The cabbage gall mosquito has caused increasing damage to cabbage crops in the last ten years. The practical research center for agriculture and horticulture in Flanders Inagro therefore investigated the control of the cabbage gall mosquito together with a number of CSA (Community Supported Agriculture) organic growers, Koster plant nursery, Pherobank and Howitec in 2019 and 2020.
Contarinia nasturtii
The cabbage gall mosquito (Contarinia nasturtii) is a species of mosquito in the Cecidomyiidae family. This almost 2 mm long gall midge only lives for a few days and lays eggs on the stems or in the hollows of the heart leaves of plants in the cruciferous family. The larvae attack the leaf tissue and the growth point. This infection causes vertigo. Cruciferous weeds, such as shepherd's purse, white herd, cress and common barbara herb, are also visited by the cabbage gall mosquito.
The cabbage gall mosquitoes are usually seen in May-June, when they hatch from the pupae after wintering in the ground. Since the lifespan of the cabbage gall mosquitoes is short (3-5 days), they look for a partner to mate as quickly as possible. After mating, the females each lay around 100 eggs in clusters of 2-50 eggs near the hearts of the plants. The eggs are small and transparent and hatch after 1-6 days. From these eggs, larvae about 2 mm long emerge, which feed at the base of the heart leaves. The life cycle of the cabbage gall mosquito is 24-31 days, depending on the weather. In quiet and warm weather, the cabbage gall mosquito is most active. Below 15 °C, the cabbage gall mosquito is not very active. Since the cabbage gall mosquito is active from May to the end of September, 3-5 generations per year can be counted on. The last generation of larvae overwinter in a spun cocoon in the soil and pupate there.
The larvae of the cabbage gall mosquito feed at the base of the heart leaves. During feeding, the larvae produce a substance that results in swollen plant tissue, abnormal growth and brown corky wound tissue. This wound tissue is characteristic of the infestation of the cabbage gall mosquito. This disrupts the plant's sap flows and the formation of the main stem. This means that there are twisty (young leaves that keep growing twisted), callous plants (the heart of the leaves dies and rots in humid weather) and multiple shoots and heads.
Issues
The cabbage gall mosquito is difficult to observe because of its hidden way of life. In addition, the larvae have usually already left the plant when the damage becomes clearly visible. In addition, the larvae of the cabbage gall mosquito are not easy to distinguish from other larvae. In contrast to the larvae of the cabbage fly or cabbage leaf miner, the larvae of the cabbage gall mosquito have no black mouthparts. Due to the rapid development and several overlapping generations during the cultivation season and the combination with a low damage threshold, it is difficult to control the cabbage gall mosquito.
Monitoring
To gain insight into population dynamics, pheromone traps can be used. This captures the males and can be identified using a microscope. Even when crops are covered, it is recommended to install traps. This is to determine the best time to carry out the work, for example weeding. If the pressure is high, it is advisable to wait before removing the nets.
Controlling
The advice to control the cabbage gall midge in the plot is as follows:
Pheromone confusion and the use of natural enemies are also being investigated in America, but still appear to be too expensive or insufficiently efficient for use in practice.
The solution for organic growers
For organic growers, there remains only one solution and that is to cover with fine-mesh insect screen. A number of CSA growers have tested Howitec's lightweight insect screens Ornata addu 80100, Ornata addu 5080 and Ornata light. In addition to effectiveness, ease of use was also considered. The Ornata addu is praised for its flexibility and nice connection to the ground and offers sufficient protection against the cabbage gall mosquito; even at high pressure. Ornata light is smoother and stiffer to use, but offers comprehensive protection against the cabbage gall mosquito. In addition, Ornata light appears to be stronger than the Ornata addu in practice, although the Ornata addu's UV stability is again better. Howitec has now released Ornata light 2.0. This is an optimized version of the light.
Conclusion
The cabbage gall mosquito is becoming an increasing problem in certain regions, which is why monitoring is very important. To control the cabbage gall mosquito properly, you should limit the conditions for reproduction and survival of the insect as much as possible and cover crops (at the sensitive stage) with a lightweight fine-mesh insect screen from Howitec.
Authors: Femke Temmerman, Inagro and Anton Hulsegge, Howitec.
