What LED lighting means for pest monitoring

Let’s face it – Canada’s greenhouse industry is eager for new technology. A key example is the use of LEDs in the greenhouse industry, which ranges from preliminary research tests to widespread use as auxiliary lighting in less than 20 years. The use of LED lights over conventional lighting brings all sorts of advantages – from production yields to energy savings. But are we sure we are not leaving anything behind?

When technology can be breakthrough, we tend to focus our efforts on studying its direct effects, such as growing time and crop quality. But we often don’t stop exploring what the indirect effects could be, both positive and negative.

With LEDs, the main indirect effects we need to consider are the insect and mite pests that affect the plants we light and the IPM strategies we use to control them.

Previous articles in Greenhouse Canada have covered some interesting preliminary results on the effects of LEDs on the biology and behavior of natural enemies (see Arthropods and Greenhouse Lighting: Like Moths to a Flame by Dr. Rose Labbé and Cara McCreary, Greenhouse Canada, October issue 2020). Today’s article asks an even more fundamental question – do we know how LED lighting affects something as simple (and so important to IPM decision making) as sticky card pest monitoring?

A question of perception

Sticky cards used for crowd surveillance and trapping are generally yellow or blue for good reason. These wavelengths are two parts of the visual light spectrum that insects are most responsive to. Thrips in particular react to blue, which is why this trap color is sometimes used in vegetable and ornamental greenhouses (visit ONfloriculture.com for more on thrips and optimal trap colors).

But what happens if we illuminate a yellow or blue trap card with an essentially purple light (i.e. an LED with a mix of red / blue)? At least to the human eye, this certainly seems to have a significant impact on the color of the trap. But what colors do the insects perceive? Does this make the cards less – or more – attractive to malware?

Comparison of the performance of yellow and blue mass capture cards under HPS light [left]

To the same cards under LED lights with a red: blue mix of 50:50 [right]. Credit :: OMAFRA

Get answers

To answer these questions, we conducted an experiment in research greenhouses that looked at the effectiveness of surveillance cards under traditional HPS lights compared to red / blue LEDs. Here is a summary of our lineup.

Insects and plants: we used chrysanthemums as a model plant. They don’t need additional lighting, but they harbor a wide variety of pests. Thirty thrips per bank were released into the culture and allowed to propagate for two weeks. The mushroom mosquitoes came free!

Lights: We hung a single light (LED or HPS) over the center of each bench, which is filled with 100 pots of 6 inch mother plants. We used a 50:50 red to blue light ratio for the LED.

Blackout curtains were hung between the benches and on the walls shared with other greenhouse compartments to prevent interference from other light sources other than natural sunlight from above.

To make sure that we see a result (positive or negative), we set the lights to a moderately high intensity: 350 µmol / m2 / s at the focal point (see diagram on the next page). In the case of vegetable production in the greenhouse, the light intensity would be closer to 200 µmol / m2 / s. In cannabis production, the light intensity can exceed 500 µmol / m2 / s.

Insect monitoring: We set up two yellow and blue adhesive cards at the focal points of the lights and at different distances on the bench, which recorded significantly lower values ​​with a light meter. This way we could see how the cards reacted in the “bleed zones” of the lights as well as directly below.

Banks have also been split into two sections (A & B) with equal numbers of card treatments in both to give us more data points to work with.

At various intervals, cards were collected and put in clear plastic bags, then thrips and fungus gnats were counted at a later point in time.

A diagram of the experimental set-up for studying the effects of light on mass capture / surveillance maps. Each bank contained 12 adhesive cards that were placed directly over the crop. The light intensity directly under the light (ie the focal point of each light, represented by the darkest circle) was 350 μmol / m2 / s for both types of light. In the first bleeding zone, the light intensity was approx. 215 μmol / m2 / s. This was reduced to 150 μmol / m2 / s in “Bleed Zone 2” for the LED lights. However, the HPS light had a shorter range; The light intensity in bleeding zone 2 was about 115 μmol / m2 / s, similar to the ambient light conditions.
Credit: OMAFRA

What am I spying on with my little eye?

After all this work, what did we find interesting in the preparation of our experiment with our insect pests? Well the answer depends on what day you might have asked us …

Our first experiment was conducted over two weeks to see how the light sources affected the attraction of insects on maps on different cloudy and sunny days. This was our “real-world test” as the variation in light source and intensity is similar to what we would expect in commercial greenhouse production with additional lighting.

From previous research in Southern Ontario, we know that yellow cards catch more thrips than blue cards over a two-week period at a ratio of 70:30 in ambient light. In our first lighted experiment, this was the case for thrips caught under both HPS and LED lights. If there weren’t any differences in our map colors in attracting pests, you’d expect about 50 percent of the insects to be caught on each color – but we’ve seen an average of 68 percent of thrips turn yellow. This was also shown in the case of mushroom mosquitoes, which preferred yellow cards in a similar ratio.

Overall, this is good news for growers in traditional greenhouse production facilities. You can still use yellow surveillance and bulk trapping cards that effectively trap a wide variety of flying pests – regardless of their lighting type.

Proportion of thrips caught on yellow or blue trap cards under additional lighting from A) HPS lights or B) LED lights over a period of two weeks. The dashed line at 0.5 represents where we would expect the bars if thrips did not favor a map color. A “*” indicates that the data differed significantly from the expected ratio of 50:50 at P ≤ 0.10, which means that the color with the higher bar was preferred.
Credit: OMAFRA

Although the proportions of thrips and fungus gnats caught on yellow and blue maps were similar for both types of lighting and lighting areas, the actual number of thrips caught in the different lighting areas varied in ways we did not expect. Under HPS lights, the highest number of thrips (1,072) were caught in the focus area, compared to only 236 thrips in bleeding zone 2. This trend was reversed under the LED lights – most thrips were caught in the farthest bleeding zone, with the least under the focal point. We don’t yet know why this is so, but it certainly warrants further investigation as the optimal placement of surveillance or trap cards under HPS and LED lights may be different.

We also performed a second attempt over a 24 hour period with extremely cloudy weather. This was by design (what we call the “worst case scenario” attempt) so that we could see what would happen when most of the light was coming from additional lighting. These conditions tend to mimic storage style cannabis or microgreen production or the greenhouse conditions in latitudes with little natural light in winter.

As you can see in the graphs to the right, we got some interesting results. Although cloudy conditions did not change the response of thrips or fungus mosquitoes under HPS lights to the map color, it did under the LED lights. In the bleed areas, the purple light of the mix of red: blue LEDs seemed to cancel out the effect of the card color – yellow and blue were equally effective!

Percentage of thrips (AB) or mosquito fungi (CD) caught on yellow or blue bulk catch cards under HPS or LED lighting. The purple marking indicates areas where LED lights have changed the attractiveness of the cards tested. The dashed line at 0.5 represents where we would expect the bars if insects didn’t have a preference for the map color. A “*” indicates that the data differed significantly from the expected ratio of 50:50 at P ≤ 0.10, which means that the color with the higher bar was preferred.
Credit: OMAFRA

It’s unclear why we didn’t see this effect in the focus of the LED light – perhaps the vertical position of the cards directly under the light protected them from drastic color changes on the sides of the cards; we’re just not sure yet.

However, this certainly suggests that if LEDs are the only source of light, the lighting recipe chosen can drastically affect what we know about something as simple as the optimal color of the surveillance card. In systems such as warehouse production and vertical farming, more testing is needed to see how the types of lighting affect the usefulness and optimization of traps for surveillance and pest control.

Until the IPM researchers have a little more time to understand the effects of greenhouse lighting on insects, it is a good idea to test the colors and types of traps in your own facility to get the answers they need. A short two-week test with pairs of cards placed at various distances from your light source (at least four pairs) is all you need to shed some “light” on the subject!

Sarah Jandricic, PhD, is the IPM Greenhouse Floriculture Specialist for the Department of Agriculture, Food and Rural Affairs of Ontario (OMAFRA). She runs the ONFloriculture blog (ONfloriculture.com), which provides flower growers with the latest information on pest management. You can reach them at [email protected]

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