If you found iris borer pupae under the Siberian irises, they had eaten
some of your rhizomes during the summer. They do not eat the rootlets,
but, it they eat all the rhizomes, you will lose the plants. This has
happened to us. Entire clumps have disappeared because we didn't notice in
time. I don't know of any non-chemical way to prevent this except for
constant vigilance to detect the beginnings of damage and catching the active
borers. Some growers burn over the Siberian beds in the fall to destroy
any eggs that have been laid. The eggs begin to hatch in zone 6 in April
and continue for a number of weeks. Hope this helps.
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