Thrips; small pest, big impact

When it comes to insects, it often seems the smaller they are the more trouble they are.

They don’t come much smaller than thrips and it’s difficult (nearing on impossible) to separate the many thrip species. Plague thrips are found on stone fruit, citrus, also on ground cover plants such as strawberries and lots of plant types in between.

Western flower thrips get a lot of headlines, and they are also found on many and varied crops. Perhaps the best you can do when monitoring your crop is just recognise it is a thrip and apply systemics or contact pesticides… Its name or species doesn’t really matter.

And how do you recognise a thrip? Simple; the adults have four wings parked along their backline over the abdomen and these narrow wings are fringed with hairs all the way around. If you get a magnifying glass, you can see that fringing hairs around the perimeter of the wings are actually longer than the width of the wing itself.

See the illustration above. This wing is the easy-to-spot difference to all other insect groups. Thrips are a separate group of insects: Order Thysanoptera (‘Thysan’ is Greek for fringe and ‘ptera’ is Latin for wing) and yep, they are the only insect group with fringed wings. I told you it was simple. Oh, and if there are slimline and smaller wingless insects nearby… they’d be either the first or second stages of thrip nymphs.

The next two stages in the life cycle are the non-feeding propupa and the pupa – sometimes on the plant hiding in dead petals or the axis of leaves but mostly found in the soil or leaflitter, down 100–300mm below the surface where they often overwinter.

To begin the timeline, the young are either ‘born’ alive or, if from eggs, these hatch in a few days. For the next 10 or so days (depending on season/temperature) these nymphs feed and moult on those tender tissues… and then drop off the plant to go into a type of pupal stage in the soil.

In dry summer climates and in winter, there can be a pause of months until conducive conditions arrive again. Adults typically live 2–6 weeks and lay in the range of 100–300 eggs in that time.

Getting off the plant when they are in the last stages before becoming an adult is a significant factor that not everyone knows about.

Spraying the plant has no effect on this pupal stage. If, in just a few days later they emerge, mate and began feeding and egg-laying again, you can easily mistake this rapid reappearance as a failure of the spray application… or wrongly blame insecticidal resistance.

Thrips can also literally ‘arrive out of the blue’. Adults become airborne and are carried long distances in breezes. You might do your pest monitoring today and not find much, then at the next check, there are hundreds of these longish specks.

It is easy to notice them on flowers because most flowers are pale, and thrips are dark but thrips also get their jollies from buds and younger leaves. This is because their mouthparts are designed to rasp the surface cells so that they can lap up the puddles of juice. Soft and juicy petals, new leaf growth and buds are easily rasped tissues.

Little pest… big damage

Rasping or abrading the soft tissue leaves very noticeable scarring and sometimes malformation blemishes. Leaf surfaces can show pale patches where chlorophyll has disappeared (which also inhibits photosynthesis). Thrips can damage the sexual parts of flowers meaning the prevention of fruit development and the setting of seed.

However, it is not just the blemishes; wholesalers, retailers and their customers are understandably reluctant to buy added ‘livestock’ and exporters definitely will not. Some thrip species are vectors of bacterial and viral transmission. Fungus grows in the faeces.

Controlling thrips

Predatory mites and insects are widely and successfully used but sometimes plague thrips and western flower thrips arrive in… you guessed it… plague proportions. If the beneficials don’t control the plague within a few days and it is going to cost you money in lost production, insecticides become your option.

If there is still sufficient withholding time to harvest, systemic insecticides are applied and absorbed into the sapstream which is lapped up by thrips after disrupting the surface. They die. The label withholding period needs to be observed if choosing systemics.

Contact spays such as a natural pyrethrum have only a 1-day withholding period and can be used right up to the day before harvest or anytime you see the ‘invasion’ begin… to give you an immediate kill, preventing the population explosion.

Application options include sprays, mists, or ULV. The aim is to take out all the pests on the plant at the time, giving respite of about a week before the population rebuilds to profit-threatening levels.

A 100% kill is almost a feasible expectation because droplets, whether really fine or a coarse spray will reach thrips which almost always are feeding on the topmost growth. The ‘rule’ is to apply when necessary… not because it’s Friday (or any other set, regular day).

The other application which can make a big difference is to treat the soil. Depending on the soil, the pupae can be as far down as 300mm; in growing media such as coir, they can be anywhere in the bag.

Residual insecticides will usually last the whole season, meaning pre-pupae that enter for their off-plant experience never re-emerge…  and these residues will also kill curl grubs and other beetle larvae feeding on grass roots before they emerge as scarabs, Christmas beetles, weevils, Chrysomelid beetles and others, to attack your trees.

Resistant western flower thrip?

Resistance is best confirmed by some laboratory or controlled testing. I’m not saying it isn’t a problem, but so often ‘Resistance’ is declared by growers that haven’t understood that they haven’t contacted – and therefore killed enough of the population as explained in the paragraphs above.

Written by Ion Staunton of Pestech Australia. Ion is an entomologist and the manufacturer/marketer of PyBo. Questions? Email him on ion@pestech.com.au or phone him on 0407 308 867.

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