As chilly nights of October arrive, the perennial onslaught of insects invading our homes begins. Last week I stumbled into the bathroom for a morning shower and was greeted by a rambunctious weevil strolling across the sink. This impressive beast is commonly known as the vine weevil or black vine weevil. While some weevils like the boll weevil have more notoriety, this little rascal is not necessarily the lesser of two weevils. Black vine weevil is a serious pest of ornamental plants worldwide. It eats more than 100 species of annual and perennial herbaceous plants, vines, shrubs, and trees. It is a serious pest in nurseries, greenhouses, and landscapes.
The adult weevil is a curious creature indeed. To fly, most beetles open their hardened outer wings and spread membranous wings beneath. However, the outer wing covers of the black vine weevil are permanently fused together making flight impossible. This pest gets around by foot or transport by unwitting humans.
Most of the animal life on this planet procreates through the union of males and females. Not so with black vine weevils. This is a species where males are unknown. By a reproductive process called parthenogenesis, female weevils beget only daughters also capable of reproduction without the assistance of males. An ancient association with a strange bacterium called Wolbachia likely facilitated this unusual reproductive strategy.
Adult black vine weevils are quite common in eastern landscapes where they hide on the earth by day and climb plants to eat leaves by night. As they feed, they remove chunks of foliage on the edges of leaves. This injury is commonly referred to as leaf notching and it is highly diagnostic for the presence of black vine weevils and a few close relatives. Females deposit eggs on the soil and from the eggs hatch legless grubs with a penchant for eating roots. After consuming roots, weevil grubs may strip the tender bark from the stem of a shrub. Both types of injury may cause plants to die.
Rhododendrons, azaleas, yew, astilbe, and heuchara are among the favorite foods of black vine weevils in my yard. Infestations of black vine weevil can be mitigated by reducing excess soil moisture in June and July when adult weevils lay eggs onto the soil. Moist soil supports viability of eggs so watch your irrigation and don’t use excessive mulch in flower beds. Weevil grubs can be controlled with applications of entomopathogenic nematodes. These tiny roundworms can be purchased commercially, mixed with water, and applied to soil where grubs are present. The nematodes enter the body of the grub and release a deadly bacterium that brings a painful end to these nocent creatures. You may also purchase insecticides to be applied directly to foliage or to soil beneath the plant. Systemic insecticides are transported from soil to leaves via the plant’s vascular system. Systemic insecticides or those applied to foliage will poison adult weevils as they feed. Normally, I recommend returning insect visitors to the wild. Black vine weevils garner less compassion and earn a “thumbs down.” I recommend disposing of them permanently.
To learn more about black vine weevil and entomopathogenic nematodes, please visit the following web sites: