Thrips

Thrips – an overview:

This page provides an overview of thrips that cause damage in vegetable crops. The related tools listed at the end of the page provide detailed information about their identification, damage and management. It is important to be able to identify insect pests such as thrips, and to have unknown species expertly identified so that they can be appropriately managed.

Thrips are small, slender yellow, green, grey, or black torpedo-shaped insects. Out of the 5,000 known species of thrips, fewer than 100 are important pests of crop plants and cause significant damage to flowers, foliage, and fruit when feeding. Most species feed on fungi or plants; others efficiently transmit virus in the Tospovirus group of plant viruses.

Adult thrips are poor fliers, but are easily dispersed by wind and on plants, people, or equipment. Thrips like climbing into small, protected places. They are usually found in flowers or in the growing tips of young seedlings, though they may also be present on the undersides of leaves. Thrips numbers in crops can be assessed by using yellow sticky traps placed at several locations in a crop.

Thrips as sap suckers

Thrips cause direct damage by using their piercing and sucking mouthparts to pierce plant cells and suck the cell contents, which results in the deformation of flowers, leaves, stems, shoots, and fruits. Damage appears as silvering and flecking on the leaves of seedlings. Severe infestations can result in bronzing, yellowing and stunting of leaves. Thrips can also damage buds and flowers through laying eggs, causing deformed fruit to develop.

Thrips as virus vectors

A small number of thrips – currently less than 20 – can transmit virus in the Tospovirus group of plant viruses. Thrips species which transmit Tospoviruses include Western flower thrips, Tomato thrips, Melon thrips, and Onion thrips. They can infect a wide range of vegetable and weed hosts. This has a considerable influence on the distribution and abundance of these insect pests.

Thrips species, hosts and damage

The following table provides information on example host vegetable crops, adult appearance and primary damage for key thrips species.

Thrips species	Host vegetable crops	Description (adults)	Primary damage
Western flower thrips (WFT) Frankliniella occidentalis	Capsicum; eggplant; tomato; beans; peas; lettuce; celery; potato; parsley; beet; spinach; choy sum; bitter melon.	Body is black, brownish, yellow, white, or orange; abdomen extends beyond wing tips at rest; thick hairs at the tip of the abdomen.	Feeds on flowers and new plant growth. Leaves become stippled and scarred. Most efficient vector or carrier of TSWV.
Tomato thrips Frankliniella schultzei	Tomato; lettuce; celery.	Small; dark-brown to black abdomen with banded appearance; clear wings and dark legs	Damage to leaves and young fruit. Virus vector of TSWV and CaCV.
Melon thrips Thrips palmi	Cucurbit and solanaceous vegetables including potato; eggplant; capsicum; melons; zucchini	Cigar-shaped insects; pale green to orange in colour and mostly found on the undersides of leaves.	Damage to leaves and growing points, scarring of fruit, and fruit drop. Virus vector of TSWV and CaCV.
Onion thrips Thrips tabaci	Garlic; onion; capsicum; celery; peas; hairy melon; choy sum; bitter melon; Chinese broccoli; long melon; snake bean.	Smallest species; yellow to dark brown body with more uniform colour of abdominal segments.	Stippled and scarred leaves and other plant parts. Vector of TSWV and IYSV.
Plague thrips Thrips imaginis	Lettuce; beans; tomato; cucurbit vegetables.	Female is yellow to light brown; male is smaller and yellow.	Feeds by rasping plant tissue, mainly attacking flowers causing petal scorching. Flowers may not open properly or may be severely distorted. Also feeds on young leaves.

Pest management

Virus infected plants cannot be cured. Therefore control measures to prevent or reduce the levels of disease in crops by removing or avoiding sources of virus infection and minimising spread by thrips is critical.

Integrated Pest Management (IPM)

IPM is the best strategy to manage and control the source and incidence of both thrips and the viruses they transmit. It uses the least ecologically disruptive techniques to manage pests within economically acceptable levels.

Crop/farm hygiene: Old crops infested with thrips are a major source of virus and should be sprayed for thrips and removed as soon as possible, particularly if young crops are to be planted nearby.
Crop rotation and spacing: Avoid overlapping sowings of susceptible crops and sequential plantings side by side to minimise virus spread from one crop to the next.
Weeds removal: Manage weeds along the headlands, irrigation channels, in fallow land, under hydroponic tables and around greenhouses as they can host thrips and viruses. Weeds that are flowering are particularly attractive to thrips as they feed on pollen. Destroy weeds well before planting, not as crops are planted, as virus infected thrips may migrate from the wilted weeds to the young plants.
Healthy planting material: Viruses can be introduced in infected seedling plants, which then provide a virus source throughout the life of the crop. Seedling plants should be located well away from production areas, kept weed free and systematically monitored for insect pests and diseases with a regular spray schedule in place.
Biological control: If predator numbers are high enough, they can control thrips. Biological control using predatory mites (Hypoaspis and Montdorensis) is an option in protected cropping.
Use of resistant varieties: Varieties of capsicum and tomato resistant to tomato spotted wilt virus (TSWV) are available. The resistance operates by the leaf cells around the point of virus inoculation and preventing virus movement from the immediate area.
Use of thrips proof mesh: Can prevent movement of thrips into greenhouses.
Use of sticky traps: Use blue and yellow sticky traps to capture adult thrips.

Chemical control or use of insecticides

Insecticides can be used to reduce virus spread by controlling thrips. Note that frequent use of insecticides may also lead to the development of insecticide resistance in thrips populations so it is important to rotate chemical groups and follow a WFT insecticide resistance management plan. Monitor for thrips and only use insecticides when required. The overuse of insecticides also kills natural enemies which help control thrips.