A Monitoring System and Development of Ecologically Sound Treatments for ELM Leaf Beetle

Temperature monitoring

The best times to sample and treat ELB can be determined by monitoring heat accumulation, expressed in degree-days (DD) (9; also see reference 1). We sampled eggs, three larval stages, and adults throughout the season at numerous sites and years, while monitoring DD above the ELB developmental threshold of 1ΓC (50°F) starting 1 March at each site. Accumulated DD was recorded at the time of peak density of each life stage, and the mean DD for each stage calculated for all sites and years. These values (Fig. 1) can be used to predict when a stage will be most numerous at a particular site. Weather data can be obtained from nearby weatherstations, on-site instruments (some of which calculate DD directly), or daily maximum/minimum temperatures from a local paper. Computer programs are available (see reference 1) to calculate DD from these data. In our studies, we measured accumulated DD at some sites using Omnidata Inc. Biophenometers. At other sites we calculated DD using data from nearby weather stations connected to theUniversity of California IPM computer system (12).

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Figure 1.

Elm leaf beetle degree-day development in California. Life stages vs. degree-days above 11 °C from March 1 (means and standard deviations for 19 sites over a 6 year period).

ELB monitoring

A 30 cm (1 ft) elm branch terminal was the sampling unit on which the monitoring system is based. ELB eggs were used for damage prediction because of convenience (they hold still) and sampling them allows time to make treatment decisions before the damaging larval stage occurs. After originally counting individual eggs, then clusters, we have now determined that recording presence or absence of eggs on each sample unit is adequate for damage prediction. Because we found significant differences in egg density between cardinal directions but not between upper and lower crown (16), branches are now taken from the more easily sampled lower crown in 8 segments per tree. These are north, east, south, and west, in both the inner crown (from trunk halfway to drip line) and the outer crown.

We began by sampling a small number of trees intensively (up to 120 branches/tree) on small pilot study sites, but found that damage was not adequately predicted for larger sites because of variation between trees. After a computer simulation of different combinations of numbers of trees and branches per tree, we developed a scheme that offers a good compromise between sampling effort and precision for sites of different sizes. This scheme (Table 1) gives the number of trees to be sampled and the number of branches to be examined per tree, depending on the site. Trees for sampling (the same trees should be sampled on each date) should be chosen randomly. Large sites may be divided into sections and trees chosen randomly within those sections so that sample trees will pick up any spatial variation in ELB density. For example, a 60-tree site could be divided into three 20-tree sections and five sample trees chosen per section.

We rate combined adult and larval damage on each 30 cm branch terminal, on a scale of zero to ten, where ten equals 100% defoliation. Damage is rated by comparing it to a visual standard (Fig. 2). Managers may wish to rate damage using this method, or may use the less labor intensive and less accurate method of visually rating the whole tree from 0-10 to estimate damage from each ELB generation. For sites with one ELB generation, generation damage is measured at the end of the season in fall. For sites with multiple generations, damage around the egg peak (or at the end of the season for the last generation), minus damage at the prior egg peak, reflects the effect of the previous generation.

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Figure 2.

Standard for visual comparison to estimate elm leaf beetle apparent damage (rated 1-10 where 10=100%). Defoliation of 60-90% was estimated by comparison to the darkened areas of 4010% of leaf area removed (i. e. 70% damage is the dark area of the “30% of leaf area removed” figure).

Originally we hoped to predict ELB damage and the need for control throughout the season by sampling eggs in the first generation only. However first generation sampling did not predict season-long damage adequately due to differences in generational patterns between years and sites. The damage in all four years at Cloverdale, CA (Fig. 3) was well above the acceptable level but it would have been impossible to predict from the very low first generation samples in 1988 or even the moderate egg level for the first generation of 1989. Thus each ELB generation needs to be sampled; we recommend sampling weekly starting about 50-100 DD before the predicted egg peak for each generation (eg. 183-233 DD for generation one, 853-903 DD for generation two, etc.). Weekly Sampling should continue until the percentage of branches with eggs peaks and then falls, or until 100 DD have acuumulated after the predicted peak time. With accurate weather data, this will usually mean sampling three to four times per generation.

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Figure 3.

Elm leaf beetle egg cluster and damage means per sample vs. time on English elm for 4 years at Cloverdale, Sonoma Co., CA, 1987-1990.

Since there were no established treatment thresholds we arbitrarily chose a damage rating of 4 based on our experience with homeowners and tree managers. In the first generation, if less than 45% of the branch samples have viable eggs then the damage at the end of that generation will be in the acceptable range (damage rating 4 or less than 40% defoliated), with a probability of error of 10% (Fig. 4). In generation 2 the maximum percentage of branches infested for acceptable damage was about 30%. This difference may be due to errors in determining the magnitude of the second egg peak in our early work, when the second generation was sampled only every two weeks. Continuing research will clarify the relationship between egg density and damage in the second and later generations.

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Figure 4.

Percent of samples with viable elm leaf beetle eggs at egg peak and resulting damage to English elm in generation one, northern California, 1986-1989. Each point is one tree with 40 branch tip samples; less than 10% of trees below the 0.45 line had damage above level four.

Treatments

Parasitoid introductions

Tetrastichus gallerucae is a tiny wasp that parasitizes ELB eggs. Female Tetrastichus gallerucae parasitize from 50-90% of the eggs in each egg cluster (17) and usually also feed on remaining eggs, effectively destroying the cluster. The parasitoid was credited with largely suppressing an ELB outbreak in Paris, France in 1904 (10). In southern California the parasitoid has been established and has significantly reduced ELB defoliation (R. F. Luck, University of California at Riverside, personal communication. We developed a rearing method to supply large numbers of T. gallerucae for release (2). To date we have released approximately 300,000 parasitoids, of 5 different European strains, from 6 sources, at 25 locations in several different northern and central California climate types (Fig. 5) (5).

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Figure 5.

Locations in California with English or Siberian elms where we have released Tetrastichus gallerucae, an egg parasitoid of eim leaf beetle, from 6 sources.

The parasitoid has the ability to build up high populations during the season. At one of our sites, 95% of ELB egg clusters were parasitized after a single release of 1000 parastiods in May, 1986 (4). The following year only one egg cluster was found and no detectable defoliation occurred. However at most sites it has failed to overwinter, and when it has overwintered has not been detectable until late in summer, sometimes reaching high populations then. In contrast, when T. gallerucae was being collected in Europe it was easily found in the first ELB generation in spring. The reason for its different performance in California is unknown, as the climate is similar enough that overwintering should not pose a problem. Previous laboratory and field studies indicate that few individuals of the introduced strains have the longevity to overwinter.

In the absence of overwintering populations of T. gallerucae, inoculative releases of the parasi-toid in spring could be combined with compatible early-season control measures until parasitoid populations increase later in the year. A large-scale insectary program would be required for widespread use of such releases. Another option would be to ship field-collected parasitoids from southern hemisphere release sites in the fall, for release in the northern hemisphere in our spring.

Bacillus thuringiensis

Foliar application of two different formulations of this bacterial insecticide (Mycogen Corp. M-one and M-Trak) was made at several sites each season from 1989 to 1991. Use of degree-day monitoring was effective in timing Bt applications to affect the vulnerable early larval stage. Although there were reductions in ELB populations in each trial, damage to the treated areas did not differ significantly from untreated areas. It is possible that multiple applications may be effective but the cost might be prohibitive for most managers. Several companies are working on other Bt formulations that may be more effective than those we tested in this study.

Summary

Each ELB generation must be monitored individually. Time of egg peak is predicted and egg sampling scheduled by monitoring heat accumulation measured in degree-days above 11 °C (Fig. 1). Sampling in each generation should begin 50-100 DD before the predicted egg peak and continue weekly until eggs peak or 100 DD after the predicted peak. Number of trees sampled, and branches per tree, must be adjusted depending on site size (Table 1); the sample unit is a 30 cm (1 ft) branch terminal for which only presence or absence of eggs is recorded. If more than 45% of the branch terminals have ELB eggs at the first generation maximum, treatment should be planned for that generation, using methods described here or other methods. For the second generation the unacceptable damage threshold is 30% branches with eggs. Treatments should be timed using DD monitoring. Damage may be recorded on a per branch or pertree basis in orderto monitor accuracy of damage prediction or the success of any treatments applied.

We emphasize that this monitoring method is still being refined and may change with further field testing, especially for damage prediction beyond the first generation. Also, the method was developed for English elm in northern and central California and adjustments may need to be made for other areas. However, these techniques now permit tree managers to determine if control is necessary; their use alone will result in a substantial decrease in the use of chemical insecticides for ELB control.