Black & Green Gram
Thrips
Thysanoptera
Insect
In a Nutshell
- Small silver patches on upper leaf.
- Yellowing of leaves.
- Deformation of leaves, flowers and fruits.
Black & Green Gram
Symptoms
Small silver patches are seen on the upper side of leaf blades, an effect known as 'silvering'. The same patches can appear on petals where the pigment has been removed. On the underside of the leaves, the thrips and their larvae sit together in groups alongside their black dung spots. Leaves of affected plants yellow, wither, deform or shrivel. Feeding during bud or flower development later results in scarred, stunted or deformed flowers or fruits respectively and loss of yield.
Recommendations
Organic Control
Some biological control measures have been developed for specific thrips. The insecticide spinosad is generally more effective against thrips than any of the chemical or other biological formulations. It lasts 1 week or more and moves short distances into sprayed tissue. It can, however, be toxic to certain natural enemies (e.g., predatory mites, syrphid fly larvae) and bees. Therefore, do not apply spinosad to plants that are flowering. In the case of flower infestation, a combination of garlic extracts with some insecticides also seems to work well. For species that attack the leaves and not the flower, try neem oil or natural pyrethrins, especially on the undersides of the leaves. The use of highly reflective UV mulch (metalized reflective mulch) has been recommended.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. Due to the high reproductive rates and their life cycles, thrips have developed resistance to different classes of pesticides. Effective contact insecticides include fipronil, imidacloprid, or acetamiprid, which in many products are combined with piperonyl butoxide to enhance their effect.
What caused it?
Larvae and adults feed on plant tissues. Thrips are 1-2 mm long, yellow, black or both in color. Some varieties have two pairs of wings, while others do not have wings at all. They hibernate in plant residues or in the soil or on alternative host plants. They are also vectors for a broad range of viral diseases. Thrips infest a broad variety of plants. Dry and warm weather conditions favor population growth, while humidity reduces it. Adults can be easily carried by wind, clothes, equipment, and containers not properly cleaned after fieldwork.
Preventive Measures
- Plant resistant varieties.
- Use virus- and thrips-free transplants from certified greenhouses and plant nurseries.
- Avoid planting susceptible plants next to weedy areas and alternative hosts.
- Add plastic or organic mulch along the rows to reduce the incidence and progression of thrips.
- Monitor fields regularly.
- Use sticky traps over a large area for mass-catching.
- Prune by cutting plants just above branching points and nodes instead of shearing off terminals.
- Keep plants well irrigated, and avoid excessive applications of nitrogen fertilizer.
- Remove infected plants and any plant debris and destroy them.
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