Thrip Control in Your Garden

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). Chilli thrips (Scirtothrips dorsalis) were first reported in Western Australia in the early 2000s and can infest roses, vegetables, citrus and other fruit.

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 and leaf litter where they develop wing buds. Newly emerged adults are sluggish at first, but quickly become active and fly off on their fringed wings. They are relatively poor flyers, but can fly from plant to plant. During warm weather, the entire life cycle can be completed in 10 days, and there may be as many as 15 generations per year, so populations of Thrips can grow rapidly.

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 host plants including:

• Vegetables such as Beans, Capsicums, Chillies, Cucumbers, Onions, and Tomatoes
• Rockmelon and Strawberries
• Fruit trees, including Citrus and Olives
• Ornamental plants, such as Carnations, Chrysanthemums, Fuchsia, Rhododendron and Roses
• Thrips also breed in several broadleaf weeds, including Morning Glory, Pig Weed, Shepherd's Purse, Thornapples and Wild Radish.

It used to be thought that Thrips first rasped and scraped away at the leaf surface, and then sucked up the plant 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.

• Flowers, fruits and leaves can be distorted by Thrips feeding in buds.
• Characteristic ''silvering' on the upper surface of leaves caused by Thrips feeding on the underside of leaves.
• Dark droppings on leaves, especially on the undersides of leaves.
• Brown blemishes on flowers, particularly on light coloured blooms.
• Tomato Spotted Wilt Virus (TSWV) symptoms, such as large circular blotches on tomato fruit and ring-shaped brown patches on the leaves of plants, including Calendula, Capsicum, Dahlia, Lettuce, Potato and Tomato. Many broadleaf weeds also harbour TSWV.
• Chilli Thrips cause leaves, stems and flowers to become distorted and discoloured, leaf veins can brown and leaves may appear dried. Chilli Thrips and are most active from spring to autumn. 

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.

Keep an eye out for Thrips while you are watering your plants. For this to work, you will need to know what Thrips look like, 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.

On vegetables like Tomatoes, Beans, Broccoli, Brussels Sprouts, Cabbages, Cauliflowers and Peas, as well as ornamental (non-edible) plants like Roses and Fuchsias, spray foliage with Yates Advanced Garden Insect Killer. Spray foliage thoroughly as soon as Thrips or their damage are seen. Spray both sides of foliage, including on the undersides of leaves. 

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 an insecticidal soap that works via contact, 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.

Control Chilli Thrips (Scirtothrips dorsalis) with the same products mentioned above for general Thrips control. On a range of vegetables and ornamental plants, use Yates Advanced Garden Insect Killer. In organic gardens, use Yates Nature's Way Vegie & Herb Spray. 

• Keep an eye out for the presence of Thrips, plant damage or TSWV symptoms on susceptible plants.
• Watch for tiny insects appearing on light-coloured items on the washing line.
• It is easier to control small populations of Thrips rather than trying to treat large infestations. 

• Remove weeds to reduce breeding sites.
• Prune off and dispose of infested buds and blooms. If they are full of Thrips, they are likely to open up distorted anyway, so you may as well dispose of them to reduce the numbers of Thrips on your plants. Put the prunings in your green waste bin, not the compost.
• Encourage natural enemies of Thrips in your garden.
• Pulling up plants with TSWV will remove the chance of Thrips spreading the disease further.

• Ladybird beetles, Lacewing larvae, Minute Pirate Bugs and predatory Mites are all predators of Thrips. Growing a range of flowering plants in your garden helps attract beneficial predators. 
• Minute Pirate Bugs and predatory Mites are available commercially for the control of Thrips.