Rid your gar­den of pesky thrips

Sunday Tasmanian - Tassie Living - - HOME - Peter Cun­dall

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.

UN­DER AT­TACK: Above, streaked glad­i­o­lus leaves in­di­cate a se­ri­ous at­tack from thrips; left, an in­fected rhodo­den­dron leaf.

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