreated (P < 0.55, 2 0.34). The proportions of severely damaged leaves on treated and untreated trees were 0.02 ± 0.001 and 0.02 ± 0.001, respectively. In 2004, there was no significant interaction between the application of imidacloprid and the height of leaves in the canopy of the tree. This indicates that defoliation related to feeding by Japanese beetles was consistent between high- and low-canopy branches within each insecticide treatment. How- ever, in 2005, there was a significant insecticidal treatment by height interaction (P < 0.0003, F1,72 14.63) (Figure 1). In 2005, leaves in the lower canopy of trees treated with imi- dacloprid sustained very low levels of injury, whereas ap- proximately 50% of leaves in the upper canopy of treated trees were severely damaged. In both years, defoliation was significantly affected by the height of the leaves in the tree canopy with a clear pattern for greater levels of defoliation in the upper canopy (2004, P < 0.0001, F1,72 97.73; 2005, P < 0.0001, F1,72 34.87) (Figure 1). This result supports and confirms observations made previously by Fleming (1972) and Rowe and Potter (1996). Rowe and Potter (1996) suggested that Japanese beetles aggregate in the upper canopy of trees as they orient to the silhouette of their hosts during colonization. Once feed- ©2007 International Society of Arboriculture Figure 1. Percent of leaves severely damaged by Japa- nese beetles in 2004 and 2005 in the upper and lower canopies of Tilia cordata trees treated with imidacloprid and untreated controls. Bars represent means and verti- cal lines represent standard errors. Within each year, means that share the same letter do not differ by Tukey test (P < 0.05). ing begins, the release of floral and fruit-like volatile com- pounds such as linalool and aliphatic esters attracts ever- increasing numbers of beetles to the upper canopy of the tree (Loughrin et al. 1996, 1997). An interaction between direction and insecticidal treatment (P < 0.016, F3,543.76) governed the degree of defoliation in 2004 (Figure 2). Defoliation was suppressed and similar at all cardinal points on trees treated with imidacloprid, but severe defoliation was more common on western exposures than on northern ones. No such pattern was observed in 2005 when levels of defoliation exceeded 74% and the degree of defoliation was uniformly distributed around the canopies of both treated and untreated trees (Figure 2). It is not clear why defoliation was more intense on western exposures and less so on northern or other exposures. Beetles may have initially colonized the western sides of trees from surrounding habi- tats. Feeding would increase volatile release and attract other beetles to these portions of the trees (Rowe and Potter 1996; Loughrin et al. 1996, 1997). During both 2004 and 2005, we observed dead beetles beneath the canopies of trees treated with imidacloprid but not beneath untreated trees. Moreover, Japanese beetles were observed on the foliage or treated trees expressing symptoms of poisoning, including trembling, uncoordinated movement, and arrested movement. Soil applications of imidacloprid re- duced defoliation in the year of application and at least 1 year thereafter.
November 2007
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