FEW weeks ago you described how to control fungus gnats on polyanthus,
but mine have larger worm-like bugs that seem to have eaten away
almost all the base of the plants. Any ideas?
SOUNDS as if the problem is weevil larvae, sometimes called vine
weevils. There are many different types of weevils, most of which
are grey or black beetle-like insects but with more prominent "snouts"
than real beetles.
Adult weevils can feed
on a wide range of plants, often causing distinctive holes in leaves,
and the larvae can be destructive root feeders on many annuals and
Adult weevils are relatively
easy to control with insecticides like Mavrik and Confidor, but,
protected by the soil, the larvae are a more difficult target. The
most widely recommended insecticide for soil use is diazinon, which
is available in liquid and granular forms from most garden centres.
Follow the label instructions carefully and try to restrict use
to the affected areas only as it can kill most soil insects - the
good as well as the bad.
Some gardeners claim
to have had success reducing weevil larvae damage by cultivating
the soil thoroughly, to destroy pupating larvae, immediately after
removing affected plants.
Avoid planting the same
type of plants in that area for at least a year or two in an attempt
to break the cycle. You could sow a green cover crop like oats or
mustard and leave it to grow over winter, then dig it in, before
it sets seed in spring, to improve the soil. Then plant vegetables
or unrelated ornamentals for the next year or two.
Gardener, Issue 172, 2005, Page 24
Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.