June 2022 - 12

A MIDWEST
CHALLENGE
Transplant drench fights
devastating carrot weevil
By Dean Peterson
VGN Correspondent
Carrot weevil is an extremely
sporadic pest, but can devastate
members of the Apieceae family -
particularly carrots, celery and parsley.
" Carrot weevil is such a niche pest,
some people don't even know about it, "
said Elizabeth Long, assistant professor
in Purdue University's Department of
Entomology.
The adults are small snout beetles.
They're dark brown with light brown
or copper-colored scales and are
perfectly camouflaged to hide in
the soil.
" No one really looks for carrot
weevil and if they do, they can't find
it, " Long said.
Long has worked with Ohio growers
who've had up to 100% losses in
parsley from carrot weevil - despite
repeated insecticide applications.
Long has also worked with a Michigan
celery grower who has had trouble
with the pest and evaluated a
transplant drench as a management
option at that site in 2021.
Long spoke on the results of that
study in the celery session of the
2021 Great Lakes Fruit, Vegetable &
Farm Market EXPO in Grand Rapids,
Michigan.
Carrot weevil is native to North
America, but is more often a pest in
the Great Lakes region of the U.S. and
Canada. The adults can live a year and
overwinter. They emerge in the spring,
walk to new fields or plantings, and
lay eggs. The eggs can hatch in 5-16
days depending on how warm the
temperatures are.
The adults are active early and lay
eggs for most of the growing season.
The eggs are laid in the stem, creating
egg scars. These egg scars are covered
with a dark, frass-like substance to
protect the larvae, which makes it very
difficult to get a foliar insecticide to
the larvae.
The larvae are legless, cream-colored
and have golden-colored head capsules.
The larvae cause damage by tunneling
into the plants. Wilting and leaf
yellowing are the typical signs of
plant injury.
12 | VegetableGrowersNews.com
One full generation takes about
30-40 days and there are generally two
overlapping generations each year.
" You can find eggs, adults and larvae
in the field all at the same time, " Long
said. " Carrot weevil is a long-lived
insect that is incredibly cryptic. "
Carrot weevils can feed on other
members of the Apiaceae family,
including dill and parsnip. The weeds
wild carrot, broadleaf plantain,
patience dock, sour grass and
pineapple weed can also be hosts and
perpetuate a carrot weevil problem.
Field sanitation and disking or tilling
crop residue at the season's end will
reduce overwintering survival. Good
weed control is important, too.
The first step in population
management is to monitor adult
numbers in the early spring to
determine the amount of pest pressure
and to properly time insecticide
applications. It sounds simple, but
it's not.
The best monitoring tool is a
modified version of a Boivin trap -
also known as a radiator trap. The
modified version is a wooden base
with openings, or 'teeth', on the sides
allowing adult weevil entry to a
carrot bait.
Put the traps out early - no later
than two weeks before transplanting -
and replace the carrot every three days.
" The carrot is the lure, " Long said.
Place the traps on field margins, or
where adults may have overwintered,
including previously infested fields.
The goal with foliar insecticides is
to adequately cover the foliage and
crown to kill the adults before they
can deposit eggs. However, the adults
appear to have limited susceptibility to
some insecticides at their labeled rates.
Because the crop canopy becomes so
dense as these plants grow, it's hard to
get adequate coverage and penetration
with insecticides, making it hard to
contact the adult weevils.
" Timing is everything, " Long said.
Foliar applications must be made
early and target young plants.
Treatment thresholds are very difficult
to determine because trap catches don't
correlate well with field populations.
Once the plants have three true
Adult carrot weevil. Photo: Ohio State University
leaves, trap monitoring changes to
scouting for egg scars.
" The adults will disappear; you can't
find them, " Long said.
Make scouting the youngest plants
in the field - the three- to four-leaf
stage - the priority because they are
the most vulnerable.
Scouting for egg scars is difficult.
The egg scars appear as tiny, dark
marks and can be in the petioles of
parsley and in the petioles and crowns
of carrots and celery. Scouting is
tedious, time-consuming and requires
correct identification of the scars.
Finding egg-laying scars also means
the plants have already been damaged.
The best scouting strategy is to walk
an X-shaped transect in the field,
scouting 10 plants in 15 different spots
along the transect. The treatment
threshold in parsley is 1.5% of the
plants having egg scars. There isn't
much data on thresholds for carrot
and celery, but a presence of egg scars
generally means treatment is needed.
The effectiveness of contact foliar
insecticides is limited against larvae
because they are typically concealed
in the plant tissue. Soil insecticides, in
theory, don't have this limitation and
can protect the young roots by killing
eggs and larvae inside the plant. The
goal is to get the insecticides into the
root zone early enough to be in the
tissues prior to egg hatch.
Verimark was the transplant drench
evaluated in Michigan on celery in 2021.
" Verimark is our insecticide of
interest, but the product is very
expensive, " Long said.
The goal was to determine if
Verimark killed adult weevils or
deterred egg laying and if it killed eggs
or hatched larvae inside the plant. In
the study, four- to five-week-old celery
seedlings were drenched with Verimark
24 hours prior to transplanting at a low
rate of 7.5 fluid ounces per acre and a
high rate of 13.5 ounces.
The preliminary results were solid.
There were seven times fewer eggs in
plants treated with the high rate of
Verimark than untreated, and about
one-half as many eggs with the low rate.
The high rate also produced the highest
number of dead weevils - about onethird
more than the low rate.
The tests were repeated in the
lab. In controlled studies where the
weevils were forced to feed on treated
plants, at both the low and high rate of
Verimark, eggs hatched but no larvae
survived on the plants.
" We have pretty solid evidence
something works, " Long said. " Nine
days after the drench, we were still able
to kill larvae that hatched in the plant. "
Verimark is also effective on other
insects that feed on the germinating
seed, and those pests are listed on the
label. VGN
http://www.VegetableGrowersNews.com
June 2022

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