The tomato leafminer, a pest from the South.

The tomato moth: what exactly is it?

Behind every caterpillar there’s always a butterfly. And the butterfly responsible for the damage to your tomato plants is the tomato moth, also known as the armigera or Helicoverpa armigera. This lepidopteran is a nocturnal moth of tropical or subtropical origin. Having arrived during the heatwave of 2003, it established itself in the south of France, where it began to make headlines. And, with the latest heatwaves, it is gradually moving north across the territory. Thus, I, living and gardening near Saint-Étienne in the Loire, at about 650 m above sea level, observed its presence for the first time in the summer of 2023, on my tomato plot.

A nocturnal moth active in spring

Male and female Helicoverpa armigera are slightly different. Both are about 4 mm across, and they differ in the colour of their wings, orange-brown in the female, grey-green in the male. The forewings are also edged with a row of black spots. The female is slightly longer than the male, and also more robust. Their mating flight takes place at dusk or at night as soon as the soil temperature reaches 17–18 °C, i.e., in April–May in the south, a little later elsewhere. During their brief lives, the male and female feed on nectar from surrounding flowers and quench their thirst with dew drops. Then, the female can lay between 300 and 500 eggs, typically on young shoots, the undersides of leaves and the flower buds. Spherical and flat at the poles, these eggs change from pale yellowish-white to brown. Incubation lasts only four days! Then the larvae hatch.

Adult female Helicoverpa armigera

The larval development cycle

Tomato moth larvae pass through six developmental instars, the duration of which depends on temperature. From 18 days at 22 °C, larval development can last up to 50 days at 17 °C. Upon hatching, the young wandering caterpillars feed on young leaves. They can be very aggressive, even cannibalistic, towards their conspecifics. In the second developmental instar, they penetrate the fruits, green or already ripe, by boring a small hole, often quite near the peduncle. And there they feed and defecate. Which renders tomatoes unfit for consumption. Towards the end of development, these caterpillars, variously coloured green, yellow, or brown, measure 30 to 40 mm in length.

Once well fed, after the six developmental instars, the caterpillars burrow into the soil to pupate. There they spend the winter as chrysalises. And in the following spring, they return!

What damage occurs to tomatoes and other vegetables?

Tomatoes are the main target of these noctuids, though they can also attack peppers and aubergines, squashes, beans and maize. Sous d’autres latitudes, elles font aussi des ravages sur les plantations de coton ou de tabac. Elles entraînent chez les maraîchers des pertes commerciales énormes, car une fois que “le ver est dans le fruit”, il devient totalement immangeable. Parce que les chenilles grignotent la chair de la tomate, souillée par leurs excréments. En revanche, les ravages sur les feuilles, au premier stade larvaire, sont nettement moins impactants.

As it is fairly difficult to see the moths in flight, or even the eggs, it is when the damage is done that detection occurs. Et le premier indice qui ne trompe pas est le trou dans la tomate. Souvent un seul par tomate, en général bien rond. En découpant la tomate, suivant le stade larvaire, on voit la chair grignotée. Et parfois la chenille encore présente.

A small, perfectly round hole in the tomato is a sign of tomato moth attack

Si les conditions climatiques s’y prêtent, plusieurs générations de noctuelles de la tomate peuvent se succéder sur une période de culture.

How to control the tomato leafminer?

It is relatively difficult to combat the tomato moth. Because, when the first symptoms appear, the caterpillars are quietly lodged inside the fruits of their host plant, completely inaccessible.

To combat them, you therefore need to intervene at other stages of their life cycle. But natural control methods remain few. The only truly effective treatment is Bacillus thuringiensis (Bt), a microbiological insecticidal effective against adults and, above all, the larvae at the first stage of their development. But you must not miss them, as they are vulnerable for only four days, often well hidden under the leaves. As for the adults, they fly at dusk or at night. Needless to say, they generally go unnoticed. That is why market gardening professionals precede Bt sprays with pheromone traps to monitor their flights. It is also recommended to renew Bt sprays every 10 days on all parts of the tomato plant, including, and more so, on the undersides of the leaves.

By carefully observing your crops (and with a good pair of glasses), you can also spot the caterpillars at the last stage of development, when they try to burrow into the soil for their diapause. You can also try to find the eggs!

Finally, the main means of control is the systematic destruction of infested tomatoes. Do not let them rot on the ground on which they fall!

Here, this tomato seems to be attacked by the tomato moth. In fact, it is simply hail damage. No sign of a caterpillar!

And then, you can count on the valuable help of a few natural allies, namely the blue tits (which relish caterpillars of all kinds!), the bats, very effective at dusk, and the hoverflies. That is why it is essential to promote biodiversity to attract, to your garden, blue tits and other predators.

To find out more :

• Bacillus thuringiensis (Bt): a natural insecticidal
• Pheromone traps: why and how to use them in the garden?

Preventive measures to avoid Helicoverpa armigera outbreaks.

As the biological control of the tomato leafminer looks set to be challenging, it’s best to focus on prevention! And preventive measures are numerous. By deploying them all, you will increase your chances of deterring the tomato leafminers from establishing themselves in your garden:

• Install insect nets or insect-netting as soon as you plant your tomato plants, in April or May depending on the region, to prevent eggs from being laid directly on the host plant. If tomatoes are grown under glass, closing openings in the evening and at night is enough to deter the adults.
• Place pheromone traps for the tomato leafminer to monitor the adults’ flight and act accordingly.
• Remove weeds from the base of the tomato plants carefully. After all, the adults can find a good food source there and drink dew.
• Encourage the establishment of natural predators of the tomato leafminer by installing feeding stations and nest boxes, or by planting berry-bearing shrubs for the tits. You can attract hoverflies by planting melliferous plants or by building bat shelters.
• Strictly apply crop rotation. Indeed, the caterpillars overwinter in the soil at the base of their host plants.
• Destroy all crop residues: leaves, stems and unripe tomatoes.


However, here the larva is indeed inside the fruit !

Personally, as every year, I let my four hens into the vegetable garden in winter when crops are scarce. By scratching the soil and pecking, they fairly consistently destroy all the larvae. This season, for now, there are no small holes visible in the tomatoes…