|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.