The larvae of several kinds of root weevils can cause serious damage
to strawberry roots, leading to reduced yield and in at least one case
this year in southern Vermont the complete demise of a previously healthy
field. Black vine weevil (BVW) is probably more common than strawberry
root weevil or rough strawberry root weevil in New England. The life cycle
and management of these weevils are the same. Their larvae are whitish,
crescent-shaped larvae and 1/4 to1/2 inch long with no legs. Adults emerge
and feed from May through August, laying eggs as late as October that hatch
and overwinter as larvae. Adult feeding causes characteristic scalloping
or notching of the leaf edges, but rarely does this cause economic damage.
(Feeding on the interior of the leaf, causing holes, is caused by Asiatic
garden beetles or Japanese beetles.)
Adults weevils hide in the crowns during the day and feed at night.
They are not easy to kill with insecticides so a better strategy is to
kill the larvae with applications of beneficial nematodes. If adults are
numerous (i.e. more than 50 out of 100 leaves sampled across the field
have notching) then a spray may be warranted. The pyrethroid bifenthrin
(Brigade) provides some control if used at the highest labeled rates. The
best timing for this spray is at night during the peak feeding activity
of adults, before they start laying eggs, or about 1 week before harvest
ends. Neem-based products containing azadiractin (such as Aza-Direct) may
be acceptable for organic production, and while neem will not kill the
adults it can disrupt egg-laying if applied at high rates at least twice.
While Admire is very good for controlling some white grubs it is mediocre
against Asiatic garden beetle and very poor against BVW.
Although bifenthrin claims to kill spider mites, many twospotted spider
mite populations are resistant to pyrethroids. Spraying this product or
other pyrethroids usually exacerbates spider mite problems by selectively
killing off predatory mites. Growers challenged with black vine weevil
problems should plan well ahead, and use horticultural oil (SunSpray UltraFine
Oil) early in the growing season. If applied with an airblast mist blower,
oil can be inexpensive, effective, and non-toxic to predatory mites. This
strategy can then reduce the risk of spider mite problems later. Be sure
to use oil about 2 weeks before any Captan sprays, because the two products
are extremely phytotoxic. Alternatively, Brigade may be applied with oil
2 to 3 days after mowing the foliage during renovation. This approach should
jointly control spider mites and root weevil adults.
The key to successful use of beneficial nematodes is sufficient time
for multiplication of the nematodes in hosts (weevil larvae) and dispersal
of nematodes throughout the soil. Early- to mid-May application has given
excellent results, especially when the numbers of larvae of the next weevil
generation are evaluated in the autumn. Research in CT, NJ and elsewhere
has shown that the appropriate nematode species properly applied can effectively
infect and suppress weevil populations. Heterorhabditis bacteriophora (Hb)
appears to be the best candidate for control of root weevils when the soil
temperature is above 60 degrees ('J-3 Max Hb' from The Green Spot; 'GrubStake
HB' from Integrated Biocontrol Systems; 'Larvanem' from Koppert Biologicals).
Beneficial nematodes can also be applied in late summer (August 15 - September
1), and in that case, Steinernema feltiae ('Nemasys' from Griffin Greenhouse
Supply, 'Gnat Not' from Integrated Biological Control Systems, 'Entonem'
from Koppert Biological) should be considered in northern locations since
it tolerates cooler soil temperatures and completes its life cycle so quickly.
Other beneficial nematodes may also control weevils but these 2 species
were most commonly found established in CT strawberry fields. There
is no point in applying beneficial nematodes in early or mid-summer since
few larvae are present.
Nematodes are living organisms and they can be killed if they are misapplied.
Order nematodes ahead of time and be ready to apply them through a sprayer
or irrigation soon after they arrive, refrigerating if delay is necessary.
Do not apply nematodes using a sprayer with a piston pump. Use clean
equipment, removing all screens finer than 50-mesh. Apply nematodes in
early morning or evening in a high volume of water to already moist soil,
pre-irrigating if needed. Apply another 1/4 inch of irrigation after application
to wash them onto and into the soil. Although references suggest rates
of several billion nematodes per acre, I found researchers and suppliers
recommended 250 (if banded in the row) to 500 million per acre, at a cost
of about $100 to 200 acre depending on volume and source. Green Spot says
their formulation requires lower numbers of nematodes but the cost ends
up about the same. Ironically, nematodes probably work best in the worst
weevil-infested fields. High populations of weevil larvae allow explosive
growth in nematode populations, while low populations of larvae may not
permit efficient nematode reproduction. Strawberry plants can recover their
vigor remarkably well if crown feeding has not occurred and diseases haven't
taken over the roots.
Root weevils cannot fly, so they infest new plantings by wandering into
fields from surrounding weedy and woodland vegetation, or in large numbers
from recently plowed, infested strawberry plantings. Even plantings several
hundred feet away can become generally infested as a result of mass migration
from plowed fields. A good rotation program with substantial distance between
strawberry fields can help to manage root weevils. Also, when turning under
old, infested strawberry plantings, it is critical to leave a row or two
at the perimeter of the field as a trap crop to protect other plantings.
Adult weevils will be intercepted in these rows before they leave the field
and thus lay their eggs where the larvae will not do any damage. At the
end of the season the trap rows should be turned under prior to planting
winter rye. Do not spray the trap rows as this may repel weevils and result
in more migration to other fields.
Some Beneficial Nematode Suppliers:
(Mention of pesticides and biological controls is for information purposes
only; no endorsement of materials or brands is intended. Always read and
follow instructions on the label.)