Thursday 22 August 2019

Diarmuid Gavin on pest patrol - how greenhouse creepy-crawlies can be nipped in the bud


Get your boots on and check for unwelcome pests...
Get your boots on and check for unwelcome pests...

Diarmuid Gavin

Our gardens, whether nicely kept or on a sliding scale of messy, are teeming with growth and life just now. Plants are growing, blooming and fruiting and many of the critters which live around them or feed from them are enjoying this season of plenty.

Those of us who enjoy a bit of greenhouse gardening know that the issues of pests and diseases inside under the protection of glazed panels can be even more exaggerated than the problems they create outside. Glasshouses are a great addition to any garden, allowing the keen grower to propagate nearly all year round. They're also attractive to insects who enjoy the snug indoor accommodation and plentiful supply of food. While low levels of such pests can be tolerated, what do you do when there's an infestation and how can you prevent this from happening?

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Glasshouse hygiene is a good starting point. An annual clean in autumn or winter and disinfecting of benches and floors will disrupt the lifecycles of many that choose to overwinter in your digs. Good ventilation will help with humidity and try not to overcrowd the place with plants. Hanging yellow fly traps can control fly populations too.

Weeds can be a host for a variety of unwanted visitors so keep on top of them. Regular patrols will spot problems before they spiral out of control and many of these creatures can simply be removed by hand. However, sometimes it can feel like you're overrun by them so here's a guide to some methods for the most common culprits.

When you brush past a plant in the greenhouse and a cloud of white flies comes out, then you have whitefly. A common glasshouse pest, it's a sap-sucker, and you'll be able to detect it by sooty marks on your leaves - this is a mould growing on their sticky excreta. Leaves turn yellow and eventually drop off with large infestations.

Insecticides, whether organic or chemical, will need repeated applications. Organic insecticides are made from plant extracts so can be used on edible crops. However, with any insecticide, you run the risk of killing beneficial insects as well so it can upset the natural balance in your garden. An alternative to sprays is using biological controls. This is where you release beneficial insects that will eat the undesirable flies. In this case it's a parasitic wasp called encarsia which gobbles up the larval form of whiteflies. It's important that you don't use insecticide at the same time as that will kill the wasps as well. As for the black mould, just wipe this off with a damp cloth.

Red spider mite is another sap-sucking insect. Usually found in warm glasshouses and conservatories, it can holiday outside in the garden when the weather's right. It's not fussy so ornamental and fruit and veg are all prey. The mites are microscopic but the damage they cause is visible to the eye - a fine mottled yellow appearance on the leaves and sometimes they weave a fine web across the foliage which becomes pale and dries up. The solution? They hate water so a good hosing down will help and regular misting is a good idea. There is a biological control for this - a small mite called phytoseiulus which, when released in your glasshouse, will gobble up the red spider mite.

Glasshouse thrips are tiny black insects that cause a silver mottling on leaves and flower distortion. Ask for blue sticky sheets from your garden centre and hang these above your plants. There are a number of predatory mites that you can order online such as amblyseius, or use organic sprays.

There are also lots of homemade deterrents. These involve mixing varying concoctions of liquid soap, garlic, neem oil, peppermint, vinegar or baking soda diluted in water in spray bottles so if you're a keen DIYer, get mixing in the kitchen and I'd love to hear from you if you've had success going down the chemical-free route.


Top Tip

Check new arrivals for any insect baggage they are carrying with them - the undersides of leaves are often a hiding place.

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