Heucheras, or coralbells, are known for their colourful purple, bronze, silver or burgundy leaves and their bell-shaped flowers. They're easy to grow and do well in light shade or full sun, as long as they're protected from afternoon heat. They're tolerant to drought and prone to few diseases. Not many insects attack them, either. However, if you have insect holes in your hueuchera, they're probably caused by the black vine weevil.
Black vine weevils are small black beetles about 1/4 to 3/8 inch long. They have short snouts, narrow heads and dull grey or brownish-black bodies. Their wing covers are well rounded and covered with small pits and fine yellow hair. In spite of having wings, the adults can't fly since their wing covers are fused together. Black vine weevils reproduce asexually, without needing a male to fertilise their eggs, which they lay in the dirt or leaves at the base of the plant. The C-shaped larvae are cream-coloured with brown heads and measure 5/8 inch when they reach full size.
Damage caused by adult weevils will appear on the lower leaves first, since the weevils don't fly and must crawl up from the dirt where they spend their days hiding. Look for notches and holes in the margins of the leaves, where the adults feed. Black vine weevils never chew holes in the centres of leaves. Damage caused by adult beetles isn't severe, but is very unattractive. Damage caused by grubs is more serious. Grub, or larvae, can kill heucheras by eating the roots and girdling the main stem. Symptoms of larva attack include yellowing or off-colour leaves and stunted growth. A severe attack can even leave a plant without any roots at all.
Adult beetles can be removed from your plants by hand, but grubs are harder to get rid of. Remove excess mulch and only water plants when necessary, because grubs do better when the soil is moist. Small, microscopic worms called nematodes are an effective way to control grubs. When they're added to the soil, they parasitise the beetles. Order them from your local garden supply store.
Heucheras are extremely hardy plants and, as long as you act before they dry out completely, they can even recover from the total loss of roots. If your heuchera is completely rootless, inspect it for weevils and then wash it off before repotting it in clean soil. Keep your plant quarantined in a shady spot for a few weeks and see if it recovers.
- University of Arkansas Plant of the Week: Coralbells
- The Primrose Path: An Introduction to Heuchera
- Penn State: Woody Ornamental Integrated Pest Management -- Black Vine Weevil
- University of Rhode Island Greenshare Fact Sheet: Black Vine Weevil
- Huecheraholic: Evil Weevils -- Is Vine Weevil Really That Bad