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:
The Green Spot: 603-942-8925 or www.shopgreenmethods.com
Griffin Greenhouse Supplies: 978-851-4346 or www.griffins.com
Integrated Biological Control Systems: 888-793-4227 or www.goodbug-shop.com
Koppert Biologicals: 800-928-8827 or www.koppert.com
(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.) 7/02
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