The tomato leaf miner – a nightmare for farmers in Kenya and Switzerland

The tomato leaf miner moth "Tuta absoluta", which originates from South America, became a nightmare for tomato producers worldwide within a few years. Luckily today, there are effective organic methods of pest control, as a visit to the tomato canton "Geneva" in Switzerland shows.
 

An infestation of Tuta absoluta makes tomatoes inedible. The moth can destroy entire harvests. (Picture: Agroscope)

By Claire Muller 

Its name alone sounds terrifying. The moth Tuta absoluta, also known as the tomato leaf miner, first appeared in Switzerland in 2009, most likely introduced via foreign imported tomatoes destined for our supermarkets. In the canton of Geneva, where 20% of Swiss tomatoes are produced, this insect, which originates from South America, spread in no time. It then disappeared almost completely until 2018, when it suddenly reappeared and caused considerable damage in several greenhouses. "The moth took control of an entire greenhouse within a few weeks, which led to enormous yield losses," reports Gaëtan Jaccard, who advises and accompanies farmers in Geneva and Vaud on behalf of the fruit and vegetable production unit.

Entomologist Serge Fischer has dedicated part of his scientific career at the Agroscope Research Station in Changins to insect pests, including Tuta absoluta in particular. "This moth likes it warm and is therefore perfectly adapted to the living conditions in the greenhouse, where - pupated - it spends the winter protected from frost. It hatches as soon as the temperatures rise, precisely when the tomatoes begin to germinate. This way it finds food immediately and can reproduce." 

Well camouflaged and fertile

The Tuta absoluta is perfectly camouflaged, a big advantage over other moths and bugs that also pose a threat for vegetable growers. "This nocturnal small butterfly remains invisible despite its 5 to 7 mm length as long as you don't set up pheromone traps for biological control," explains Serge Fischer. Thus, the females lay their eggs on the backside of the leaves, and the larvae immediately penetrate the plant after hatching. "They dig tunnels that quickly penetrate large parts of the plants, preventing any photosynthesis, which slows down their growth." They then infest the fruit, making it unmarketable. Another <trump> of the Tuta absoluta is its above-average rapid reproduction – a female lays up to 250 eggs. 

"In sub-Saharan Africa, the international insect research institute icipe, with the support of Biovision, has developed various methods of integrated pest management using a parasitoid wasp species that originates from South America but is widespread in Kenya and Uganda and thus adapted to local conditions." Switzerland is also trying to use auxiliary insects such as the Macrolophus, a predatory bug that has proven to be an excellent ally for vegetable farmers in eliminating mites, whiteflies and butterflies. "This bug is an efficient opponent of the Tuta," confirms Samuel Hauenstein of the Research Institute of Organic Agriculture (FiBL). "However, it must be used in sufficient numbers, namely one to two bugs per square metre, and it needs optimal living conditions." In recent years, vegetable growers in Switzerland have increasingly resorted to insecticides. Although these are natural agents that are permitted in organic farming, they also affect beneficial insects – a problem that Biovision also encountered in the project to control the tomato leaf miner in Kenya.

  • A pheromone trap covered by Tuta absoluta: Since the insect is almost invisible, its occurrence in a greenhouse can only be controlled with the help of these traps before it causes damage. (Photo: Agroscope)
  • Damage to leaves and fruits: The tunnels dug into the leaves are easily recognisable and lead to the rapid withering of the plants. Fruits infested by Tuta absoluta are no longer marketable. (Photo: Agroscope)

Protection through confusion

"Fortunately, the sexual confusion method of controlling Tuta absoluta is now permitted in Switzerland," says Gaëtan Jaccard. The application of pheromone traps has been permitted since two years. The male moths are attracted by the female scents  and stick to the traps, thus preventing mating. Although this method has been used in fruit and wine growing for 25 years, it first had to be tested for vegetable growing. The approval of pest control methods is a lengthy process in which different data, for example on the effect, must be examined and included. Although the use of pheromone traps is expensive because it is time-consuming to install the traps, producers can use them to keep the devastating pest under control - and thus offer greater safety to all those involved along the tomato supply chain in Switzerland.