Rid your garden of pesky thrips
DESPITE being in the middle of winter, thrips and other pests are lurking, waiting for spring. If we can deal with them now before they start to multiply, we’ve already won the first battle.
Thrips are deadly. They may be hard to see but inflict immense damage to our plants.
Once they start breeding, vast numbers of these relentless sapsuckers are created. Thrips work undercover, so a great deal of damage can be inflicted before we become aware of their activity.
They mainly live and breed out of sight beneath leaves or within the foliage of some plants.
Too often it is only when leaves or flowers shrivel that we become aware of an attack. The first early signs can be a gradual silvering or streaking of some leaves.
Some flowers wither away even as the petals start to open. Luckily most of the 5000 species of thrips leave our plants alone. In fact, some species help us by preying on aphids, two- spotted mites and other small, soft- bodied insects.
The most destructive thrips seem to target specific groups of unrelated plants.
For example, onion thrips may attack cucumbers, brassicas, cineraria, begonias, cyclamens, gerberas and even sweet peas.
Gladiolus thrips are enormously destructive and occasionally feed off irises too.
Greenhouse thrips, a serious pest of cool- climate greenhouses, thrive outdoors in Tasmania. Over the past couple of decades they have become a nightmare in many gardens, feeding off and weakening rhododendrons, azaleas, some viburnums and Lily of the Valley trees. They also get stuck into liliums, ferns, fuchsias, dahlias, roses and even tomatoes.
Greenhouse thrips, often mistakenly described as Red Spider mites, are only 2- 3mm long, dark brown or yellow. Some are winged. A magnifying glass reveals them, usually folded straight alongside narrow bodies like thin vertical stripes.
Adult thrips are usually accompanied by numerous nymphs, some of which look like tiny aphids.
Thrips have powerful, rasping mouthparts so they can penetrate leaf cells. They draw out the sap, emptying one cell after another.
As they do so, the exhausted cells turn pale. These become the silvery leaves commonly seen on infested rhododendrons and Viburnum tinus. Deciduous azalea leaves become mottled.
When infested leaves are turned over, we can clearly see evidence of these culprits as the masses of rust- like reddish- brown specks.
Most of these specks are not the pests but their numerous droppings.
A closer examination will reveal that some of these tiny “particles” are alive and moving, especially as temperatures rise. These are thrips and their young.
If not controlled, they cause target plants to remain stunted and eventually die an early death. Some keen rhododendron growers have been so inundated with these pests they have virtually given up.
Some control can be achieved by carefully raking up and collecting all fallen leaves now and destroying them.
The ground around each plant and the undersides of all leaves should be sprayed with a contact insecticide such as pyrethrum. I add a tiny amount of household detergent as a wetting agent.
Where serious infestations exist, additional spraying is necessary, preferably at seven- day intervals.
Thrips thrive in dry conditions, which are why the worst infestations occur where the foliage is close to house walls, sheltered from rain. They hate wet conditions, which is why I occasionally place a sprinkler directly beneath vulnerable plants and give the undersides of the leaves a 30- second blast. This method works wonders.
With gladiolus and to a lesser extent iris, evidence of a thrip infestation shows up as vertical silvery foliage streaks. All gladiolus species are vulnerable and damage from thrips can be heartbreaking because flowers wither away even as they begin to open.
Gladiolus thrips spend the winter in crevices and loose, tunic materials around corms.
The dried remnants of old stems are a favoured hiding place. At the end of summer all leaves must be cut off, flush with tops of corms. As the corms become dry during storage, all loose, flaky materials, dead stalks and leaf bases must be rubbed off.
Once thoroughly cleaned, the corms should be soaked in an insecticide solution of pyrethrum and water. Simply add four tablespoons of the insecticide to half a bucket of water, mix well then drop in the corms for an hour- long soak. Later, store the cleaned gladiolus corms on a rack until spring. Immediately before planting, give them another pyrethrum soak.