38 Thrips in a changing climate by TOM WALKER
ABBOTSFORD – Thrips have the potential to be a significant threat to a number of BC crops. ES Cropconsult Ltd. has been monitoring the presence of thrips in potato fields in the Fraser Valley. Kiara Jack shared results of a three- year study that evaluated the damage to potatoes by thrips “in a changing climate” at the Pacific Agriculture Show earlier this year. The good news is that the
current risk of thrips reducing yield in potato crops is low, but that may change. “The reason we are looking at thrips is that climate predictions suggest we will have hotter and drier summers in the Lower Mainland and thrips will thrive under those conditions,” says Jack. She adds that milder and wetter winters are also predicted, which might increase thrips winter survival.
“With the combination of
those two factors, we will probably have more thrips out in fields.” “This project looked at potatoes but thrips affect a lot of different crops including cabbage, strawberries and greenhouse crops,” says Jack.
Thrips damage potato
leaves directly by piercing and sucking out nutrients and by laying eggs in leaf material. Thrips are also potential vectors of viruses, particularly tomato spotted wilt virus (TSWV).
Thrips identification can be a challenge. Adults are only 1 to 1.5 mm long. “If you’re not looking
carefully, it can be hard to tell,” says Jack. She suggests using a hand lens and looking on the backsides of potato leaves. Growers should look for
silvery-coloured, shiny damage along the leaf veins. The damage may be on either
side of the leaf but does not show through from one side to the other. Look for the dark specks of frass that thrips leave behind. The insect has a very short life cycle – as little as nine days in hot dry weather. Just because they haven’t been seen doesn’t mean they aren’t present in the field.
Thrips damage has to be distinguished from lygus nymph damage which is likely to break through the leaves and cause leaves to pucker, spider mite damage (visible through both sides of the leaf and likely to include webbing) and aphid damage (aphids present, or their corpses, together with yellowing/ purpling leaves). Thrips have been shown to
spread TSWV in other regions. TSWV is difficult to identify in the field, says Jack. Concentric rings on the tubers or on leaves are one indication. She advised growers to look for leaky lesions similar to early
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blight or late blight. “If you are not seeing the other indicators of leaf blight, get it looked at,” says Jack.
Guidance rare The study looked at the
presence of thrips and the damage they can cause. Historical data from ES Cropconsult shows that from the late 1990s to early 2000s, thrips recommendations were rarely made for more the one field per year in the valley. However, since 2003, thrips management
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recommendations have been made in 5% to 20% of fields monitored by ES Cropconsult. “The years with the highest
recommendations had more growing degree days,” Jack points out. In 2015, thrips caused a
significant, though small, increase in yield in test plots, (likely due to plant
compensation) while in 2016, there was not a significant impact on yield. “Currently, the risk of thrips
reducing yield appears to be low,” the study concludes. TSWV has been found in
the Fraser Valley on weeds and some crops but two years of testing has not found evidence of TSWV being transferred by thrips into potatoes.
The study also found that farms in Delta are more likely to have higher thrips numbers at the edges of their fields than those in Abbotsford. “Organic growers have
more thrips but they also have more predators,” Jack points out. “So, you don’t necessarily have more damage if you are growing organically.” Variety selection has some
bearing. “We can say if you have a
Satina crop, you are more likely to have thrips than if you have Orchestra, Kennebec, Imola or AC Peregrine,” says Jack. “We have seen over the
years that thrips move in from neighbouring grass and grain fields,” says Jack. “Growers could avoid a late planting of potatoes when neighbouring grain is about to dry out.” Similarly, she suggests waiting until potatoes are harvested before you harvest grain if thrips are a concern. Attracting predators with a
diverse set of plants and conserving them by minimizing chemical sprays and controlling dust around fields will help reduce pest issues, Jack says. Chemicals are effective as a
last resort. Growers can use seed treatments and foliar sprays.
“But make sure you are doing it at the right time and getting good coverage,” says Jack. “They’re tiny and you need to hit them directly to kill them.”
COUNTRY LIFE IN BC • MAY 2017
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