Get Growing: Leaping lily leaf beetles!

  • Mating adult lily leaf beetles in Amherst. Photographed June 8, 2017. Tawny Simisky, UMass Extension

For the Gazette
Published: 5/13/2020 1:25:46 PM

Springtime brings us many gifts in the garden, as perennials and bulbs break their dormancy and send tender green shoots forth from the earth. But here in New England, warmer weather coaxes more than waking plants from the earth; it also rouses the dreaded lily leaf beetle from its winter slumber. In mid-April, these bright red beetles start emerging from the soil to begin their destructive life cycle. If you have lilies in your garden, chances are you have encountered the Lilioceris lilii.

The lily leaf beetle is the subject of UMass Extension Service’s newest installment in its fascinating and informative InsectXaminer series. Produced by Tawny Simisky, Extension entomologist, the video documents the dastardly doings of the lily leaf beetle in all its phases of development.

Like many of our garden pests, the lily leaf beetle is not native to North America. According to Simisky, the beetle is a native of Eurasia and showed up for the first time in Montreal in 1943, probably in a shipment of bulbs from Europe. It made its first documented American appearance in Cambridge, Massachusetts, in 1992 and since then has spread throughout New England.

As Simisky explained to me, “true” lilies and fritillaria are the beetle’s two most significant host plants. As some readers may know, a host plant is one that both feeds a mature insect and provides it with a place to lay eggs and feed its larvae. Milkweed, for example, is a host for monarch butterflies. True lilies are in the genus Lilium that include hundreds of species including tiger lilies, Asiatic and Oriental lilies and martagon lilies. The beetle has no interest in daylilies, which are of a different plant order altogether, despite their name. Lily leaf beetles will also feed on Solomon’s seal, lily of the valley, potatoes and a few other plants, but they do not lay eggs on those plants and thus do them far less damage.

Simisky said she chose to highlight the lily leaf beetle now because this is the best time for gardeners to address the problem. As beetles emerge from the soil, usually in the vicinity of the host plants but not necessarily right next to them, they head for their hosts and begin feeding on them. “They tend to be found hidden in the shaded areas of plants, so you might not notice them at first,” said Simisky.

Once they start eating, they begin mating. According to Simisky, adult lily leaf beetles can readily be seen mating on host plants such as true lilies. (There’s a terrific image on the video of the beetles doing their thing.) The females lay their eggs — small, tan ovals — on the undersides of leaves, usually in lines of from two to 16 eggs. One female typically lays between 250 and 450 eggs per cycle. The eggs take four to eight days to hatch, turning orange and then deep red in color as they prepare to hatch.

When the eggs hatch into larvae, things get ugly. To protect themselves from predators, the larvae coat themselves in their own poop — the technical term is “frass” — creating a “fecal shield.” Basically, the larvae look like dark-brown, slimy blobs of poop with hungry mouths that devour growing lily plants as fast as they can. After a feeding frenzy of from 16 to 24 days, the larvae pupate, spinning white cocoons around themselves as they change into florescent-orange pupae. Around 20 days later, the adult beetles emerge from their cocoons and start chowing down once again. They will continue to eat until the weather turns cold, when they will burrow into the ground for the winter, preparing for next year’s feeding and breeding cycle.

So what’s a lily-lover to do? If you have a manageable number of plants, hand-picking is probably your best bet. Given their bright-red color, the beetles are easy to spot when they wake from their long winter nap. Unfortunately, said Simisky, they’re not so easy to catch because they can fly. But now is the best time to get rid of them. Simisky advises catching as many of the beetles as possible and dropping them into soapy water. You should also examine the undersides of leaves and remove their eggs. This strategy interrupts the breeding cycle so that you will have fewer poop-coated larvae to pick off.

If you have a large number of lilies that are heavily infested, you might resort to chemical pesticides. These include products containing neem oil, a botanical insecticide, as well as products containing spinosad, a microbial insecticide. But spinosad is toxic to pollinators when it’s wet, so never apply it to blooming plants. If you go the chemical route, follow the label directions carefully.

Not surprisingly, the animal kingdom has its own ways of dealing with lily leaf beetles. Researchers at the University of Rhode Island and their collaborators have found three parasitoid wasps that are natural predators of the lily leaf beetle. Parasitoids are insects, often wasps, that lay their eggs either on or inside the body of the host insect, usually at the larval stage. As the eggs hatch, the parasitoid larvae feed on the host insect and kill it. The lily leaf beetle’s fecal shield actually may attract these parasitoids to the larvae.

But in the meantime, until these beneficial wasps are introduced to our area, you’ll need to put on your gloves and assume the predator role yourself.

For more information about the beetles and how to control them, here is a link to UMass Extension’s fact sheet on the lily leaf beetle:

And don’t forget to check out the InsectXaminer video: It delivers!

Mickey Rathbun, an Amherst-based lawyer-turned-journalist, has written the “Get Growing” column since 2016.
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