The Efficacy of Micro-Injected Imidacloprid and Oxydemeton-Methyl on Red Gum Eucalyptus Trees (Eucalyptus Camaldulensis) Infested With Red Gum Lerp Psyllid (Glycaspis Brimblecombei)

MATERIALS AND METHODS

Study Site

A stand of heavily infested red gum eucalyptus trees lining a golf course at Industry Hills, California, was utilized as a study site beginning July 1999. This site consisted of several hundred densely planted, irrigated trees approximately 20 years old and resembled a natural hillside-forested area. Trees were initially infested with RGLP for less than a year, and at the time of the study, most trees were visually estimated to have been 60% to 80% defoliated. New foliage was under constant attack from egg laying, flying adult pysllids emerging from infested leaves.Forty trees were selected at random to serve as test subjects. Mean tree diameter measured at breast height averaged 17 cm (6.7 in.), ranged between 10 and 60 cm (3.9 and 23.6 in.), and was used as a criterion to determine standard dosage rates to be micro-injected into the tree trunks at crown level.

Micro-injected doses were applied on July 8, 1999. Each application treatment followed the standard protocol established by the J.J. Mauget Co. A 4.4-mm (11/64-in.) drill bit was used to penetrate the crown bark approximately to a 10-mm (3/8-in.) depth into the xylem tissue. Commercial pre-measured capsules containing prescribed pesticide doses were attached by feeder tubes that were inserted into each treated tree. Imidacloprid-treated trees received a dose of 3 mL of 10% active ingredient per 5 cm (2 in.) of trunk diameter. Oxydemeton-methyl was applied at a rate of 3 mL 50% active ingredient per 5 cm of trunk diameter. A combination of both materials consisting of a dose of 2 mL of 10% imidacloprid and 3 mL of 50% oxydemeton-methyl was applied as a separate combination treatment. Capsules and feeder tubes were removed within 24 hours following treatment. Each treatment and untreated check was replicated 10 times.

Following treatment, ten leaves were randomly sampled weekly from each tree. After 3 months of weekly sampling, leaves were sampled on a monthly basis for 1 year. Collected leaves were immediately bagged and placed in a cooler and kept refrigerated prior to microscopic examination. Population levels of RGLP nymphs on leaves were determined by microscopically examining both sides of leaves. Lerps were carefully removed and live nymphs were counted. A pre-treatment census of RGLP nymph populations was taken on July 1, 1999 (Table 1). Post-treatment levels of pesticide efficacy were determined by comparing the mean population levels of live RGLP nymphs found under lerps on sampled leaves. Dead or living nymphs found outside lerps were not counted.

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Table 1.

Pre- and post-treatment monthly means^z (4-week intervals) of numbers of live red gum lerp psyllid nymphs per leaf in trees treated with insecticides^y.

The total average percentage reduction of RGLP nymphs due to treatments, compared to untreated trees, was calculated using the following formula:

% reduction = (untreated average - treatment average/untreated average) × 100

RESULTS AND DISCUSSION

Efficacy results were based on comparisons of population levels of live RGLP found under lerps among leaves sampled over this 15-month study. Direct RGLP nymph mortality following treatment was too difficult to measure because newly killed early instar nymphs often failed to establish a protective lerp and fell off the leaves undetected. Tables 1 and 2 summarize the monthly RGLP nymph densities among treatment and untreated trees. A statistical analysis (5% error rate) was performed among treatments using Stateview® ANOVA and Fisher’s PLSD.

Significant reduction in RGLP nymph populations occurred among most treatments when compared to untreated trees on a weekly basis (Table 1). However, significant differences between various materials, at the 5% error rate, could not be detected until the third month following application. During the first month following treatment, oxydemeton-methyl showed early efficacy when compared to untreated trees. However, after the first month following treatment, population levels of RGLP nymphs began to rise and no significant difference was detected between the population levels of this material and untreated trees after two months (Table 1). Therefore, residual effectiveness in reducing RGLP nymph populations, under the conditions of this study, seems to be limited to approximately 2 months. The overall percentage reduction of RGLP nymphs compared to untreated trees for 3 months was 52.9%.

Imidacloprid-treated trees had significantly lower RGLP populations than untreated trees for 8 months (Tables 1 and 2). Averaged reduction of RGLP nymphs, when compared to untreated trees, was 81.4% over 3 months, 73.2% over 8 months, and 40.7% over 15 months.

The combination of both materials showed significant lower population levels than the untreated trees over 8 months (Tables 1 and 2). Overall reduction of RGLP nymphs, when compared to untreated trees, was 74% over 3 months, 64.1% over 8 months, and 4.1% over 15 months. The lower overall difference of the combination treatment may be due to the reduced amount (2 mL instead of 3 mL of 10% active ingredient) of imidacloprid applied in combination with oxydemeton-methyl.

Nymphal population variances among treatment trees were too high to consistently detect significant differences between treatment averages on several dates after 8 months.

Some trees were observed to have longer residual activity than others, which may indicate possible differences among individual trees sustaining levels of imidacloprid beyond 8 months. Differences may be due to several factors that may interfere with material uptake. Such factors may include tree health, age, root and crown structure, crowding, and the degree of nutrient or water stress. Tattar et al. (1998) also found the translocation of trunk micro-injected imidacloprid varied considerably among and between three tree species. These factors need to be the subject of further investigation to improve the implementation of micro-systemic methods used in tree care.

CONCLUSION

All materials showed significant reduction of RGLP nymph populations when compared to untreated trees. Oxydemeton-methyl showed effective control for the first month following treatment, but showed less effectiveness thereafter. Imidacloprid and imidacloprid combined with oxydemeton-methyl showed statistically significant reduction of nymph population levels up to 8 months. Given the ability of RGLP to quickly reinfest trees, longer residual materials, such as imidacloprid, have a distinctive advantage over short residual materials for controlling RGLP infestations.