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Root weevils

Both the adults and larvae of root weevils can seriously damage seedlings, especially in coastal nurseries. Adults of the strawberry root weevil, Otiorhynchus ovatus, the rough strawberry root weevil, Otiorhynchus rugosostriatus, and the black vine weevil, Otiorhynchus sulcatus, all cause stem girdling. Less frequently, the small gray grass weevil, Trachyphloeus bifoveolatus, and Strophosoma melanogrammus girdle stems. Larvae of the strawberry root and black vine weevils are soil inhabitants. They feed on and seriously damage seedling roots (Figure 61). All root weevils have a similar general appearance and life history.

Hosts and damage

Adult weevils primarily affect young 1+0 container seedlings growing in greenhouses. Spruce seedlings seem to be preferred, but cedar, hemlock, larch, true firs, and pine are also attacked. Stem girdling occurs in a band about 1 cm wide, usually on the fleshy, uppermost part of the hypocotyl, and may resemble damage by Tehama bonifatella. The latter differs by the presence of a fine silk webbing at the soil line. Feeding damage occurs primarily in June and July before stems become woody, when seedlings are approximately 8 weeks old and 8-15 cm in height; adult weevils feed all summer. Damaged seedlings often occur one at a time, near the outside edges of styroblocks and at the perimeter of greenhouses. Presence of weevils may go undetected because they feed at night and hide during the day.

Black vine weevil larvae mainly affect container stock in greenhouses, where high humidities provide favorable living conditions for larvae. To date, container nurseries in coastal areas have sustained the most damage; Douglas-fir and hemlock appear to be favored host species. Larvae can seriously damage grafted stock, which is frequently kept in heated greenhouses for several years, enabling weevils to reproduce year round, or where 2+0 container rotation allows population build-up over 2 years. Usually one larva (Figure 62) is found per container cavity as early instars are cannibalistic. Larvae feed throughout the fall and during warm periods in winter. One larva can consume almost all the roots of a seedling and may girdle the stem just below ground line. Damage often goes undetected, because the dormant seedlings exhibit no shoot symptoms in the winter. Thus, often neither the presence of the weevil nor damage is evident until the stock is lifted in the spring.

Although larvae of the strawberry root weevil are primarily a pest of strawberries, they can damage many species of 2+0 bareroot conifers. Most infestations occur at the edges of 2+0 panels, where larvae normally occur in patches. Larvae develop in the top 25 cm of soil around host plants and can number as many as 3000 per m2. Larvae feed on the fibrous roots of seedlings, stripping most laterals and, in heavy infestations, girdling stems at the root collar (Figure 63). Again, damage may go unnoticed until seedlings are lifted, although seedlings may become chlorotic in the fall. Heavily damaged seedlings can be easily pulled from the soil. To date there has been no notable damage in 1+0 stock.

Life history (Figure 64)

The following generalized life history is based on outdoor, coastal conditions, where all weevil species pass through one generation per year; however, the timing and number of generations per year may be different in heated greenhouses. Overwintering occurs as larvae or adults. Oviposition by overwintered adults begins as early as May. Adults, which develop from overwintering larvae, appear about the end of May to early July. Both overwintering and emerging adults oviposit for about 7 weeks, and all oviposition ceases by early September. All adults are females, and generally egg-shaped, with short snouts, elbowed antennae, and stout, hard-shelled bodies. They are unable to fly. (Figure 65).

Black vine weevil adults are 8-11 mm long and black with scattered patches of distinctive yellow hairs; strawberry root weevil adults are 4-6 mm long, and dark brown to black and slightly shiny. The rough strawberry root weevil is intermediate in size and generally a dark, chocolate-brown color. Eggs are laid mainly beneath duff, mulch, or the soil surface and have a 10- to 20-day incubation period. Larval size varies according to adult size (Figure 66). Larvae are slightly curved and creamy white with brown heads. They have no legs. Overwintering larvae pupate in earthen cells during the spring and early summer, emerging as summer adults.

Management

Weevil control is aimed at killing adults or preventing them from laying eggs. Larvae, once established, are difficult to control. In bareroot and container nurseries, surveys for adult weevils should be conducted during May and June when the majority are emerging. Detection of weevils is difficult, but may be done using bait stations or pit-fall traps. Broad leaved plants such as rhododendrons and azaleas, which are preferred hosts for weevils, can also be used for weevil detection. Growers may detect weevil presence by watching for damage on potted specimens of these plants placed around the nursery site. Signs of fresh girdling on conifer seedlings in June and July will also indicate that weevils are present and actively feeding. If not managed, these adults will likely oviposit on the stock.

In bareroot nurseries, weevil boards have been used successfully to determine the length of the adult emergence period, the distribution of weevils in the nursery, and the effectiveness of control programs. A weevil board consists of a 30-cm length of a "two-by-four" which has been flagged with tape, numbered, and placed flat on the soil surface. These simple traps work because adult weevils feed on foliage at night and seek a cool, dark hiding place during the day. Best results are obtained when boards are placed in open areas because they provide an only source of shelter for weevils. More weevils are caught in 1+0 and fallow panels, than in 2+0 panels.

If surveys detect adult weevils; or if nurseries experienced serious problems with them during the previous growing season, control is advisable. Insecticidal sprays should be applied around the end of May - i.e. as soon as adults become active. A second application about 4 weeks later will control late-emerging adults before they oviposit. Disking bareroot panels in April and May will kill fragile pupae. Removing weeds, which act as reservoirs for adults, will help lower endemic weevil populations.

Selected References 

Garth, G.S. and C.H. Shanks, Jr. 1978. Some factors affecting infestation of strawberry fields by the black vine weevil in Western Washington. J. Econ. Entomol. 71: 443-448.

Gerber, H.S., N.V. Tonks, and D.A. Ross. 1974. The recognition and life history of the major insect and mite pests of ornamental shrubs and shade trees of British Columbia. B.C. Dep. Agric., Bull. 74-13, Victoria, B.C.

Nielsen, D.G., M.J. Dunlap, and J.F. Boggs. 1978. Controlling black vine weevils. Amer. Nurseryman 147: 12, 13, 89-92.

Summary

Root weevils

Principal, locally grown hosts

Host age and season when damage appears

 

Nursery type and location

 

 

 

     

Bareroot

 

Container

 
 

Age

Season

Coastal

Interior

Coastal

Interior

Adults
All species, but prefer spruces

1+0
2+0 Transplants

Late summer through fall

Yes

?

Yes

Yes

 

1+0

Early summer

?

?

Yes

Yes


Figures

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Click on this image to see a larger version     Figure 61. Container-grown 1+0 spruce girdled by adult root weevils.

 

 

 

 


Click on this image to see a larger version     Figure 62. Black vine weevil larva on container-grown Douglas-fir.

 

 

 

 

 

 


Click on this image to see a larger version     Figure 63. Strawberry root weevil girdling of bareroot spruce.

 

 

 

 

 

 


Click on this image to see a larger version     Figure 64. Life history of root weevils (one generation per year).

 

 

 

 

 


Click on this image to see a larger version     Figure 65. Adults of the black vine weevil (top), rough strawberry root weevil (middle), and strawberry root weevil (loser). Note size and colour differences.

 

 

 


Click on this image to see a larger version     Figure 66. Larvae of the strawberry root weevil (left) and black vine weevil. Note the larger size of the latter.