Yews in Connecticut
Richard S. Cowles
Conn. Agric. Expt. Station, Valley Lab.
P. O. Box 248
Windsor, CT 06095
INSECT PROBLEMSBlack vine weevil, Otiorhynchus sulcatus
The larvae of this weevil feed on roots from July through May. The small, legless grubs devour the small roots and progress to chewing the bark from the larger roots, often girdling them. The tops of girdled plants first turn yellow, then brown, and the severely injured plants die. Large larvae overwinter, then pupate and emerge as adults from May - July. The 1/2" long adult weevil is black, with a beaded appearance to the thorax and has scattered spots of yellow hairs on the wing covers. Only females are known. Adults are flightless and feed nocturnally for 3 - 4 weeks before being able to lay eggs, but they then continue to lay eggs until early October, overwinter, and start laying eggs again the next spring. During the day, adults hide under dropped needles or soil under shrubs, or within the center of large, dense shrubs. Notching by adults of needles can be unsightly, but damage to the root system of yews is much more important, often making the plants unmarketable. Infested plants or plants with damaged root systems often die following harvest, making replacement at retail centers or residential sites necessary.
Of the yew production in Connecticut, nearly all fields are infested with black vine weevil and nearly 50% are unmarketable due to root feeding by black vine weevil, resulting in an annual loss in excess of $1,000,000 per year. Not all growers spray to control this pest. Out of the 9 growers interviewed, 7 applied insecticides to control black vine weevil. One of these growers applies 14 insecticide applications through each growing season, 10 to kill black vine weevil adults. Two growers (with less than 5 acres apiece) never spray for black vine weevil because the populations never have become damaging in their fields. One grower (50 acres) has quit spraying to control black vine weevil adults because the sprays had become completely ineffective.
Insecticides usually are applied as foliar sprays with a hydraulic sprayer, boom sprayer, or mist blower when there is adult feeding and before egg laying starts. The usual timing for these foliar sprays is during May, June and July at 2- to 4- week spray intervals, which theoretically should break the life cycle (Hanula 1988). However, not all adults feed on foliage each night, so residues must remain effective for several days. Some products (acephate in particular) have too short a residual to be effective (Nielsen and Montgomery 1977, Cowles 1996). The optimal strategy is nighttime spraying when adults are exposed. However, residential development adjacent to growers' fields limits night spraying because of noise complaints.
Insecticide resistance is common for organophosphates, carbamates, or pyrethroids (Nielsen and Montgomery 1977, Cowles 1996). Furthermore, knock down and recovery following pyrethroid exposure is common (Cowles 1996). These weevils are flightless and parthenogenic, which leads to locally adapted, insecticide-resistant strains. Therefore, one field may have organophosphate resistant populations, and a field a few miles away may have an organophosphate sensitive population. Growers try to alternate sprays of different chemical classes (making all the materials listed below of potential value for resistance management) to avoid insecticide resistance development. Black vine weevil's parthenogenesis allows a resistant individual to found a clonally resistant population of offspring. Combined with high fecundity (averaging 250 ? 600 eggs per adult), resistance development allows rapid exploitation of resources once they no longer are restrained by chemical control. In the most intensively managed fields, weevils may be resistant to all of the major classes of insecticides, and some growers resort to tank mixes of pyrethroid + organophosphate or pyrethroid + carbamate insecticides.
Insecticide labels available to nursery growers often include use rates based on quantity of insecticide per 100 gallons, rather than setting a maximum limit on a per-acre basis. This can lead to extraordinarily high application rates of many products. For example, the highest field crop use rate for acephate is 1 lb per acre, while nursery growers may apply as much as 7 lb a.i. per acre per application.
Available Chemical Control
Acephate (Orthene) 70SP. The Orthene label suggests a use at 1 lb of product per hundred gallons. Growers may apply 100 - 1000 gallons per acre, or 0.7 - 7 lb/Ac of a.i.
Azinphos-methyl, (Guthion) 2S or 50WP. The label rate is 1.5 - 2 pints/100 gal of the 2S formulation. With a range of 100 - 1000 gal/Ac, this translates to 0.375 - 5 lb/Ac of a.i.
Bendiocarb (Turcam 76W). The label suggests a use rate of 1.3 lb of product per 100 gal. With a range of 100 - 1000 gal/Ac, this translates to a range of 1 - 10 lb/Ac of a.i. Turcam is the only product currently available that can reliably provide excellent control of black vine weevil adults, especially when tank mixed with pyrethroids.
Bifenthrin (Talstar 0.67 F). This product is used at a rate of 20 fl. oz. per acre, or 0.1 lb a.i. per acre, with a limit of 0.6 lb. a.i./Ac per year.
Fluvalinate (Mavrik 2F). This product is applied at a rate of 10 fl. oz. per acre, which translates to 0.16 - 1.6 lb/Ac of a.i.
Phosmet (Imidan) 70WSB. This product is used at a rate of 0.75 - 1 lb/100 gal. With a 100 - 1000 gal/Ac range, this translates to 0.5 - 7 lb/Ac of a.i. Imidan was not applied in any fields during 1999.
Permethrin (Astro) 3.2E. The labelled rate for this product is 8 fl. oz. per 100 gal, or a range with 100 - 1000 gal/Ac of 0.06 - 0.625 lb/Ac of a.i. Permethrin is limited to 2 lb a.i. per acre per year.
AlternativesCryolite (Prokill Cryolite 96, Gowan Cryolite Bait [10 G])
The bait formulation has not been available in the northeast. Bait formulations of insecticides appear to be highly toxic to carabids (RSC, unpublished data). The sprayable formulation does not stick well to surfaces, and also is abrasive to spray equipment. However, because it requires ingestion it may be selectively toxic to black vine weevil. No cryolite was applied in 1999.
Oil (SunSpray Ultrafine Oil, Lesco Horticultural Oil)
Oil on yew can be applied at a 1 - 2% concentration (v:v). This translates to use rates of 2 - 20 gal/Ac of product. As a non-residual contact insecticide, oil may only be useful for controlling black vine weevil adults during the brief interval following peak emergence when they feed intensively and are exposed throughout Taxus foliage during the day (mid-June). Alternatively, oil can be used to kill weevils knocked down by pyrethroids before they have a chance to recover. Oil can be somewhat phytotoxic to the Taxus x media variety 'Brownii', but is not apparently toxic to varieties 'Densiformis', 'Hicksii', or 'Tauntoni' (RSC, unpublished data). Summer use of oil may control Fletcher scale, too, so oil may be able to replace a tank mix of two other insecticides, and at lower cost.
Treating the soil with insect pathogenic nematodes (Steinernema carpocapsae, S. feltiae, or Heterorhabditis bacteriophora), in May to control overwintered larvae and pupae, or in late July through mid-September can result in dramatic weevil population reductions in strawberry plantings (Cowles 1997). Other less available species of nematodes (such as H. megidis) may also prove useful. Whether this option can be made to work well in nurseries remains to be investigated. Field-grown nursery production, and non-irrigated sites in general have been viewed as difficult sites for applying insect pathogenic nematodes. The lack of irrigation and the cool soil temperatures at the time large black vine weevil larvae are present may limit host finding by nematodes and nematode reproduction (Evenhuis, Rutherford et al. 1987, Georgis and Gaugler 1991). Barriers to adoption of this method are the lack of research data from field-grown nursery stock demonstrating effectiveness, and the poor quality control, availability, and cost of these nematodes. Ten acres were treated with insect pathogenic nematodes in 1999.
Carabids, or predatory ground beetles, are also implicated in maintaining low black vine weevil populations (Evanhuis 1982, Crook and Solomon 1996). Unfortunately, carabids may be more susceptible to the insecticides currently used in foliar sprays than are the weevils (Çilgi et al. 1996, Obadofin 1977). Therefore, foliar sprays may inadvertently release black vine weevils from biological control by carabids. There is a pressing need to discover foliar insecticides or spray strategies that will selectively kill weevils rather than carabids, and to find other cultural practices that can help to increase the effectiveness of these predators.
Post-harvest control practices
A post-harvest Dursban 4E root ball disinfestation dip procedure is registered but is not used by growers. The reasons for not using this procedure are 1) high cost (chemicals, labor, and equipment), 2) requirement for specialized dipping tanks, 3) great potential for phytotoxicity, and 4) hazards to workers.
It is possible that insect pathogenic nematodes can be applied to replace the need for an intensive adulticide program for control of black vine weevils. In addition to nematodes, several components of a multifaceted research program (Cowles and Abbey 1999) may make elimination of organophosphate and carbamate foliar sprays possible. These components include (1) a preplant bifenthrin root dip procedure, to potentially protect the root system of yew liners for 2 - 3 years following transplanting, (2) adoption of selective insecticide use to suppress black vine weevil adult populations; this will involve measuring the mortality of black vine weevils vs. carabids with conventional insecticides vs. oil, azadirachtin and cryocide treatments, and (3) conservation of carabid populations through ground cover management in the driveways within nurseries.
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