There are about 6,500 species of thrips found throughout the world, with about 900 species described in Australia so far. Thrips are members of the order Thysanoptera which means ‘fringed wings’, a trait common to thrips species that have wings.
The behaviour of thrips is varied. About 50% of thrips species feed only on fungi, and some species are predatory on other insects (including on other thrips species). Among the remaining species – the plant feeders - there are some species that only feed on leaves and other species that only feed on flowers. There are very few species of thrips that feed on both flowers and leaves.
Some pest species of thrips are resistant to many of the chemicals registered against them. Even worse, some pest species of thrips transmit plant viruses as well as physically damaging plants while feeding. The most common thrips-transmitted plant virus in gardens is Tomato Spotted Wilt Virus (TSWV).
Thrips are notoriously difficult insects to control because of their small size and rapid breeding. The secret to controlling thrips is to keep an eye on your plants, so that you can spot thrips incursions early.
For example, you could keep an eye out (see Monitor) while you are watering your plants. For this to work, you will need to know what thrips look like (see Description), so that you know that the insects you observe are actually thrips and not something else. A clue to their presence is the damage they cause – see Symptoms of Thrips.
Prevention is always better than cure. There are several things that you can do to prevent thrips damage (see How to Prevent Thrips Appearing).
Regular monitoring of your plants, either indoors or out in the garden, will ensure that a major infestation of thrips doesn’t occur ‘overnight’. Managing thrips is much easier if you can catch an infestation in its early stages. Don’t wait for an infestation to build up to unmanageable levels.
To control thrips, spray pests thoroughly with Yates Nature's Way Vegie & Herb Spray or Yates Nature’s Way Vegie & Herb Spray Concentrate. This is a contact insecticide, so you will need to ensure the spray thoroughly contacts the thrips for the product to be effective. This can be difficult when they are feeding within buds and blooms, but keep an eye on out for symptoms and spray at first sight.
Thrips are tiny, slender insects usually only a millimetre or two long. Because of their small size, you may need a magnifying glass to see them properly. The word ‘thrips’ is used for both single and multiple insects - an individual is not a ‘thrip’.
Thrips pest species to be aware of in gardens, include Greenhouse Thrips (Heliothrips haemorrhoidalis), Western Flower Thrips (Frankliniella occidentalis), Tomato Thrips (Frankliniella schultzei), Plague Thrips (Thrips imaginis), Onion Thrips (Thrips tabaci), and Melon Thrips (Thrips palmi).
Most of the species listed above have a wide distribution, except Melon Thrips (Thrips palmi) which is currently restricted to parts of the Northern Territory and Queensland. It is extremely difficult to distinguish between most of these thrips without the aid of a microscope.
Adult thrips are usually about 1-2 mm long, narrow-bodied, and they have two pairs of fringed wings and six legs. Adult thrips vary in colour from yellowish to black, depending on the species.
Larvae of pest species of thrips are pale cream or yellowish, wingless, but with 6 visible legs.
Thrips develop through complete metamorphosis, meaning their lifecycle takes them through the stages of egg, larva, pupa and adult. Female thrips lay 20-40 transparent eggs in the tissue of leaves, flowers and fruits, and in closed flower buds. Larvae hatch, feed and grow for a few days and then moult into a larger larval stage.
Larvae then pass through two motionless and non-feeding immature stages (‘pre-pupae’ and ‘pupae’) in soil where they develop wing buds. Newly emerged adults are sluggish at first, but quickly become active and fly off on their fringed wings. During warm weather, the entire life cycle can be completed in 10 days, and there may be as many as 15 generations per year.
Thrips can reproduce parthenogenetically (i.e., without males), so in some species (such as Greenhouse Thrips) all adults are female. With Western Flower Thrips, males do occur, but females outnumber males about 4:1. Adult females usually live for about 4 weeks - twice as long as males if they occur.
Thrips of various species damage a wide range of plants including:
Image above: a group of Thrips larvae,
including the symptoms of silvery leaves and black droppings.
(image courtesy of Elise Dando)
It used to be thought that thrips first rasped and scraped away at the surface of a leaf, and then sucked up the sap. Thrips are actually sap-sucking insects - they suck the contents from individual plant cells – there is no rasping or scraping involved. Thrips will often be found feeding on the underside surface of leaves.