| From 1984 to 1993, the lab has worked
on the development of a user-friendly sampling technique to monitor
for the presence of ELB and predict subsequent defoliation. During
this period, we sampled over 200 trees in more than 25 cities in central
and northern California (Dahlsten
et al. 1994). Since English elm is the most susceptible elm,
we have focused most of our efforts on this elm.
The appropriate times to sample and/or treat ELB are determined by monitoring heat accumulation, expressed in degree-days (DD) above 11°C starting March 1 (Fig. 1). At the predicted egg peak, 16 branch tips 30 cm long are taken from the lower crown; two from each cardinal direction in both the inner and outer crown (Dahlsten et al. 1994). The presence or absence of viable (unhatched) egg clusters on each sample unit provides good damage prediction for that generation (Fig. 2). We developed a rating system to determine foliage damage levels by ELB. We rate the combined adult and larval damage on each 30-cm branch terminal on a scale of 0 to 10, where one equals 10% and ten equals 100% defoliation. Damage on a foliage sample is rated by comparing it to a visual standard, which shows examples of leaves with each damage rating (Dahlsten et al. 1993). Since there were no previously established treatment thresholds, we chose a damage rating of 4 as our threshold based on our experience with homeowners and tree managers. Recent work in Sacramento has indicated that 40% defoliation is unacceptable to city tree managers and homeowners there. In 1997 we began using 20% defoliation as our threshold. |
Fig. 1 Accumulated degree days above 11 degrees
C from March 1 in northern CA and the occurrence of ELB eggs and larval
stages.
Fig. 2 Comparison of the percentage of samples
with elm leaf beetle egg clusters and the resulting damage to English
elm in beetle generation 1, 1986 -1989. Each point is the mean of 40 30
cm branch samples from one tree. Red lines represent updated treatment
threshold.