PSI: Using beneficials to stop black vine weevil

Meet the suspect

The black vine weevil, Otiorhynchus sulcatus, is one of the most common weevil species encountered in horticultural cropping systems, and feeds on more than 200 different plant species. Black vine weevil is distributed throughout most of North America, and is primarily spread by the transport or shipping of infected containerized greenhouse or nursery plant material. Larvae cause the most significant 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, if extensive, may result in an economic loss by affecting marketability. Many perennials, annuals and shrubs are susceptible to attack by black vine weevil including astible, azalea, begonia, bergenia, cyclamen, epimedium, geranium, heuchera, hosta, rhododendron and yew.

Adults are 1½ 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 a quarter to a half-inch 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.
 

How they do damage

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 or age. In fact, one larva per plant was reported to kill cyclamen, whereas as few as three larvae can kill a rhododendron plant. Established plants are more tolerant of black vine weevil larval damage compared to young plants. This is because the root system biomass is greater, thus allowing established plants to withstand feeding without suffering adverse effects on growth.
 

Stopping the pest

The primary strategy used to manage black vine weevil is to apply broad-spectrum insecticides, such as bifenthrin (Talstar), to kill adult females before they lay eggs. However, an alternate strategy is to target the larval stage in the growing medium by using either beneficial (entomopathogenic) nematodes or fungi. Beneficial nematodes are microscopic roundworms that, in the infective juvenile stage, enter the larval stage of insects through natural openings such as the mouth, anus or breathing pores (spiracles). Once inside the larva, the nematode releases a bacterium that attacks the midgut by producing protein destroying enzymes, resulting in septicemia within 24 to 48 hours. The nematodes reproduce inside the black vine weevil larvae and eventually release a new generation of infective juveniles that disperse and search for additional larval hosts. The nematodes require moisture for survival. Beneficial fungi infect the insect skin by enzyme degradation or mechanical pressure. Once inside the insect, the fungus colonizes the body cavity where blood flows, which negatively affects the integrity of the insect immune system. Death may occur after three to 14 days of exposure.
 

Treating with beneficials

Beneficial nematodes are applied as a drench to the growing medium where temperatures should be between 50° and 85°F. The beneficial nematode species, Steinerema kraussei (Nemays L) can tolerate growing medium temperatures of 41°F. This makes the species effective in suppressing black vine weevil larval populations because the larvae tend to be more active under cooler temperatures. Multiple applications may be required, although the frequency of application is contingent on the extent of the larval infestation and larval age. Furthermore, it is important to irrigate the growing medium the day prior to making applications of beneficial nematodes to provide a moist environment conducive for survival. Beneficial nematodes have demonstrated effectiveness against black vine weevil larvae under greenhouse conditions.

Beneficial fungi, such as Metarhizium anisopliae (Met52), may be applied as a drench to the growing medium. This fungus infects the larvae of black vine weevil. In fact, studies have demonstrated that M. anisoplieae may persist in many different types of growing medium. However, the type of growing medium may impact efficacy. Some studies have reported 85 to 100-percent control of black vine weevil larvae with a drench application. Furthermore, combining beneficial nematodes and M. anisopliae may result in synergistic activity, leading to better control than using either agent alone. Thorough saturation of the growing medium is important so the nematodes and fungi make contact with black vine weevil larvae. It should be noted that multiple applications may be warranted in order to obtain high mortality rates over an extended period of time as there may be instances where declines in fungal persistence occur. Therefore, it is important to read the label of the given product (whether it be associated with a beneficial nematode or fungi) to ascertain the frequency or intervals of application. Overall, the use of either beneficial nematodes or fungi may be a viable option to manage black vine weevil in horticultural production systems.

Raymond Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.