Targeting Thrips in Scandinavian strawberry crops

10/05/2020 - As the weather warms up in Scandinavian countries such as Denmark and Sweden, thrips are beginning to appear in glasshouse and tunnel grown strawberry crops. As a result, it is vital to have sticky traps in place to monitor thrips emergence and choice of trap colour is important.

To control Western Flower Thrips (WFT) (Frankliniella occidentalis), the strategy at this time of year generally includes Hypoaspis-System (Strateolaelaps scimitus). This predatory mite eats thrips pupae in the soil or growing media.

Thrips control strategies can also include predatory mites such as Amblyseius-System (Amblyseius cucumeris) or Swirskii-System (Amblyseius swirskii); These are introduced onto the crop leaves together with Nutrimite™, the pollen-based supplementary feed to enable strong populations to build up. Swirskii-System in particular responds very well to Nutrimite™, laying numerous eggs to populate the crop with the next generation of eight legged warriors.

Growers have a choice of yellow or blue traps. At this time of year, control strategies have tended to focus on non-flying predatory mites. In the absence of winged beneficials in the crop, yellow sticky trap cards and rolls are generally deployed to attract a range of flying insect pests; they are also thought to attract WFT better than blue traps in the lower light levels of spring. Blue sticky trap cards and rolls tends to attract only thrips. Growers usually switch from yellow to blue traps later in the season when winged natural enemies, such as Orius-System, are introduced to the crop.

However, in a bid to combat some thrips varieties, increasing numbers of growers are introducing the winged predatory bug Orius-System (Orius majusculus / Orius laevigatus) earlier and earlier into the crop. This needs to be taken into account when selecting trap colour.

Know your thrips species
To optimise control, it is crucial to know which thrips species is in the crop before devising an appropriate control strategy.

Sweden
A study carried out in early summer in Swedish strawberry crops found amongst others: Thrips fuscipennis, Frankliniella intonsa and Thrips major.   

“WFT do not seem to be a problem for us, at least not in open field or tunnel production,” says Victoria Tönnberg from the Swedish agricultural advisory Hushallnings Sallskapet. “WFT can’t tolerate our outdoor winter temperatures. We are not aware of a single confirmed case, or even suspected case, of WFT in tunnels. As things stand, it is only known as a problem in greenhouses in Sweden, and then mainly in ornamental potted plant production. Our challenges come from other thrips species, which tend to migrate in from nearby fields and surrounding vegetation.”

Denmark
The situation is different in Denmark, as Danish technical specialist Lars Stubsgaard - from Borregaard Bioplant (part of the Biobest Group) - explains.  

“Working with Jude Bennison of ADAS in the UK, we recently analysed thrips appearing in strawberry tunnels in Denmark, he says. “We were surprised to find most thrips were Frankliniella intonsa, as well as some WFT. Entering the crop from the ends of the tunnels, they appeared very quickly; by week 20 there were suddenly as many as 10 per flower!

“While WFT can be managed using strategies including Amblyseius-System (Amblyseius cucumeris) and Swirskii-System, (Amblyseius swirskii), thrips species such as F. intonsa and T. fuscipennis tend to be less attractive prey for these predators,” explains Lars. “In addition, these thrips tend to pupate in the foliage, rather than falling to the soil or growing media. Soil-dwelling predators have less opportunity to eat the pre-pupal and pupal stages.

“As a result, Danish growers rely more on early season sprays, where available. There isn’t as much insecticide resistance in these thrips species, compared to WFT, however there are very few products available. Therefore, they can’t rely on pesticide sprays.

“Growers also look to establish predatory bugs, such as Orius, earlier in the season. Orius (majusculus) establishment can be poor if applied too early. We recommend delaying introductions until it is warm enough, which is usually early June.”  

Generalist thrips predator - Orius
Tilmann Brunnberg, technical specialist of Biobest Germany, has found benefits in introducing Orius-System earlier in recent years.
“We find it is beneficial to start with a soft thripicide spray, where available, when the first thrips emerge,” he says. “We then introduce Swirskii-System/Swirsikii-Breeding-System and apply Nutrimite™ around the release points.

“The resultant strong Swirskii population takes care of the WFT. However, we also introduce Orius earlier in the season to work as an effective generalist thrips predator. With Orius-System, regardless of which thrips species are in the crop, this predator will eat them!”

Orius-System is an aggressive thrips predator and can contribute to the control of spider mites, aphids and Lepidopteran pests. Orius-System can be introduced as flying adults that will immediately begin to lay eggs in the crop; or as nymphs, it stays where released to focus on hot spots.

Orius hunts within flowers, but also under the calyx of the strawberry fruit where F. intonsa tends to hide. However, the predator can be difficult to establish early in the season due to lower temperatures and shorter days.

Establish early
“We need to work hard to establish Orius early, introducing up to 10 per metre, but the benefits are clear to see,” says Tilmann.

Lars concludes by saying: “We have learnt that placing plenty of sticky traps at the ends of tunnels is important to help stop the thrips influx. We mix yellow and blue traps, as F. intonsa seems more attracted to blue.

“In an IPM system we need to integrate all the tools we have to combat these pests, and our tailor made strategies have to be flexible to deal with local pressures.”

Work on F. intonsa from Korea* found that white sticky traps and sky blue traps (similar to Biobest’s Bug-Scan^® Blue and Bug-Scan^® White) were found to be particularly attractive to F. intonsa.

*Mi-Ja, Seo & Sun-Jin, Kim & Eun-Jin, Kang & Myong-Ki, Kang & Yong-Man, Yu & Myeong-Hyeon, Nam & Suk-Gee, Jeong & Young-Nam, Youn. (2006). Attraction of the Garden Thrips, Frankliniella intonsa (Thysanoptera:Thripidae), to Colored Sticky Cards in a Nonsan Strawberry Greenhouse. Korean journal of applied entomology. 45.