The black vine weevil (Otiorhynchus sulcatus) is found throughout the world and in some areas causes severe damage to commercial crops. With the increased use of container plants, it has spread from nurseries and now flourishes in our gardens.
Adult weevils are about 1/2 in (1 cm) long and black, with a number of lighter spots on the shell. The first adults emerge in early spring and continue to do so over a period of about two months. Although they are flightless, they are evidently excellent climbers, as larvae are often found in tubers of Pendula begonias hanging many feet above the ground.
The larvae, which are legless, are about 1/4 in (7 mm) in length. They are crescent-shaped, white or cream in color, with a light brown head. Pupation occurs in mid- to late winter in a smooth-walled cell in a tuber, in the potting mix, or in open ground.
The weevils enjoy a varied diet, with the adults feeding at night on the leaves of any of some 100 different plants, where they leave a characteristic notched pattern on the edges. But the larvae are the most destructive during late summer and fall, with their unobserved voracious attack on the roots and, in the case of begonias, on the tubers. The growing plant, under attack by these larvae, may wilt and not thrive, as the small roots so important to growth are being eaten.
It pays to check a wilting plant for any signs of infestation by knocking it out to check the roots rather than applying more water immediately. A dormant tuber infested by the weevil larvae may have anything from one small hole to many holes, or may even be just a husk where the weevils have dined out in style.
Prevention: The most sensible way to combat all pests and diseases is with prevention, and this is particularly so with the vine weevil, since most controls are very toxic.
Avoid growing begonias with plants such as cyclamen, which have a reputation as one of the weevils’ favorite foods. Check the root ball of any growing Tuberhybrida purchased or received during the season, and be vigilant for evidence of activity when repotting tubers. Potential hiding places, such as plant debris and loose boards, are best kept clear of the begonia growing area. It may be possible to catch the adults by looking in such places, but they are shy and nocturnal, so a flashlight search is the only way to find them.
Careful cleaning and inspection of lifted dormant tubers, with perhaps a dip in insecticide, will prevent the weevil larvae transferring from one tuber to the others in their storage area. When potting, fresh mix from a reputable source is recommended; the re-use of old potting mix should be avoided. As few as six larvae in a container can produce 6000 offspring in a season-thus, even one left in old mix can lead to problems.
Most controls are intended for attacking the larvae and are mixed with potting mixes or in the soil to give protection throughout the growing season. The active ingredient in such products is chlorpyrifos, a contact and ingested organophosphorus-be warned that these are toxic chemicals, and any leftover mix cannot be used on the vegetable garden.
Biological controls are also available in the form of nematodes. These microscopic creatures enter the bodies of the larvae, kill them, and continue to multiply, killing any new grubs that hatch during the growing season. The downside is their very short shelf-life and the large quantities they are marketed in, making it difficult for the home gardener to use them.