Photo by Catherine Eckert | Dreamstime.com
thrip damage
This photo shows typical damage from Thrips to strawberry flowers.
Strawberry growers currently have two insects on their most wanted list; lygus and thrips. These generalist pests don’t just go for strawberries. They are equal opportunity insects enjoying everything from alfalfa and peaches to vegetables and other crops. They are far from a harmless annoyance as both can lead to berries destined for the compost heap.
Arlan Benn, berry IPM supervisor and organic research coordinator with E.S. Crop Consult, a firm which provides IPM services in the Fraser and Pemberton Valleys of BC, notes that both pests live on a wide range of hosts beyond berries and are easily dispersed throughout farming fields. This makes monitoring and control all the more important – and difficult.
Deformed and scarred strawberries are sometimes victims of poor pollination or wind or mechanical damage, but thrips, which love hot, dry weather, are common and abundant and may be overlooked when it comes to similar appearing damage. Thrips leave a shiny tan or brown patch on the berry’s surface. It may look like mechanical damage but as Benn notes, if the berry is fairly sheltered and not touching the ground, it’s far more likely to be thrips.
“We can expect [thrips] to be a growing issue with changing climates,” he says. “Thrips are predominantly a quality issue and reproduce very quickly under ideal conditions meaning they need to be monitored closely.”
Lygus leads to the “cat-faced” berries some growers may confuse with pollination issues. The deformation is caused by the sucking mouthpart of the insect which punctures the berry and makes it pinch inward as it grows.
“Poor pollination causes a similar effect, except you’ll be able to notice that the seeds are very small near the deformation – this is because they weren’t pollinated, so they don’t contribute to the fruit’s growth,” he explains. “With lygus injury, the seeds will all be roughly normal size.”
The damage from lygus can be extensive even at low counts of the insects. It is able to impact both yield and quality in strawberries and there is no “down-time” with these pests – they are around during the entire season. Adding insult to injury, they are very mobile and will fly to fruit trees and other crops in the spring.
Monitoring and control are the keys for both insects, though even these tried and true methods can be challenging with lygus and thrips.
“With lygus, some growers were finding difficulty with some product [control] groups and their lack of efficacy this past season,” Benn says. “Densities were also extremely high in many fields. This stresses the idea that farmers need to be scouting fields before and after product applications to ensure they are managing the problem.”
Photo by ES Cropconsult
Thrips damage
Thrips damage to a strawberry.
Thrips are more difficult to pin down in terms of aligning monitoring counts to crop damage.
“Sometimes we can measure very high levels and see no damage, other times there are very few pests and noticeable damage,” he notes. There are many complex interactions in the field that could be partially responsible for this. For example, some thrips are known to be omnivorous, so they could be distracted from the crop and feeding on spider mites. It could be as simple as weather affecting the activity level of thrips feeding. There are still a lot of unknowns with this pest in Lower Mainland strawberries.”
Despite this, the best approach with thrips and lygus continues to be regular monitoring. Benn recommends at least weekly.
“You need to know when the first lygus nymphs [immature insects] arrive as they are most susceptible to control measures when young, so get out there as blossoms show up in the field,” he says.
Field monitoring for insect levels is crucial to confirming insect damage versus pollination issues, wind or mechanical damage. By directly measuring pest levels, growers will have a better awareness of whether thrips or lygus are the cause of their strawberry quality issues. This is a better gauge of potential issues than looking at damage and making assumptions.
“If you grow more than one susceptible crop they will be especially important for you to monitor and control carefully,” says Benn.
Locally recommended pest thresholds, like those noted in the BC Ministry of Agriculture berry production guide, will help. Lygus limits to trigger spraying are one nymph per 20 plants, while California recommends spraying at 10 thrips per blossom. Obviously these levels vary making it even more important to track pest numbers and know your field and crop before making the decision to spray.
For example, while some will say finding 20 lygus or thrips in a field is a spray indicator, it depends upon the size of the field.
“Track beneficial insects such as Minute pirate bug, lady bird beetles, and predatory thrips- if the levels are high, consider waiting to spray for thrips,” he says. “Lygus shows a preference for alfalfa so watch for nearby hay fields being cut and their migration into the strawberry fields.”
Photo by ES Cropconsult
Lygus Adult
Lygus Adult
Benn worked with growers in the Lower Mainland who had losses due to lygus. These growers struggled to get control of the pests throughout the season and while they didn’t track losses, Benn expects them to be significant.
“At its worst points [levels of damage were] very high and definitely impacting their bottom lines,” he says. “Less drastically, thrips were also challenging for a couple of growers.”
The best aid E.S. Crop Consult gave to these growers was to help them identify pest levels and determine the most effective times to spray. Overall, in 2018 thrips was easier to control than lygus, but both pest must be monitored closely in 2019 to avoid costly damage.
