Light Traps as a Pest Management Tool

Introduction

Light traps are playing an increasingly important role as a pest management strategy in greenhouse crops in Ontario. They provide an easy-to-use non-pesticide alternative tool for reducing and suppressing moth pests, such as adults of tomato pinworm and cabbage loopers. This article outlines some information on the use of light traps as a pest management tool in greenhouse crops.

What are light traps?

Light traps use a combination of blacklight lamps for attracting insects, and a low voltage electrocutor that surrounds the lamps. These lamps emit long wave ultraviolet (UV) radiation mainly in the region around 365 nanometres (nm). They also emit some visible light of wavelengths around 430 and 540 nm. Studies have shown that ultraviolet light is the most effective part of the light spectrum for attracting night-flying insects, hence their use for attracting nocturnal moths.

How attractive are light traps?

As an indication of the attracting power of light traps with respect to tomato pinworms, a light trap was compared with a pheromone trap equipped with a fresh lure for male tomato pinworms, in a commercial house. Both traps were placed within the crop at a similar height within the crop canopy, within the same row, and with a spacing of about 30 metres between them. The number of adult pinworms caught over a period of one night by these traps was checked. Whereas the light trap attracted and killed at least 100 adults overnight, the pheromone trap caught zero adults.

Factors to consider in use of light traps

Size

There are a few sizes of available light traps, differing in their attractive powers. Most traps have an attracting diameter range of 40 to 50 metres, but there are also larger ones that are also more expensive.

Location

Ideally, consideration should be given to their attracting range when placing them within the greenhouse, but most often, convenience dictates their location. This means that they are usually located along walkways. Growers can add on a receptacle containing soapy water, or place a large sticky board below the trap to catch both the electrocuted victims that fall off, and those that might have just bounced off the trap and fallen. Care is usually taken to place the traps at such a height that the light emitted from the trap is not blocked by plants, nor are they so high that access by smaller moths is difficult. That is, the traps are preferably placed just above the plant canopy.

Modification of traps

We have observed that removal of the metallic grid that surrounds the light plus electrocution mechanism in the trap, improves their efficiency. Although the grid size is usually large enough to allow small moths such as tomato pinworm adults to fly through, they definitely impede access by larger moths such as adults of loopers and tomato fruitworms.

Time of year

Light traps are best used during the cooler months of the year when outdoor temperatures are 100C and less. This is to eliminate the problem of attracting more flying pests into greenhouse. However, during the warmer months of the year, the traps may be better placed outside of the greenhouse, at a safe distance, to attract adults away from the greenhouse. Some growers have tried this but there are no concrete studies to show how successful this strategy is in reducing indoor pest populations.

Maintenance

Light traps have to be inspected regularly so that they can be cleared of insects stuck to them. Traps that are clean are obviously more efficient than those that are covered with insect corpses.

How do light traps affect beneficials?

A note of caution is that light traps attract and kill bumble bees. To avoid or at least minimize trapping of bees on these traps, they should be set up with a timer so that the lights are on at dusk and off at sunrise. In addition to attracting bees, results of a study in which the attractiveness of various light sources to some beneficial insects was studied, were recently reported by researchers at the University of Missouri. The light sources included blacklight, blacklight blue (dark violet-blue lamp used for testing, inspection and analysis in food and textile industries, darkrooms, discos, nightclubs, sign lighting) plant/aquarium light, and cool white light. Results of the attractiveness of the different light sources to the beneficials were as follows:

• Lady beetles favoured the blacklight source;
• The parasitic wasp, Ophion sp. was more attracted to the blacklight blue source;
• Lacewings were equally attracted by cool white, blacklight and plant/aquarium light lamps;
• Damsel bugs (related to lygus bugs) showed no preferences.

These results indicate that some of our beneficials may be attracted to the lights. However, lower temperature conditions at nights should mean reduced activity of most of the beneficials at this time, and therefore, reduced chances of trapping. Many of our moth pests, on the other hand, tend to be more active at nights. Generally, use of light traps during the cooler months of the year has been a good non-chemical strategy for managing moth pests.

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