Choosing the right colour for your crop plant may provide an alternative to chemical pesticides


<p>Selecting the right coloured variety of crop plant may become an alternative to the chemical control of pests. Colour affects plants' ability to withstand the pests that prey on them and infect them with viruses, bacteria and fungi. For example, the cultivation of dark-coloured crop plants would probably reduce losses by spoilage.</p>

Every year, insects cause major damage to crop plants and thereby to horticulture and agriculture. They eat crop plants and infect them with harmful viruses, bacteria and disease-causing fungi.

“Because plants are unable to escape pests, they need to be able to defend themselves. It has been noted that colour plays a part in plants' survival,” says Inspector Kim Tilli of the Plant Health Unit of the Finnish Food Safety Authority Evira. 

Insects avoid red plants

The internal and external characteristics of plants either attract or repel plant-eating insects which may be omnivorous or specialise in a single plant genus or species. Selecting the correct plant variety enables farmers to reduce the damage caused by insects without resorting to the use of toxic chemicals. Carefully considered choices of crop plant colour may become an alternative to chemical pesticides.

“Colours clearly affect the number of insects a plant attracts. Insects avoid red plants because red-leafed or otherwise dark-coloured varieties apparently contain substances that are harmful to insects. Certain varieties may have inbuilt, natural pest control mechanisms that render chemical pesticides unnecessary,” Tilli says.

If farmers favoured red-leafed or otherwise very dark plant varieties, they could reduce the losses caused by insects and even improve the commercial quality of plants. Light-coloured plants can be used to draw insects away from actual crop plants, after which the pests can be removed from the trap crops.

The European Union is seeking to reduce the use of pesticides for pest control. The number of permitted chemicals is also being reduced on a continuous basis. A range of agricultural techniques can be used instead of chemicals. In addition to trap crops, other means of chemical-free pest control include crop rotation, mulching and insect gauze.

Colour also matters in plant breeding

By influencing insects' feeding and egg-laying choices, a plant's colour also has an impact on their growth, development and vitality. The development of crop plants capable of reducing the damage caused by insects would also increase harvest sizes.

“Since colour seems to affect insect behaviour and choices, it may also be an important factor in plant breeding. A feeding experiment with cabbage butterfly larvae revealed that the weight of the larvae did not increase when they ate red cabbage. The experiment's results suggest that the larvae fed with the light variety of cabbage gained most weight, those fed with the green variety weighed slightly less, while larvae that ate the red variety accumulated the least weight. Larvae with the weakest development had fewer chances of surviving and going on to breed,” Tilli says.

Kim Tilli investigated the feeding and egg laying choices of different insects by performing a field experiment as part of his Master's Thesis at the Faculty of Agriculture and Forestry of the University of Helsinki. He also investigated the effects of plant colour on the food consumption and growth of Pieris brassicae larvae.

The effect of leaf colour on the attractivity of plants to insect pests (Master's Thesis, in Finnish). 
Abstract on the Master's Thesis.

For further information, please contact:
Inspector Kim Tilli, Plant Health Unit, tel. +358 400 808 521


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