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Polydrusus
Berry plants are susceptible to the predations of weevils, but not all berries have the same issues with the busy creatures. These nocturnal pests can be controlled, according to entomologists Tracy Hueppelsheuser of BCAGRI in Abbotsford and Hollis Spitler of Washington State University in Mount Vernon, but it takes work and careful methods. Both have studied the pest for a number of years.
Berries and expected damage
It’s post-harvest when strawberries will show the signs of hungry weevils. After harvest, growers will spot frass, or a sawdust-like material, as well as tunnelling and tracking in and around the fleshy roots.
“In strawberries, you really recognize damage after harvest,” says Spitler.
Blueberries are in the most danger in their younger years and it’s hard to spot the hungry pests in either their larval (weevil) form or as adults (snout beetles). Weevils don’t seem to care as much for gnawing on the more mature woody plants, so they tend to avoid the more established fields.
“The outer surface of wood at the root is eaten off,” Hueppelsheuser says of the damage in blueberry plants. “You might even see girdling of whole stems.”
If there’s a lucky berry, when it comes to weevils, it’s the raspberry. Robust growth is their best defense in that it may allow them to escape some of the damage of the pests.
“Raspberries can sometimes outgrow the problem and you won’t see them,” Spitler notes.
Spotting weevils can be like a bad game of Where’s Waldo.
With a pest that’s nocturnal, finding and confirming their presence is often an extensive job. Scouting is best done in the early spring or late fall and obviously at night. In strawberries and blueberries, Spitler advises taking the time to search for signs of damage or the pests themselves.
One of the key indicators in the field, according to Hueppelsheuser, is that plants will be weak, have poor growth and may even have notching in the leaves from the adults looking for a bite to eat of grower’s crops.
“It’s a good idea to map or look for where those weak spots are,” she says. “Because they will be in patches.”
These signs apply most to blueberries, but to a lesser degree in strawberries as well and perhaps the odd unlucky raspberry plant. The location and severity of these types of damage will help to identify the weevil species which can be typed by a lab if weevils or snout beetles can be caught and sent in for identification.
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Otiorhynchus
Macro photography of Otiorhynchus, an evil weevil and common berry pest.
Larvae
Unfortunately, the damage may not always be seen in the field, or it may look like something else. If a grower suspects weevils, Hueppelsheuser advises them to pull up the plants – especially in spring – to see the roots and shake the plant over a drop sheet to see if there are weevils.
These sneaky pests may not be where they are expected, either. Weevils will make hibernation dens in the soil as they go from larva to pupa (in the soil) then emerge as adults. However, as Spitler advises, they won’t always be in the rows, sometimes they will be between rows in the soil.
“They come out at night and you can collect them,” he says. “You’ll find healthy adults on the underside of leaves. They are mostly nocturnal. They don’t like light. It’s a lot of on the ground work.”
Because they are so hard to spot, scouting is about confirming their presence, and not about numbers as is the case with other pests.
“The number that you find isn’t really going to help you determine the amount of damage you’re going to see,” she cautions. “It’s more of a ‘find or don’t find’ situation.”
Control methods
And finding them, in any amount, means action must be taken, yet, there are very few choices in weevil control when it comes to chemical options.
“We do have some choices, limited choices,” says Hueppelsheuser says. “So we have to use them carefully, limitedly and judiciously.”
Because weevils may be on the undersides of leaves, Spitler adds that it takes the right kind of spray equipment to get the treatments to the right place. He advises to spray just the weevil problem areas and the bordering spaces of those areas without going “crazy” with the treatment.
“Directed spraying is key,” he says. “Air spraying isn’t going to help with weevils. We’ve tried injections with varied results.”
He has also found that some of the spraying for Spotted Wing Drosophila (SWD) in berry plants has killed some of the adult weevils as well.
“We’ve been chasing weevils for quite a few years,” he says.
A weevil identification factsheet can be found on the BC Ministry of Agriculture website and the WSU berry production guide is another good source of information.
The following is advice and information from the Weevil Identification & Management In Blueberry fact sheet found on the Ministry Of Agriculture website.
Plant damage
Basic Weevil Life Cycle:
Weevils have 4 life stages; adults, eggs, larvae and pupae. Weevils are primarily nocturnal, feeding in the evening and night and seek cover during the warmer days in the soil, or in shady plant foliage. There are 6-7 larval instars. Weevils spend about 10 months in the soil as larvae and pupae, where they are well protected. There is one generation a year. Adults can live for more than one year.
Most root weevil species that affect blueberries do not fly, but are strong walkers. Some species are all female, and lay viable eggs without mating, while some species have both males and females.
Damage
Weevil larvae feed on and girdle the roots, causing plants to decline, reduce yields, and can kill young plants. Weevil Adults feed on new growth, which can kill new buds and branches. Small plants can be heavily compromised or killed by weevils after only a year of feeding. Mature plants can decline and under-produce, or be killed after years of larval feeding. It is difficult to find larvae in root systems, but damage is relatively easy to find particularly if populations are increasing. Overall, blueberry plant decline appears to be a result of various factors including weevils, disease organisms, sub-optimal soil type, pH, and deep planting.
Monitoring
Adults: Look for ‘notching’ on leaves. Adults can be difficult to find on blueberries. Use a drop sheet or beating tray to collect adults from foliage. Do after dusk for best results for most species. Count the weevils that drop onto the sheet or tray.
Larvae: Look around plant roots for white larvae, and ‘tracking’ on roots from larvae feeding. Either pull up weak plants or excavate around roots. It is difficult to see damage on unwashed roots. Rate the damage as either low, medium, high, and old or current feeding damage.
Sample many locations within a field: Weevil distribution in a field is uneven, and the worst areas tend to be near field edges. Record the number found, and the species. If you are unsure, take the insects found or the entire plant (with roots) to the Plant Health Lab (B.C. Ministry of Agriculture) for verification.
Management
If you find weevils, or recent damage to roots when monitoring, take action. If a weevil problem is occurring in an established field, it will take a few years of diligent management to clean it up. A multi-year, ongoing approach to weevil management is necessary.
Prevent weevil establishment: Weevils usually move in from wild areas along the field edges or can be introduced in containers. Ensure planting stock is free of weevils.
Foliar insecticides for control of adult weevils are registered for use in Canada. For best results, it needs to be warm enough so that the adults are up and actively feeding, which can be a challenge if evenings are still cool or rainy in spring and early summer. Coverage is important; depending on the species, they can stay down low on plants in sucker growth (i.e. rough strawberry weevil), or be up high during day time (i.e. green weevil).
Beneficial nematodes: Some species have shown adequate suppression of weevil larvae. Plan to use nematodes when larvae are young and close to the soil surface (Aug-Sept).
Met 52 (Metarhizium anisopliae) mixed in media nursery stock will protect plants for two years. Met 52 drenched in established in-field blueberry plants does not work well (trials in Oregon). Met 52 is currently registered for use in Canada in some crops including nursery stock (non-food plants) and strawberries.
Ministry of Agriculture, Province of BC.
