Editor’s note: this is part two of
this series. Don’t miss part one of Viburnum Under Siege.
The accidentally introduced pest,
viburnum leaf beetle (Pyrrhalta viburni) or VLB for short, can destroy a shrub in just a couple of years. Unfortunately it continues to spread through North America. So I am trying to
learn more about it, hoping that we can protect the viburnums in our
area. This is the information that I have gathered from the sites
listed and from my own observations. It doesn’t look very
encouraging.
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VLB
larvae and leaf damage
©
Beatriz Moisset
|
The grubs or larvae emerge from eggs
early in the spring. They are yellowish or greenish, with black
markings and can reach one third or a quarter of an inch in length.
They spend a few weeks feeding on the
leaves by making numerous little holes between the veins. These holes
are easy to identify. By early or mid June they climb down along the
stems and bury themselves in the ground to pupate. When I searched
for them in mid June, I found none, only the damage remained.
By the end of June or in July, the
adults emerge and start feeding on the same shrubs, making more holes
that look similar to the ones made by the larvae. The adult beetles
are about a quarter of an inch long, brown, with a golden fuzz
coating the body.
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VLB
egg caps on terminal twig
©
Beatriz Moisset
|
Females lay eggs on twigs, after
chewing up a round hole. They cover a batch of about 8 eggs with a
cap made of sawdust and their own feces. They can lay as many as
seventy batches aligned in rows near the far end of a twig. The eggs
spend the winter well protected against predators and the weather
under this nasty mess. Their only serious threat to their survival is
the crushing mechanism that many non-native viburnums possess.
Most native viburnums, such as
arrowwood (V. dentatum) and possum-haw (V. nudum) are either very
susceptible or susceptible to VLB. The only native ones that are
moderately resistant are the nannyberry viburnum (V. lentago) and the
blackhaw viburnum (V. prunifolium). On the other hand, many of the
non-native species are very resistant. So I see our native plants
being hit by a double whammy, first by the pest itself and second by nurseries
selling “pest free viburnums,” meaning non-native ones.
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Arrowwood,
Viburnum dentatum
©
Beatriz Moisset
|
The Cornell University site has a
number of suggestions on how to fight this invasive pest starting
with placing sticky traps around the base of the stems of shrubs in
the spring to prevent the full grown larvae from crawling down to
pupate. It is too late to do that now, so we will have to be ready
next year.
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Adult
beetle
©
Beatriz Moisset
|
The adults are active now, mid July, so
I have been trying to collect them by shaking the branches, catching
them on some white sheets underneath and dropping them in soapy
water. I suspect that the numbers will grow larger in the next few
weeks.
After the first frost it will be time
to start inspecting the stems for signs of eggs and to prune and
destroy infested twigs. They should be easy to identify thanks to
photos in the mentioned sites. This is perhaps the most effective
means of control, especially if your shrubs are relatively small. I fear that all these measures combined
may not be enough to stop the advance of the VLB, particularly when
there are many tall bushes among poison ivy and other shrubs; but we
plan to do our best.
The sites listed also suggest some
pesticides; but I haven’t considered those.
If worse comes to worse there are some
soaps and oils that may be of help, especially in early spring when
the young larvae are emerging from the eggs.
I hope that we can reduce the damage in
our area so that birds passing by in the fall continue to find the
nutritious berries of arrowwood and so moths also continue to find
food.
I am interested in other suggestions. Have you detected this invasive pest in
your gardens? I suggest that you look for the characteristic leaf
damage and egg cases. Let us know if you find any.
Examples of native species highly
susceptible to VLB:
- Arrowwood viburnums (V. dentatum complex)
- Possum haw or smooth witherod viburnum (V. nudum)
- American cranberrybush viburnum (V. opulus var. americana; a variety of the European cranberry viburnum, V. opulus)
Examples of non-native species highly
resistant to VLB:
- Doublefile viburnum (V. plicatum). Eastern Asia
- Tea viburnum (V. setigerum). Central and Western China
- Siebold viburnum (V. sieboldii). Eastern Asia
- Koreanspice viburnum (V. carlesii). Eastern Asia
Resources
Bugguide
Cornell University
University of Florida
Ohio State University
Purdue State University
© 2011, Beatriz Moisset. First published in Beautiful Wildlife Garden, 2011
Cornell University
University of Florida
Ohio State University
Purdue State University
© 2011, Beatriz Moisset. First published in Beautiful Wildlife Garden, 2011
