Controlling Lily Beetles

The lily leaf beetle (Lilioceris lilii) on a lily leaf.

There's a new insect pest in town that's wreaking havoc in flower gardens throughout the Northeast. The scarlet lily leaf beetle (Lilioceris lilii) was first discovered in Massachusetts during the summer of 1992, and has spread rapidly throughout most of the New England states.

The lily leaf beetle focuses its attention almost exclusively on two species of garden flowers: Lilium and Fritillaria. The pest eats and lays its eggs on any of the true lilies (daylilies are not affected) which means Asiatic, Oriental, Rubrum, Tiger and Trumpet lilies as well as Oriental Trumpets (Orienpets) and Turk's Cap lilies are all vulnerable.

Your first indication that this pest has arrived, will probably be a defoliated lily plant. Both the adult and juvenile phases are active feeders and they can do a lot of damage in a very short time. For this reason, you should observe your lilies carefully at least once a week, beginning as soon as the plants emerge from the ground. Adult beetles overwinter in the soil or in plant debris, and will begin laying their eggs early in the growing season. Since one female can lay up to 450 eggs, a concerted effort to curb the insect's reproductive cycle is important.

The adult lily leaf beetle is brilliant scarlet in color and has an elongated body with relatively long legs and long antennae. It measures just under a half inch overall. Though these adults do feed on the foliage, their primary concern is reproduction. If you see the adult beetles, it is very likely that you'll also be able to find egg masses on the undersides of the leaves. The eggs are very tiny and are laid in narrow, irregular rows.

The lily leaf beetle is at its most damaging in the larval phase. Hundreds of larvae may hatch at one time, and they begin eating immediately. Though the leaves are their preferred food, they will also devour buds, flowers and stems. Most feeding takes place under the leaves or at leaf nodes along the stem. The larvae are bright orange and would be relatively easy to spot if they didn't have such an effective and disgusting way of disguising themselves: As the larvae eat, they pile their excrement on top of themselves. This makes the larvae difficult to see (look for soft, brown masses), difficult to destroy (squishing the mass doesn't necessarily kill the larvae) and very unpleasant to touch (some gardeners wear rubber gloves). The larvae typically feed for 16 to 24 days and then enter the soil to pupate. Adult beetles will emerge 16 to 22 days later and continue feeding until fall.

Rose Rx is a ready-to-use form of neem oil.

The lily leaf beetle is currently under relatively good biological control (using parasitic wasps) in France and Switzerland. The University of Rhode Island has released one species of parasitic European wasp in two New England sites, and biologists are monitoring the results. Other parasitic wasps are being tested in laboratory conditions.

Until there is an effective biological control, gardeners have only a few options for dealing with this destructive pest. Hand-picking the adult beetles can be effective if you have only a few lily plants. But you need to be vigilant and FAST. The beetles are very quick and as soon as they sense movement, they will immediately drop to the soil level and lie on their backs, which makes them extremely difficult to find. Egg masses must be destroyed quickly—they hatch in just 7 to 10 days. The eggs are usually laid on the undersides of leaves or along the stem. They are very tiny and hard, so it takes some focused effort to make sure they've been crushed.

The larval phase is the most difficult to control. Hand picking is an option, but it requires almost daily patrols and very careful observation. Squishing the larvae is tricky as they're smooth and will tend to slip between your fingers. Yuck!

Larvae covered in a "fecal shield."

There are two topical controls that have proven to be relatively effective. The first is neem oil, a botanical insecticide made from the neem tree. Neem kills larvae and repels adults. It is most effective early in the season and must be diligently applied every 5 to 7 days. The larvae's "fecal shield" seems to provide it with some protection from sprays, so spray coverage must be heavy and complete. Late-season larvae seem to be somewhat resistant to neem.

The control that has proven to be effective for the lily leaf beetle is the insecticide Merit (imidacloprid). This is a systemic insecticide that is absorbed into the plant, which means one application usually lasts a few weeks. Merit is available in several formulations from Bayer, and their Rose and Flower Spray has been given high marks for effectiveness. It may be applied in the early season as a soil drench around emerging plants, or be used as a foliar spray. Again, early season treatments are critical for keeping populations under control. As with all insecticides, it is important to follow label instructions and avoid spraying when bees and other beneficial insects are active.