February 2021 - 6

Overwintering pests
Asparagus beetles thrive in stalks and leaf cover
By Dean Peterson
VGN Correspondent
Asparagus beetles are endemic to
asparagus fields. The beetles emerge
in the spring and growers spray for
them. After spraying, the beetles keep
coming back, but from where? Where do
asparagus beetles overwinter?
" We used to think in the woodlots and
field borders, " said Jennifer Zavalnitskaya,
researcher in Michigan State University's
(MSU) Department of Entomology. New
MSU research confirms some beetles do
overwinter in these areas but also found
most asparagus beetles overwinter in the
asparagus fields.
Beetle numbers emerging from
adjacent woodlots were one-tenth and
beetle numbers emerging from field
borders one-twentieth of the number
emerging from within fields. The
research also
found the type
of ground cover
in the fields has
a big impact on
how many beetles
survive the winter.
Zavalnitskaya
presented the
research findings
at the recent
Great Lakes Fruit,
Vegetable & Farm Market EXPO, which
was held virtually.
Researchers studied decomposing
JENNIFER
ZAVALNITSKAYA
survival. The more asparagus stalks
in the field and the more cover from
deciduous leaves, the more beetles
survived to emerge in the spring.
" Beetles didn't survive well in tree bark
and coniferous leaves, " Zavalnitskaya
said. " Beetles survived in higher
numbers in deciduous leaves. "
Researchers also counted the number
of beetles in dead bark to see if beetles
preferred dead trees and they did not.
Live trees produce more leaf litter and
more beetles.
The obvious solution of cutting down
the deciduous trees isn't a good one
because the trees provide habitat for
asparagus beetle predators.
Asparagus beetles typically go through
two life cycles per season.
Beetles emerge from overwintering
sites in the spring and lay eggs on the
asparagus spears during harvest season.
This reduces spear quality. " It's extremely
difficult to remove the eggs at harvest, "
Zavalnitskaya said.
The eggs eventually hatch and the
larvae feed on the fern. These larvae
produce the next generation of adults
that lay the eggs that hatch in late
summer. This generation's larvae also
feed on the fern. Fern feeding can reduce
stand longevity.
The adult beetles from the last
generation seek out overwintering
sites when the weather turns cold and
the hollow asparagus stalks are the
ground covers including tree branches,
asparagus stalks, large and small pieces
of tree bark and leaf litter from both
coniferous and deciduous trees.
Deciduous leaves and asparagus stalks
were found to be the shelters which
most promoted the beetles' overwinter
preferred site.
" Stalks are a big source of
overwintering beetles, so it's really
important to chop the stalks up early in
the fall, " Zavalnitskaya said. By the time
the weather turns cold, the beetles are
already moving to overwintering sites.
" Most beetles overwinter in your
Figure 1. Asparagus beetles lay eggs on spears during harvest (A), which reduces spear
quality. These eggs do not develop into larvae during harvest, as spears are picked
before they can hatch. After harvest during the fern stage, larvae (B) hatch from eggs,
eat damage the fern and develop into adult beetles (C). There are typically two or three
generations of asparagus beetles in Michigan with the last generation of adults seeking
out overwintering shelter to survive the winter. They emerge in the spring to feed, mate
and lay eggs. Photos: Jennifer Zavalnitskaya and Zsofia Szendrei/Michigan State University
Figure 2. The MSU Vegetable Entomology Lab is investigating where asparagus beetles
overwinter. They were found hiding within fields in decaying asparagus stalks (A) and
outside fields under tree bark (B). In an experiment, researchers are placing adult beetles
in overwintering cages with different types of bark, pine needles, leaves and asparagus
stalks to see how these different shelters help beetles survive the winter cold (C).
Photos: Jennifer Zavalnitskaya/Michigan State University
field, " Zavalnitskaya said. " In terms of
management, where are there a lot of
deciduous trees and where are those leaves
ending up? " The areas with lots of leaf piles
are the areas to watch next spring.
The research also evaluated the
effectiveness of several conventional and
organic insecticides compared to a full
rate and a half rate of carbaryl against
asparagus beetles. All products evaluated
had a 24-hour, preharvest interval (PHI).
Coragen and Entrust were found to be
as effective as a half rate of carbaryl for
controlling adult beetles during harvest.
Permethrin and neem oil were less
effective. VGN
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February 2021

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