The world is full of root weevils. That’s a scary thought, isn’t it? There are strawberry root weevils, rough strawberry weevils, lilac root weevils, citrus root weevils, black vine weevils, and the ominously-named hairy spider weevil.
But that’s not all.
There are over three thousand varieties of weevils in North America alone, and more than 95,000 varieties the world over. Almost all of them are damaging pests for gardeners, farmers, and other growers.
What Are Root Weevils?
Root weevils (Otiorrhynchus spp.) get their name from the hidden damage young weevils do to the roots of their host plants. The adult form is known as snout beetles because of their long snouts. Snout beetles use their snouts as weapons – shooting projectiles through them at enemies.
They also use their snouts to chew the leaves of your plants. Root weevils have hard, beetle bodies and six legs, but other than that, they vary quite a lot in appearance depending on the species.
Most root weevils in North America are black or brown, with a hard, rounded body. They have distinctive, beetle wing covers (called elytra) on their backs. But, in weevils, the elytra are actually fused together, so they never open up into wings.
Adult weevils can’t fly, fortunately. They tend to stay relatively localized; living, feeding, mating, and dying in one area. Root weevils are a frustrating pest for gardeners – especially because there are so many varieties of weevils to attack your plants.
The Life of a Weevil
Root damage is caused by immature weevils. Root weevil larvae are small, legless, white grubs. They have dull-orange heads and small, C-shaped bodies. The larvae over-winter in the soil – alternating between a sort of hibernation and voracious feeding. These larvae feed on nearby roots, causing plenty of damage.
Early in the spring, weevil larvae complete their development and emerge from the soil as adults. Once their grown, root weevils don’t feed on roots anymore. They’re ready to devour the leaves of your plants now.
Adult weevils chew notches on the leaves of plants. They feed at night, safe from the prying eyes of gardeners, chickens, and other predators. During the day, adult weevils hide in the soil at the base of their host plant. The adults only live for a few months, rarely lasting until autumn. Most of the root weevil’s life is spent in the larval stage.
Not all weevils mate. In fact, there are some root weevils that reproduce asexually. The strawberry weevil is one such exclusively female species. Other species, like the lilac root weevil, produce male and female weevils and mate soon after the adults emerge from the soil.
Female weevils lay their eggs in late spring. They squeeze the eggs into cracks of soil and a few days later, they hatch. New weevil larvae move into the soil – spending the next 9-10 months chewing on root systems and growing into adults.
Identifying Weevil Damage
It can be hard to catch root weevils early on. After all, how often do you dig up plants and inspect their roots?
Usually, the first sign of weevil damage is on the leaves of a plant. Remember, adult root weevils love to munch the leaves of their favorite host plant. When they eat, weevils tend to chew notches into the edges of leaves.
Some people describe the notches as crescent-moon shaped, other’s a D-shaped, no matter how you describe it, your plants are being damaged. The damage adult weevils can do is primarily cosmetic, but it can provide a tell-tale sign of deeper, more permanent damage under the soil.
If there are adult weevils notching your leaves, you can bet there are larvae under the soil gnawing on root hairs, taproots, and plant crowns (the place where the root connects to the stem). These little guys can do a lot more than their elders.
Because larvae live at the base of your plant for more than half the year, they can cause consistent long-term damage. They may even kill your plant if you don’t put a stop to it. Younger plants and seedlings are most at risk from root weevils. Of course, a sustained infestation can kill off older, established plants as well.
Larvae attacks will at first simply cause a lack of vigor, slow growth, or stunted growth. Later, the plant may develop chlorosis – turning yellow. It won’t improve no matter how much you water, fertilizer, or improve sun exposure.
Eventually, the plant dies back to a more sustainable size for its reduced ability to absorb nutrients. But if the larvae keep feeding, your plant will eventually die.
Fighting Off Root Weevils
If you see an adult weevil on your plants, pluck it off. The weevil may play dead – weevils often drop to the ground as if dead when an enemy approaches. Don’t be fooled. Pick up that beetle and crush it. Then, start inspecting your roots.
If it’s late spring or early summer, look for small, white eggs. The eggs themselves are tiny – only about a millimeter across. But they’re often in clusters. If you find the eggs, remove and destroy them.
Since the eggs hatch within a few days of laying, you’re more likely to find larvae in the soil. These ugly white grubs are easy to pick out. Collect a bowl of them and bring them to the henhouse – your chickens will be delighted. If you don’t have chickens, crush the larvae like you would any other grub.
Occasionally, if you can work your soil in late winter and early spring, you may find pupae. These are the teenage weevils. They have a few adult features, like legs, to differentiate them from their younger siblings. But they’re still white and curved like the larvae.
When you buy seedlings, inspect each plant individually. Root weevils can’t fly, so they’re often introduced through new plants. Buy healthy-looking seedlings and thoroughly examine the soil before transplanting.
Clear out debris in your garden before planting. As with most pests, debris like leaves and dead plants give root weevils a place to hide during the day.
Beneficial nematodes are a fantastic way to control root weevil larvae. Use them as a soil drench – which sounds strange referring to living creatures. But soil drenching is a simple technique that can quickly introduce beneficial nematodes.
Once in the soil, nematodes will kill off the larvae. Apply them in late summer, when most of the weevil eggs have hatched and the soil is warm. Both nematodes and root weevil larvae prefer warm soil, so the nematodes will have no problem hunting down the young weevils.
Once you’ve applied your nematodes with a drench, keep the soil moist. Nematodes need a moisture-rich soil environment to thrive.
When you’re dealing with an infestation of adult beetles, it’s helpful to apply an insecticide. Snout beetles spend most of the day hiding in the soil before climbing up at night to feed on the leaves.
To stop them, apply an organic insecticide like one that contains Beauveria bassiana or Azadirachtin to the stem of the infested plant to kill the pests as they climb up to feed.
Azadirachtin works as an insect growth regulator, which means that it can stop the life cycle of an insect. Often, growth regulators stunt the insect’s drive to feed or reproduce.
Fantastic if you’re hoping to stop root weevils, but keep it away from the bees. To protect pollinators while destroying pests, only apply insecticides to the base of affected plants. Don’t apply insecticides at all while the plant is in bloom, and don’t over-dose your plants.
Often, the best way to fight off invading insects is by taking a multi-pronged approach. Start by making your garden inhospitable to pests, manually remove the weevils you do find, and only when necessary, bring in the big guns.
Root Weevils in the House
Occasionally, root weevils migrate indoors. Usually, during the hot, dry weather of late summer, the weevils start hoping for something a bit nicer. If your house is kept at a comfortable 70°F, the beetles are going to be enticed.
Fortunately, these little beetles can’t do any damage inside. Most people consider them annoying, but not destructive.
Of course, if you have a lot of houseplants, you may beg to differ. If you do spot weevils on your rubber tree plant or jade, check the pot for potential larvae.
Without soil and plants to feed them, weevils don’t last long indoors. You can vacuum them up or crush them underfoot. But if you leave them alone, they’ll either figure out a way back to your garden or die on their own.