Minute small flies, being less than 2mm in size - Thrips - cause streaky marks on flowers and foliage. They suck the sap out of leaves and flowers, and generally disfigure the plants’ appearance. Both adult flies and the young pupae feed on the plant. Thrips are very small insects - just visible - and are more active in hot dry weather – when the plant is lacking in water.
Thrips are also known as Thunder Flies, for they are very noticeable in or after thundery weather in warm temperatures.
The marks are usually a silvery colour and are pronounced on Gladiolus flowers in particular. With severe infestations of Thrips the leaves can take on a silvery sheen, but can also leave black spots on foliage and flowers alike.
It is interesting that Thrips – Thunder Flies - mainly attack actively growing plants but which are under stress for varied reasons such as poor watering regime, resulting in lack of plant vigour. (Maybe this makes the sap less dilute and therefore more tasty – and nutritious! – A Guess)
A common Thrips shown for scale on a finger. (The singular and plural name are the same with Thrips!)
Thrips are mainly a problem with indoor and greenhouse grown plants, but can also affect plants outdoors in the right dry conditions. Most greenhouse plants can be affected. Thrips can be imported into your environment with the acquisition of new houseplants or even cutflower.
Not a huge pest - unless you are going to 'show' your gladiolus blooms! Contact spray such as Bifenthrin will work as will the systemic insecticide Provado. There is also a biological preventative control with the purchase and introduction of predatory mites – Amblyseius spp. These will need to be introduced as soon as the temperature reaches 50F (10C). For practical reasons, this is only an option in the greenhouse, and probably if you are growing commercial crops – or exhibition crops.
Indoors or in the greenhouse, sticky pest trap strips can at least forewarn of the presence of Thrips though they are generally ineffective in controlling the small flies.
An early winter cleaning of the greenhouse is essential to control these and many other greenhouse pests, that overwinter in debris and crevices.
Also known as Thunder flies, Storm Flies and Corn Lice. Thrips are instrumental in spoiling leaves and flowers
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