Abstract
The elm leaf beetle, Xanthogaleruca luteola (Muller), is a serious defoliating pest of elm throughout much of North America. Petrochemically-derived insecticidal sprays have been a common management practice for this insect but increasing environmental concerns and regulatory restrictions have required that alternative insecticides be considered. Recently, new strains of Bacillus thuringiensis (“San Diego”, “tenebrionis”) which have activity against leaf feeding beetles have been discovered and become available. Field and laboratory trials indicate that these are highly effective against larvae of elm leaf beetle. Some adulticidal activity also is present with these formulations. Persistence of the tested formulations on foliage was short so proper timing of treatments is important for effective control.
Elm leaf beetle, Xanthogaleruca luteola (Muller), has established itself as a serious pest of many elm species grown throughout much of North America. Two or more generations of the insect are typically completed in a single season often resulting in sustained defoliation. Heavily defoliated trees suffer considerable esthetic damage, loss of tree vigor, and increased branch dieback.
Foliar applications of various petrochemically-derived insecticides have been a standard elm leaf beetle management practice for many years and generally give a high level of control. However, increasing public scrutiny has focused on these treatments because of perceived safety and/or environmental concerns. Moreover, liability and regulatory restrictions surrounding pesticide applications, particularly in urban areas, have spurred interest in developing alternative pest management practices for urban shade trees.
Recently, new strains of Bacillus thuringiensis have been identified which have activity against certain beetles. Discovery of B. thuringiensis var. tenebrionis was announced in 1983 by Krieg et al. (3); the “San Diego” strain was later identified by Herrnstadt et al. (2). Activity of both the “tenebrionis” and “San Diego” strains is apparently similar and is limited to larvae and adults of some beetles including the yellow mealworm, elm leaf beetle, and Colorado potato beetle (4). Lepidoptera larvae, susceptible to the commonly used “kurstaki” and “thuringiensis” strains, are not affected by the B. thuringiensis strains which affect beetles (Coleoptera).
Both “tenebrionis” and “San Diego” strains are currently under active development by several companies, largely for control of Colorado potato beetle, Leptinotarsa decemlineata (Say). The Colorado potato beetle is a serious pest of potatoes throughout North America and Europe and several strains of the insect have developed high levels of resistance to currently available insecticides. Secondary development interests have also come to include “ornamental” markets, such as elm leaf beetle.
Formulations of “San Diego” strain (M-One, Mycogen Corp., San Diego, CA) and “Tenebrionis” strain (ABG-6263, Abbott Labs, Chicago, IL) became available in sufficient quantity in 1988 to allow for field and laboratory testing. These studies were conducted to determine the performance characteristics of the new B. thuringiensis strains for use in protecting elms against elm leaf beetle.
Methods
Field trials
Field trial tests of the M-One formulation (“San Diego” strain) were conducted at two sites within the metro Denver, CO area. Precount surveys of elm leaf beetle larvae were made June 14 on marked branch terminals. At the first site (Arvada) 6 terminals were marked on each treated tree. Experimental design was a randomized complete block with 4 single tree replications. A second site (Wheat Ridge) involved a single block treatment to 2-3 trees each with 10 marked terminals. Siberian elm, Ulmus pumila, was the host at both locations.
Applications were made June 15 using a hydraulic tree spraying unit with a FMC 785 spray gun. A spreader-sticker (Loveland Bond) was added at the rate of 24 fl. oz./100 gallons. Three rates of M-One were tested, 3 qts, 4 qts, and 8 qts/100 gallons of water. Elm leaf beetle larvae at the time of application were predominantly (80-90%) in the first and second instars.
Evaluation of treatments was made June 20 by recounting the marked terminals used for precounts. Correction for natural background mortality was made using the formula of Abbott (1).
Greenhouse efficacy comparisons
Three greenhouse and laboratory trials were conducted to compare the application rate responses of both “San Diego” and “tenebrionis” strains. All trials involved applications to potted Siberian elm maintained in the greenhouse. Two larval evaluations involved 35 field collected elm leaf beetle larvae, in mixed instars, confined to each tree. Larvae were immediately confined to sample trees upon drying of the foliar applications. Three trees were used to evaluate each treatment rate.
Adult trials were conducted by confining 70 field collected adults upon two treated trees retained within a screened cage. Dead adult beetles and remaining alive adult beetles on foliage after 96 hours were used to assess mortality. Mortality corrections involved use of Abbott’s (1) formula.
Persistence evaluation trials
Persistence of the B. thuringiensis strains was evaluated by sequentially treating potted trees over the course of I-7 days prior to exposing field collected larvae to the trees. Two trials were conducted, each involving 3 trees with 35 larvae per tree.
Results and Discussion
Both field trials with M-One (“San Diego” strain) indicated a high (90-95%) level of control (Table 1) when used at the 4 and 8 qt/100 gal rates. A very sharp drop-off of activity was observed between the 3 qt/100 gal and 4 qt/100 gal rates of application, suggesting that recommended field applications of this formulation should equal or exceed the 4 qt rate. At the higher rates, most of the surviving larvae were those that had hatched within a few days of the evaluation. A background of natural mortality approximating 20% was also observed during the one week course of the trial.
In laboratory and greenhouse trials, “tenebrionis” strain (ABG 6263) also showed activity against larvae of elm leaf beetle (Table 2) giving consistently higher levels of control than M-One at tested rates. Interestingly, M-One performed less well in greenhouse tests than in the field. One possible explanation is that under field conditions larvae may succumb to increased mortality due to dislodgement from trees while suffering sublethal effects of infection. In addition, an older range of larvae were used in these tests than occurred during field testing.
Adult beetles were also susceptible to both strains of B. thuringiensis (Table 3). At tested rates, (2-4 lbs) the “tenebrionis” strain formulation (ABG 6263) gave control levels exceeding 95%; similar levels of control with the “San Diego” strain formulation were achieved at the 8 quart rate of application. These data show that the adult stage of elm leaf beetle is similarly susceptible to high-end larval stage usage rates.
Persistence of both strain formulations appeared to be short (Table 4). Sharply reduced activity was evident within 24-48 hours. The addition of a spreader sticker (Loveland Bond) did not significantly affect persistence. Observations during field trial evaluations also indicated short residual activity since newly hatched healthy larvae were found on foliage within 6 days of application. This suggests that applications of the these beetle active B. thuringiensis strains will have to be carefully timed to coincide with the end of peak egg hatch. Future improvements in formulation which allow for increased persistence or which simultaneously kill egg stages (e.g., spray oils) may allow for greater latitude in spray timing.
Control of elm leaf beetle, especially in residential areas, is one of the most important insect control services offered by many arborists in Colorado. These applicators are generally highly visible to the public because of the location of elms in 105 late instar larvae confined (35/tree) to three trees in the greenhouse. populated settings. Because of public concerns with these treatments, use of biological pesticides, such as B. thuringiensis, is a desirable attribute. Arborists are likely to use such materials if they meet performance requirements such as effectiveness against the target pest, nonirritation to the applicator, absence of significant staining or odor problems, and are priced competitively with alternative treatments. The tested beetle-active strains of B. thuringiensis (“tenebrionis” and “San Diego” strains) show substantial promise in meeting these criteria.
Acknowledgment
We gratefully acknowledge the Binder Construction Company of Arvada, CO and the City of Wheat Ridge, Parks and Recreation Department for providing study sites used in these field trials.
Footnotes
- © 1989, International Society of Arboriculture. All rights reserved.
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