Fighting leaf miners: Part 3

Tuta absoluta is resistant to a number of insecticides and can deform fruit, making it unmarketable, says Bill Kerr.

The latest leaf miner to trouble tomato growers is the tomato leaf miner, Tuta absoluta, which is usually referrred to simply as Tuta. It belongs to the Lepidoptera family, which includes moths and butterflies. Other leaf miners are Diptera (flies), which are far easier to control with natural enemies.

Tuta originates in South America and fairly recently started appearing in other parts of the world. The first specimen in South Africa was found in the Kruger National Park in 2016.

As with the other leaf miners, it has developed resistance to a number of insecticides. With a life cycle as short as three weeks in warm conditions, there is plenty of opportunity to build up resistance. This creates a problem for farmers, as new insecticides come onto the market more slowly than the pests build up resistance.

A further problem is that the larva of this moth feeds between the two layers of skin on the leaves or bores directly into the fruit. Although it does little actual damage, it can deform the fruit and make it unmarketable. It can also make the fruit go rotten, especially during ripening.

RECOGNITION

Tuta moths are small and elongated, and a mottled, brown/grey colour. They are nocturnal, but often fly out of the foliage during the day when disturbed.

Each female lays up to 260 eggs. Given the pest’s rapid life cycle, it can quickly destroy a crop.

Tuta makes blotches on the leaves where it feeds, rather than the conspicuous serpentine tunnels caused by other leaf miners.

Although it can attack a number of plant species, it has a definite preference for tomatoes. The fact that it can proliferate away from croplands can be an advantage, however, as this provides opportunities for natural enemies of the pest to gain the upper hand. This happened in the case of Liriomyza leaf miners, and the parasitoids that achieved this can provide complete control in most cases.

It remains to be seen at this early stage whether this will happen with Tuta. I have already started seeing promising signs, which will be discussed in next week’s article.

INTEGRATED PEST CONTROL

In South America, control using solely insecticides has been unsuccessful. In addition to rapid build-up of resistance, the fact that the larvae feed between the surfaces of the leaf means that many insecticides will be less effective from the outset.

For more than 20 years, a well-known organic product that was safe for humans and beneficial insects alike was available on the South African market. I personally found it highly effective. Unfortunately, the company was then required to carry out a full toxicology evaluation of the product. The cost of this was too much for the company to bear, so it stopped making the product.

Sadly, a similar product succumbed at the same time for this reason. When using crop chemicals, it is necessary to be very careful. Many pesticide control programmes are made up of a number of different, unrelated products. This is the correct approach, but all too often one of these products will kill off the pest’s natural enemies. Every product in the programme should be predator-friendly to ensure that the beneficial insects can control the pest.

ALTHOUGH IT CAN ATTACK A NUMBER OF PLANT SPECIES, TUTA ABSOLUTA HAS A PREFERENCE FOR TOMATOES

WIKIMEDIA COMMONS Signs of Tuta absoluta include blotches on the leaves of the plant.