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Larvae feed primarily on leaves and cause irregular holes (Metcalf et al., 1962; Banham and Arrand, 1970). They may also bore into the heads of lettuce, cabbage and tomatoes. Plants should be inspected for leaf damage and the presence of frass.
Control of T. ni is best achieved by regular field monitoring, applying control measures only when needed. This helps avoid the overuse of insecticides and the potential for the build up of insecticide resistance. Sampling plans should be constructed according to distributional characteristics of the insects and linked to an economic injury level. Control is not often needed because of the abundance of natural enemies, so regular monitoring of natural enemies is also important. Treatment thresholds vary depending on the crop and location and growers are advised to contact their local extension service or a private consultant for treatment guidelines, including the products, which are registered in the area. Normally, spraying should not occur when there is less than one larva per five plants.
A large number of microorganisms have been isolated from T. ni in the field (see Ignoffo and Hostetter, 1984 for a comprehensive list). Commercial preparations of bacteria (Bacillus thuringienis) and fungi (Beauvaria bassiana) can be applied. In low-input situations where it is allowed, virus-infected insects can be reintroduced into the field. This is performed by collecting diseased insects, mixing them in a blender, filtering out large tissue masses and then spraying the virus particles back onto the field.
At least 68 parasitoid species have been recovered from field-collected T. ni (Martin et al., 1984). Although all stages except the adult are attacked by parasitoids, the Trichogrammatidae may hold particular promise as they kill eggs and prevent any subsequent damage by larvae. Parasitoids and predators can be either introduced into the system by initial releases of small numbers in the hope they will multiply, or through the use of mass rearing and supplementary releases. Parasitism and predation can also be enhanced by changes in cropping practices (for example, strip cropping). Biopesticides may also be used; in Trinidad, T. ni attacking cabbage have been controlled by the introduction of Cotesia marginiventris and the nuclear polyhedrosis virus of T. ni (Yaseen, 1978).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
T. ni populations can cause both severe yield and quality losses, especially under dry conditions. Under severe infestations in cabbage, no heads may be marketable at the end of the season.