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The world is full of root weevils. It’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 hairy spider weevil in the name disturbing.
But that’s not all.
There are over three thousand varieties of weevils in North America alone and over 95,000 varieties worldwide. Almost all of them are harmful pests for gardeners, farmers and other growers.
What are root weevils?
Root weevils (Otiorrhynchus spp.) derive their name from the hidden damage young weevils cause to the roots of their host plants. The adult form is known as snout beetles due to their long snouts. Beetles use their snouts as weapons – firing projectiles through them at enemies.
They also use their snout to chew the leaves of your plants. Root weevils have tough, beetle-like bodies and six legs, but other than that, their appearance varies greatly between 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, so they never open into wings.
Fortunately, adult weevils cannot fly. They tend to stay relatively localized; live, feed, mate and die in an 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 white worms without legs. They have dull orange heads and small C-shaped bodies. The larvae overwinter in the ground – alternating between a kind of hibernation and voracious feeding. These larvae feed on nearby roots, causing a lot of damage.
In early spring, weevil larvae complete their development and emerge from the soil as adults. Once grown, root weevils no longer feed on the roots. They are now ready to devour the leaves of your plants.
Adult weevils gnaw notches on plant leaves. They feed at night, away 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. Adults live only a few months, rarely until autumn. Most of the life of the root weevil takes place in the larval stage.
Not all weevils mate. In fact, there are root weevils that reproduce asexually. The strawberry weevil is one such exclusively female species. Other species, such as the lilac root weevil, produce both 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 the cracks in the ground and a few days later they hatch. New weevil larvae enter the soil – spending the next 9-10 months chewing through root systems and becoming adults.
Identify Weevil Damage
It can be difficult 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 a plant’s leaves. Remember that adult root weevils love to nibble on the leaves of their favorite host plant. When eating, weevils tend to gnaw the edges of leaves.
Some people describe notches as crescent moon shaped, others as D shaped, no matter how you describe it, your plants are damaged. The damage that adult weevils can cause is mostly cosmetic, but they can provide a telltale sign of deeper, more permanent damage below the soil.
If there are adult weevils gouging your leaves, you can bet there are larvae under the soil chewing through root hairs, taproots, and the crowns of plants (the place where the root connects to the stem). These little guys can do a lot more than their elders.
Because the grubs live at the base of your plant for more than half the year, they can cause consistent long-term damage. They can even kill your plant if you don’t stop it. Young plants and seedlings are the most susceptible to root weevils. Of course, a sustained infestation can also kill older, established plants.
Larval attacks will initially simply cause lack of vigour, slow growth or stunted growth. Later, the plant may develop chlorosis – turning yellow. It will not improve no matter how much water, fertilizer or sun exposure improves.
Eventually, the plant dies back to a more sustainable size for its reduced ability to take up nutrients. But if the larvae continue to feed, your plant will eventually die.
Fight root weevils
If you see an adult weevil on your plants, pull it out. Weevil can play dead – weevils often fall to the ground as if dead when an enemy approaches. Don’t be fooled. Pick up this beetle and smash it. Next, 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 in diameter. But they are often in clusters. If you find the eggs, remove them and destroy them.
Since the eggs hatch a few days after being laid, you are more likely to find larvae in the soil. These ugly grubs are easy to spot. Collect a bowl of them and take them to the coop – your hens will be delighted. If you don’t have hens, crush the larvae as you would any other larvae.
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 are always white and curved like the larvae.
When buying seedlings, inspect each plant individually. Root weevils cannot fly, so they are often introduced by new plants. Buy healthy-looking seedlings and examine the soil carefully before transplanting.
Clear debris from 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 soak – which sounds odd in reference to living creatures. But soil soaking is a simple technique that can quickly introduce beneficial nematodes.
Once in the soil, the nematodes will kill the larvae. Apply in late summer when most weevil eggs have hatched and the soil is warm. Nematodes and root weevil larvae prefer warm soils, so nematodes will have no problem chasing 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 dealing with an infestation of adult beetles, it helps to apply an insecticide. Snout beetles spend most of the day hiding in the ground before climbing at night to feed on leaves.
To stop them, apply an organic insecticide such as one that contains Beauveria bassiana or Azadirachtin to the stem of the infested plant to kill pests as they climb to feed.
Azadirachtin acts as an insect growth regulator, which means it can stop an insect’s life cycle. Often, growth regulators slow the insect’s drive to feed or reproduce.
Fantastic if you’re hoping to stop root weevils but keep them away from bees. To protect pollinators while destroying pests, apply insecticides only to the base of affected plants. Do not apply insecticides at all while the plant is in bloom and do not overdose your plants.
Often the best way to control insect invaders is to take a multi-pronged approach. Start by making your garden inhospitable to pests, manually removing any weevils you find, and only when necessary bring in the big guns.
Root weevils in the house
Sometimes root weevils migrate indoors. Usually during the hot, dry weather of late summer, the weevils begin to hope for something a little nicer. If your home is kept at a comfortable temperature of 70°F, beetles will be attracted.
Luckily, these little beetles can’t do any damage indoors. Most people consider them annoying, but not destructive.
Of course, if you have a lot of indoor plants, you can differ. If you spot weevils on your rubber tree or jade, check the pot for any larvae.
Without soil or plants to feed them, weevils don’t last long indoors. You can suck them up or crush them under your feet. But if you leave them alone, they’ll either find a way back into your garden or they’ll die on their own.
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