Plant Doctor Archive

Thrips damage

CAN you tell me what is wrong with my house plant? It has had this leaf condition for quite some time with no signs of improving.

The alocasia leaf you sent is infested with thrips, tiny insects that feed by rasping away at cells on the undersides of the leaves and sucking out their juices. The dead cells create silvery patches on the leaves which can eventually turn brown, and badly infested leaves may drop off.

Adult thrips are tiny blackish looking insects, just visible with the naked eye, while young thrips are smaller and semi-transparent sometimes with a blackish head and dark markings on the body. Often easier to spot are the sticky, shiny black blobs of droppings scattered around on the undersides of the leaves where they've been feeding. Thrips are common pests on a wide range of outdoor plants and adults can fly or be blown indoors on a breeze. With indoor plants you can reduce their numbers by picking off badly infected leaves. If you only have one or two plants to treat you could take them outside and spray with one of the aerosol insecticides such as Baythroid or Confidor, according to the label recommendations. Regular washing and wiping of leaves on indoor plants is also a good way of reducing thrips damage. And for the record, the name is always spelt thrips - there's no such thing as one thrip, only one thrips!

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.