Milkweed Weevils

 If you grow milkweed for monarch butterflies, you need to know about milkweed weevils.

Milkweed weevil on milkweed bud (Beatriz Moisset) CC BY-SA 4.0

Like many other gardeners, I have researched and planted native milkweed species for my region to help offset the habitat loss of monarch butterflies (Danaus plexippus). Unfortunately, milkweed plants (Asclepias syriaca) are the food of choice for more than just butterflies.

Evil weevils

Milkweed weevils are tiny black specks that damage milkweed plants, making them distasteful or even inedible to monarch larvae.

There are tens of thousands of weevil species around the world. They all cause trouble. Boll weevils are estimated to have cost the cotton industry more than $100 billion over the past century, with inflation taken into account. On the other hand, some weevils are being employed to eradicate invasive plant species, so maybe they’re not all bad. And maybe the weevils consider monarchs to be pests.

Milkweed weevil species

There are two milkweed weevils: regular milkweed weevils (Rhyssomatus lineaticollis), which prefer the leaves of common milkweed, and stem milkweed weevils (R. annectans), prefer milkweed stems food and egg-laying.

Milkweed stem weevil (Judy Gallagher) CC BY 2.0

Milkweed weevil description

All weevils are small gray or black beetles. When I say small, I mean less than one-quarter of an inch. They may look like nothing more than specks. Get closer to see that they have long snouts called rostrums. Within those snouts are chewing mouthparts that take a significant bite out of buds, seeds, and stems.

Like many other weevils, these evil twins have elbowed antennae with knobs on the ends, hard-scale bodies, and those snouts. Milkweed weevil larvae have tiny antennae, some scales, and no legs.

Milkweed weevil behavior

Milkweed weevils are nocturnal. They start eating tender new leaves and then move towards the buds and stems, where they lay eggs. They may also sever small petioles as they feed. When threatened, these pests play dead. Don’t be fooled. As soon as you turn your back, they’ll return to feeding and damaging your milkweed.

Managing milkweed weevils

The first step in any pest control problem is identifying the culprit. And milkweed plants are not without defenses of their own. As a milkweed weevil chews on a leaf, the plant starts oozing sticky, caustic latex. In some cases (pictured below), the plant can kill the pest without any help from gardeners.

Milkweed stem weevil killed by latex from milkweed plant (Judy Gallagher) CC BY 2.0

Remember those boll weevils we mentioned above? Well, not too long ago, some researchers were trying to develop a pheromone trap for them when they discovered that milkweed weevils were more attracted to the traps than the intended victims. So, there are pheromone traps for milkweed weevils. You can also knock them into a container of soapy water.

Some people say you can inject certain nematodes into the stems of plants infested with milkweed stem weevils if you’re into that sort of thing, but I couldn’t find any science to back up the claim, and they never mention which nematode. [There are more than 25,000 species.]

Endangered milkweed

Milkweed stem weevils are partly responsible for pushing one type of milkweed toward extinction. Mead’s milkweed (Asclepias meadii) was common throughout the American tallgrass prairie. It now occurs only in sparse areas of Illinois, Iowa, Kansas, and Missouri. Of course, the weevils aren’t the only cause of this threat. It’s us, too. Agriculture, invasive plants, and the over-development of prairie grasslands have also contributed. And oleander aphids.

Endangered Mead's milkweed (Jason Sturner 72) CC BY 2.0

Check your milkweed plants regularly for those tiny black specks. And remember, if you are planting milkweed for monarch butterflies, be sure to plant native varieties, not tropical varieties. It matters a lot if you’re a monarch.

Monarch caterpillar feeding on common milkweed bud (Courtney Celley/USFWS) CC BY 2.0