By Gary Hartley

Lighting the way to improved greenhouse pest control

The great advantage of greenhouses for pest control is that they make the environment far more controllable, compared to open fields.

While growers can never hope to tame ecology, even in covered scenarios, there are various ways in which adjustments to the growing environment can make things more hostile for troublesome pests and more amenable to their natural enemies. In recent years, the use of artificial lighting as a management tool has come to prominence.

Visual signals draw pests in

It has been discovered that for some pests, visual stimuli seem to be more attractive than chemical, meaning that trapping techniques using lighting could prove more fruitful than using commonplace pheromone traps. One species pulled in by the bright lights is the European tarnished plant bug (Lygus rugulipennis), a pest with a varied diet and more than capable of causing economic damage.

A study at Wageningen University and Research explored the use of water traps fitted with a white LED, comparing them to trapping using pheromone lures. The researchers found that the LED traps caught 20-30 times more of the pest than those with pheromones in their greenhouse experiments. White light also seemed more attractive than UV.

Another advantage when targeting L. rugulipennis this way appears to be that light catches large numbers of both male and female bugs, whereas previous studies had suggested that pheromones were only of use to catch males, and even then, in modest numbers.

Using LED traps has also been tested in cabbage crops against the moths Crocidolomia pavonana and Plutella xylostella. Here too, light outperformed pheromones as a lure, as well as resulting in reduced crop damage.

Light has pull for natural enemies, too

Making use of light’s attractiveness is not only useful for trapping pest insects — it can help recruit natural pest controllers. Work in Japan has shown that the use of UV LED lighting attracts the predatory mirid bug Nesidiocoris tenuis and one of its potential food sources, the damaging whitefly pest Bemisia tabaci. Scientists showed that not only can you pest and predator in one place using the light, but it also resulted in N. tenuis having greater success as a biological control.

Building on that study, scientists in the Republic of Korea investigated whether seven more natural enemies, both predators and parasitoids, might respond positively to light. They tested different wavelengths, finding that five of the seven were highly attracted to either UV or near-UV wavelength LEDs. More recently still, scientists in China have added the ladybird Propylea japonica to the list of prolific predators which show a positive response to UV light.

Photo: Tom Blackwell/ Flickr

From pull to push

Light cannot solely be used for attraction of invertebrate pests or their enemies, with at least an equal body of work focused on whether light can be used to put pests off the idea of settling on a crop at all. The greenhouse whitefly (Trialeurodes vaporariorum), one of the most important pests of greenhouse vegetable and ornamental crops, has been a particular target species for this approach.

A study at Leibniz-Universität Hannover tested the application of blue LEDs as protection for both lettuce and poinsettia against T. vaporariorum, and lettuce alone in the case of the currant-lettuce aphid (Nasonovia ribisnigri). The scientists found that the light sources repelled T. vaporariorum in all settings, but while it showed some effect on the aphid, it was not to statistical significance.

“In fast growing crops with low thresholds for insect damage (e.g. lettuce, seeding nurseries), a combination of optical manipulation and strict monitoring could reduce the number of necessary insecticide applications or other intervening measures. This method could be applied in organic growing as in conventional practice,” the researchers explained.

Other scientists at the same university later found that adding UV light to blue increased the repellent effect against T. vaporariorum in tomato crops during both daytime and night, with light intensity increasing the effects of the treatment.

Digital angle to pest management

The inventive use of LED lighting in greenhouses is an attractive prospect when viewed through the lens of ‘agriculture 4.0’, offering opportunities for digital connection to other technologies such as artificial intelligence, which could be used to control systems or analyse the results of trapping approaches.

The main drawback of using LEDs in pest trapping and disruption is cost. As it stands, the upfront cost of pheromone traps is much cheaper than water traps incorporating lighting. However, pheromone inserts do need to be regularly replaced, and despite light traps requiring electricity, LEDs are a cost-effective lighting technology.

There are also several factors to consider when using artificial lighting as a means of manipulation in integrated pest management strategies. These include light intensity, the time of day when best to use lighting, the height of lights in relation to the movements of target species, as well as the structure of the trapping or disruption technologies themselves.

As often with research on emerging technologies, papers exploring this area are caveated with ‘further research required’ — but it’s fair to say that creative light applications in integrated pest management have a bright future.

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