Onion is resistant to pest attack due to its smell. However, one of the significant causes of losses to many farmers is thrips, the most damaging insect pests to onion plants.
Thrips are insects that suck the cell sap, killing the plant slowly. They often leap or fly when disturbed.
The pest has a high reproductive rate, short life cycle and can develop resistance to chemicals.
Last week, I visited an onion farmer on the outskirts of Nairobi and the impact of the pest on the crop was apparent to me.
The tips of the one-month-old onion crop were folding and bending. On the other side, the bulb onion leaves had whitened, and had silvery marks, an indication of a thrips attack.
There are various species of thrips, but the most common are thrips tabaci and the western flower thrips. They are found both in greenhouses and open fields.
The pest thrives well when the conditions are hot and dry, making onions their favourite crop since they grow well in warm and dry conditions.
Both the larva and adult thrips cause damage to onion plants. They feed under the leaf folds and in the protected inner leaves near the bulb and sometimes they are found when there is heavy infestation.
Under heavy infestation, they cause significant leaf damage, which results in reduction in the photosynthetic area and the plant’s ability to make food for the developing bulb. The injured plants are twisted, discoloured and scarred.
Heavy infestation during the early stages of bulb formation affects the bulb quality, size and the plants have stunted growth.
After bulb formation, the onions can tolerate heavy infestation of thrips, especially when they are almost ready for harvest.
In spring onions, the pest affects the leaf quality due to the formation of the feeding scars making the leaves to look whitened. Thrips are also vectors of the iris yellow spot virus.
AVAILABILITY OF NITROGEN
Thrips also affect garlic, but they are not a major threat to the crop as they are to onions.
To scout for thrips, tap the plant’s leaves against a white piece of paper. If the pest is present, one may see silvery tiny insects.
To minimise losses caused by thrips, use a combination of cultural, biological and chemical methods as over-reliance on pesticides leads to resistance.
Also, plant resistant and tolerant varieties to help suppress thrips in onions. Practice crop rotation and onions should not be planted next to a cereal crop as they act as sources of thrips.
Eliminating weeds also helps in controlling thrips since they act as alternative host to the pest.
Overhead irrigation and rainfall help to control thrips since they wash them away. The use of blue sticky traps plays a significant role in monitoring the adult’s thrips as they get attracted to the colour.
Predators such as lacewings and mites help in the control of thrips. While using predators, keep off chemicals as this would kill them.
Consistent availability of nitrogen in the soil results to reduction in thrip infestation thus supply nitrogenous fertiliser appropriately.
Also, other nutrients such as sulphur and potassium should be provided to the crop for bulb formation.
Use pesticides as the last option and while using the chemicals, carefully read the label and use the right quantity.
Also, different molecules should be used. However, the spraying should be done after the scouting of the pest, which should be done by sampling the leaves or the whole plants.
Thrips also affect cereal crops and vegetable fruits such as tomatoes and capsicum. Also, they attack sukuma wiki and cabbages.