Gardening with Weevils

Greetings, Bug Fans,

Members of the almost-universally-vegetarian weevil families (the straight-snouted Brentidae, the Bruchidae bean weevils, and the true weevils, Curculionidae) are infamous for their interest in our food crops and gardens. The BugLady, who doesn’t garden and has nothing on her property that even remotely resembles a horticultural planting, loves the cut of their weevilly jibs, and although most will linger in the Limbo of her “x-files” until she serendipitously IDs them, she photographs them whenever she gets a chance. There are about 3,000 species of true weevils in North America (with more than 40,000 described species worldwide, they’re the 3rd largest animal family) (numbers one and two are insects, too), 150 species of bean weevils, and 150 kinds of straight-snouted weevils. Many weevils, like today’s duo, are attached to specific host plants.

Hollyhock Weevil

The BugLady enjoys receiving small packages from BugFan Molly, because they often contain carefully-wrapped insects, offered for identification and awe. The Hollyhock Weevil arrived in that fashion, in one of those compartmentalized weekly pill organizers, with several insects sorted into the various sections. The BugLady wouldn’t otherwise have known about the HW, because she’s one of about 7 people on the planet who are “hollyhock-challenged.”

  • hollyhock weevil 1

(The BugLady tries to avoid featuring pictures of insect corpses, so here’s what they look like when they’re alive and kicking.)

The HW, in the family Brentidae, is known scientifically as Apion (Rhopalapion) longirostre. It started out in the genus Apion (the pear-shaped weevils) and later was moved into its very own genus Rhopalapion. In 2014 a proposal was made to the International Commission on Zoological Nomenclature to solidify it as Apion. Taxonomists will duke it out, but, as Professor Keeton said long ago, the animals themselves know who they are.

HWs are conquering the world. Native to Asia and southeastern Europe, they continue to spread through the Middle East and Europe (they’ve fetched up in the United Kingdom). They arrived in the state of Georgia, U.S.A., in 1914, and they now inhabit large portions of North America. The tiny larvae probably hitch a ride in hollyhock seeds.

In order to find the HW, you have to set your focus set on “mini”—they’re about 4mm long (1/8”) including the snout (rostrum). The female’s rostrum is almost the length of her body, but the male’s is shorter. Antennae are located about half way down the rostrum, and mouthparts are at the rostrum’s end. Adults do have wings and can fly.

HWs are promiscuous, and a male will often guard a female to discourage other males. Here’s a pair in happy embrace.

The two different lengths of the rostrum is an example of sexual dimorphism (two forms). Like the male, she uses her mouthparts to feed on the buds, seeds, and vegetation of hollyhocks, chewing holes in the leaves and moving up the stalk when the flower buds appear. But she also uses her mouthparts to excavate deep channels into the flower buds where she lays her eggs, one at a time (her rostrum is smoother than his). When her eggs hatch, the larvae feed on the seed embryo, eating the seed from within, eventually pupating in the seed head. Adults emerge in late summer (some may wait until the following year), but they drop down into the duff/soil around the hollyhock and spend the winter there, emerging in spring. The afflicted hollyhock produces normal flowers but little seed—not a good situation for a short-lived perennial that needs to reseed itself.

Iris/Flag Weevil

Although the Iris/Flag Weevil (Mononychus vulpeculus) is in a different weevil family (Curculionidae), parts of its biography are very similar to the HW’s (vulpeculus means fox cub, and there’s a story there somewhere, but the BugLady couldn’t find it). The BugLady found this chunky beetle feeding, uncharacteristically, on a daisy fleabane (and several on-line photos show them on daisies). Nota bene: daisy fleabanes are worth keeping an eye on, as this photo essay demonstrates.

  • iris weevil 1

The iris weevil is native, found in eastern North America. At 1/5 of an inch long, it’s a giant compared with the HW. As its name suggests, its primary host is the wild blue flag iris (Iris versicolor) though it also feeds on purple (but not white) Siberian irises.

Ms. Iris weevil chews into the iris flower’s ovary and deposits her eggs within. The larvae are rudimentary. Let 20th century entomologist/limnologist James G. Needham tell it: “The well-grown larva is very degenerate, having only the merest rudiments of antennae, eyes, optic lobes and salivary glands.” Larvae chew a hole through a row of seeds, eating seeds and seedpod tissues and leaving behind a trail of silk. The larvae eventually pupate in the seedpod and emerge when the pod splits in fall. Like the HW, the iris weevil has one generation, visible when their flower-of-choice is in bloom, and out of sight below the plant for the rest of the year.

Adults typically feed on flowers, seeds, and pods, and their feeding causes scars on the pods. The presence of iris weevils, both larvae and adults, is not un-aesthetic, and it doesn’t affect the health of the plant—irises grow back each year from a rhizome—but they’re a problem for people who breed irises for seed.

Needham, more than a century ago, was the first to study the life cycle of the iris weevil completely. What impressed him was the very fast growth of the larva (it has only the period from the time of flowering until the pod bursts to grow from egg to adult); the fact that there were only three instars (four to six is more typical); and the fact that the larval tissues are not completely destroyed during metamorphosis. Instead, Needham believed that the reserve tissue and fat were used to continue building organs/tissues in the adult that were not quite “done” when the adult emerged, and they were used to nourish the adult during its long winter hibernation.

Like Butterflies? The Riveredge Nature Center Butterfly Count is Saturday, June 27. There’s a butterfly review at 8:30 AM; the count starts at 9:30 AM, and it ends at 3:30 PM; you can spend the whole day or just part of it. Whether you’re a beginner or can call the skippers at 50 paces, your help is needed for this citizen science project that’s more than a quarter-century old. Riveredge is located between West Bend and Port Washington. To join the crew or for driving directions, please contact Mary at mholleback@riveredge.us or 262-416-1224. Binoculars are helpful; sturdy shoes a must, and BYO lunch if you’re planning to make a day of it. The event is free, but a $5 donation would sure be appreciated.

 
The BugLady