Part 1 last week covered WHY to trap and WHEN to it’s best to put up your cards. This post goes over trap colour and placement to optimize thrips catches while reducing effects on natural enemies.
Although I’ve talked about this in older posts, I’ve put this data in context of what we know now, in terms of the various thrips species that invade Niagara greenhouses. I’ll also be sharing some data on card duration.
If that isn’t exciting enough for you, next week’s Part 3 will discuss whether or not using lures is worth it to increase thrips catches.
Choosing the Right Trap Colour
Since Part 1 of this series on mass trapping showed us that we are generally dealing with TWO different thrips pest species in floriculture in Ontario (Fig. 1), the optimal card colour would need to be effective for both western flower thrips (WFT) and onion thrips (OT).
The good news is there is LOTS of scientific literature on this. The bad news is that the results are all over the place! Many studies say blue is best for WFT, while others say yellow. For OT, the contest is often between white and yellow.
Given that colour is affected by light intensity, which can be affected by both latitude and greenhouse covering, it’s important to test colours in our location (Niagara), specifically. A recent study also showed that different populations of WFT prefer different colours, This suggests there may be a genetic component to colour attraction in thrips. This further enforces that we need to do our own tests, rather than rely on information from other locales.
Western Flower Thrips – It’s Complicated
Although we previously did studies in Niagara on card colour to try and determine which colour WFT likes best, this was done before we were aware that eastern flower thrips (EFT) were muddling up the cards. Unfortunately, EFT and WFT look VERY similar, and are difficult to distinguish, even with a dissecting microscope.
However, just looking at the total thrips caught, we saw that yellow generally caught more thrips over the summer than blue cards at several commercial facilities (Fig. 2).
Trying out blue cards in an effort to reduce catches of natural enemies (see our section on this, below) can be tricky, as our data showed not all shades of blue cards are created equal. And, other than shore flies, blue cards don’t catch many other greenhouse pests. I’ve seen growers who switched to blue cards have a resurgence in fungus gnats, necessitating adding nematodes or pesticides to their IPM programs. Some switched back to yellow for this reason.
Ultimately, I would love to re-do these colour preference trials, and see EFT and WFT behave differently, but for now, yellow still seems like the safer bet for WFT control.
Onion Thrips Know What They Want
Luckily, we have a clearer picture of which card colour is best for onion thrips, thanks to Ashley Summerfield’s Masters research project. She found that OT consistently preferred yellow across all 3 of her commercial greenhouse sites (Figure 3). This even happened at the single site where the Frankliniella species preferred blue (Greenhouse C).
Will Mass Trapping Cards Trap My Bios?
I often hear from growers that they’re worried using mass trapping cards with negatively affect their biocontrol programs. This is a valid concern, given how expensive weekly releases of natural enemies can be in floriculture greenhouses.
2017 tests by OMAFA looked at this question. During our efficacy tests for thrips, we recorded the numbers of Aphidius colemani ,Diglyphus and Atheta on our yellow and blue cards. These were the natural enemies normally released by the commercial greenhouse, as part of their weekly mum production.
In those tests, we trapped between 3-5 Aphidius per large yellow card (40×25 cm) per week, depending on the manufactuter (here, only 2 were tested). None of the other bios were caught at significant rates.
We tested this question again in 2018 when were compared yellow cards to 4 different shades of blue cards. Here, we released a high number of Diglyphus and Orius. Aphidius colemani was present in the greenhouse as banker plants (1 per bench). We saw similar captures of Aphidius as in 2017 (Figure 4). Interestingly, the same shade of blue that was most effective for thrips was also most attractive to Orius. But at fewer than 2 per large card (40x25cm) per week, I wouldn’t be that considered about this bio.
Optimal Trap Placement – What Kind of Greenhouse do You Have?
Many companies and greenhouse IPM practitioners state that mass trapping cards (not just monitoring cards) should be placed directly in the crop, a few inches above the canopy. But many growers find this inconvenient, as it can interfere with production practices like pinching and even spraying. Also, putting cards on stakes can be more time consuming than hanging them on lines between the post rows. But are cards still as effective if you place them beside the crop, instead of in it?
2017 tests by OMAFA done in top-venting greenhouses confirmed that placement didn’t matter. Cards placed on strings attached to the post rows were just as effective as those directly in the crop. These test were done in 3 commercial chrysanthemum greenhouses in summer. The only time we DID see a difference was when the greenhouse was side-venting (Figure 5). In this case, colour or placement (crop vs. post row) didn’t matter. The only thing that did matter was distance from the vent.
How Long are Sticky Cards Sticky?
In 2018 OMAFA did an experiment where we hung up pairs of yellow or blue cards from different manufacturers in a commercial greenhouse in August. One card was left up for 8 weeks, with total thrips counted each week, while the other was replaced weekly. This was to see if the heat of summer negatively affected the glue on the cards, making them less effective.
Surprisingly, cards from different manufacturers behaved differently, suggesting an effect of the glue type. Company A’s “old” cards became less effective at the 5 week mark, compared to the fresh card (Figure 6). This resulted in a drop in thrips catches on the old cards by about 15% over the course of the experiment. We didn’t see this effect in Company B’s cards – both the “old” and new card caught roughly the same number of thrips each week.
Interestingly, Company A’s cards caught more thrips over all. So, even with the drop in efficacy, it was still a great card. We saw this same trend in blue cards from Company A compared to those from 4 other companies.
Some manufacturers state their cards are effective for 16 weeks, and given that all the cards were still going strong at 8 weeks in the height of summer, I have no reason to doubt this claim. However, I always suggest switching the cards out during a new crop cycle (e.g. every 10-12 weeks, depending on the crop), so cards don’t get forgotten about long past their 16 week point.
Summary: Using Cards Effectively
So how do we put all this information from last week’s post and this post on into a plan? Here are tips to follow to improve your mass trapping game this summer:
- Start early. Western flower thrips, EFT and OT start to emerge and enter greenhouses in late May, but really ramp up in July. Having your traps up in early June can act like crop insurance for seasonal fly-ins.
- Use a rate of at least 8 large cards per 1000 sqft. If you have regular problems with onion thrips, which we know mainly come in from outside, you may want to increase this.
- Use yellow cards to make sure you are trapping the widest variety of pests. This includes different thrips species, such as WFT and OT, but also other important greenhouse pests, such as fungus gnats and leafminers that just aren’t attracted to blue at all.
- If you’re using a lot of aphid parasitoids, consider blue cards. This was the only natural enemy we tested that was trapped on cards at a concerning rate. Although Diglyphus was not negatively affected, other parasitoids (i.e. those for whitefly) could also be trapped by yellow cards.
- Blue may be as effective as yellow cards for Frankliniella species (including WFT), but test them first. The shade of blue can vary widely by manufacturer. Generally, the mid shade of blue (vs. lavendar or very dark blue) seems to work best for WFT/EFT and OT in our tests.
- Optimal card placement is different in side-venting vs. top-venting greenhouses. For side-venting greenhouses, concentrate mass trapping cards or tape 2-13 feet away from vent openings. For top-venting, you can place cards within the crop, or on the post rows (whichever is easier for your operation).
- Replace cards 12-16 weeks, or when cards are full or dirty. Lots of fungus gnats or potting mix on cards could potentially reduce their attractiveness, and definitely reduces the sticky surface area!
If you want more information on using mass trapping in both floriculture and vegetable greenhouses, check out this video on mass trapping techniques from OMAFA, AAFC and the Vineland Research and Innovation Centre. Happy trapping!