Weeding out the weevil

The black vine weevil, Otiorhynchus sulcatus, is one of the most commonly encountered weevil species in horticultural cropping systems because it feeds on over 200 different plant species. Black vine weevil is distributed throughout most of the northern portions of North America, and is primarily spread by the transport or shipping of infected greenhouse or nursery containerized plant material.

The larvae cause the most damage by feeding on plant roots, whereas adults feed on plant leaves resulting in mostly cosmetic damage. The leaf-feeding or leaf notching damage caused by adults, however, if extensive, may result in an economic loss by affecting marketability. Many trees, shrubs, and perennials are susceptible to attack by black vine weevil including astible, azalea, begonia, bergenia, cyclamen, epimedium, geranium, heuchera, hosta, rhododendron and yew.
 

The Cycle of Life

• Adult black vine weevils are 11/2 inches long, brown to black in color, with small patches of yellow to orange hairs on the abdomen.
• They also have a distinct snout (mouthpart), and elbowed or bent antennae.
• Black vine weevil adults cannot fly because the wing covers are fused together.
• Larvae are ¼ to ½ inches long, legless, white to cream-colored, with a brown head, and are covered with fine hairs.
• Older larvae are ‘C-shaped’ due to the thickening of the thoracic segments.
   

The life cycle consists of an egg, larva, pupa, and adult. Adult activity varies depending on geographic location, with adults, in general, present from spring to late-summer. Adults are active at night, and create distinct notches on the margins of plant leaves during feeding. All adult black vine weevil females are parthenogenetic, meaning that they do not have to mate to reproduce. In fact, no black vine weevil males have ever been found. Adults hide during the day under containers, in debris laying on the soil underneath benches, or in cracks in the soil surface. Adult females are capable of laying up to 300 eggs during their lifetime, which is usually up to 12 months. Eggs are laid in cracks and crevices in the growing medium surface or in the upper 1 to 2 inches. Eggs hatch and larvae initiate feeding. The rate of larval development depends on temperature with growing medium temperatures between 70 and 75ºF ideal for larval activity. Larvae feed on plant root hairs, larger roots, and may also girdle crowns, which inhibits the movement of water and nutrients to plant leaves. Furthermore, larvae will tunnel into the stems of certain plants after most of the roots have been destroyed. This leads to plant stunting, reduced plant vigor, leaf yellowing, and eventually plant death.

The larval stage can last up to 12 months. In general, there is one generation per year; however, in greenhouses, and depending on temperature, several overlapping generations with different life stages may occur. Black vine weevil overwinters as a larva within cells in the growing medium that migrate to the bottom of containers, but they have been reported to overwinter as an adult in areas with mild winter temperatures. Overwintering larvae eventually transition into a pupae stage from which adults emerge.
 

Where they attack and how to stop them

Newly transplanted plant material is highly susceptible to black vine weevil larval damage, due to the lack of an established root system. Damage varies depending on the number of larvae and plant size (age). In fact, one larva/plant has been reported to kill cyclamen, whereas three larvae killed a rhododendron plant. Established plants are more tolerant of black vine weevil larval damage compared to young plants because the root system biomass is greater thus allowing established plants to withstand feeding without suffering adverse effects on growth.

Scouting for black vine weevil adults involves looking under leaf debris or soil underneath benches, and checking for adult activity in the evening using a flashlight. It is also important to remove susceptible plants from containers and inspect for the presence of larvae that may be located on the outer edge of the root system or inside the root ball. Physical management strategies include: 1) collecting or removing adults by hand during the evening to reduce populations and before females lay eggs, and/or 2) placing sticky bands on the stems of plants, which restricts adult movement.

Drench applications of insecticides and biological control agents such as beneficial nematodes may be used to alleviate problems with black vine weevil larvae. Treatments should be made in spring (March through May) to kill larvae and late-summer through fall (August through November) to kill larvae after eggs have been laid. One of the most common strategies involves incorporating insecticides with the active ingredient bifenthrin into the growing medium, which may provide long-residual activity although this is contingent on the growing medium type. Late summer to fall applications are recommended over spring applications because larvae are larger in the spring. Furthermore, lower concentrations of insecticides may be used since larvae are smaller in late summer through fall. In general, young larvae are more susceptible to insecticide applications. Insecticides also labeled for use against black vine weevil include those containing the active ingredient imidacloprid. Producers must be aware of adult activity in order to time insecticide applications accordingly. It is also important to note that repeat applications are typically required. Always read the label of any insecticide to determine if it can be used (legally) against the black vine weevil.

Beneficial or entomopathogenic nematodes are applied as a drench to the growing medium where temperatures should be between 50 and 85ºF. The beneficial nematode species, Steinerema kraussei (sold as Nemays^® L) can tolerate growing medium temperatures of 41ºF, thus making this species effective in suppressing black vine weevil larval populations because black vine weevil larvae tend to be more active under cooler temperatures. Multiple applications may be required depending on the extent of the larval infestation and age of the larvae. Furthermore, it is important to irrigate the growing medium the day prior to making applications of beneficial nematodes to provide a moist environment that is conducive for survival. Another option is drench applications of the beneficial fungus, Metarhizium anisopliae (sold as Met52^®) to the growing medium. This fungus has been demonstrated to infect both larvae and adults of the black vine weevil.

Raymond A. Cloyd is professor and extension specialist in Horticultural Entomology/Integrated Pest Management at Kansas State University in Manhattan, Kan.