admin / November 17, 2017

Pests are a major problem in greenhouses, as they are able to multiply in the sheltered environment provided for your plants. They breed so fast that by the time that you notice them, they are causing severe damage to your crops. Many of the pests are developing resistance to the common chemicals in sprays. If that happens to you, try and seek out a spray with a different active ingredient.

Be particularly careful before bringing new plants into your greenhouse, and inspect them to make sure that they are free of pests. In my experience, many of the plants that you see in garden centres or shops, already have infestations of pests. If you have any doubt, spray them first, before introducing to your greenhouse.

Organic sprays. It is possible to buy organic pesticides, which are usually sprays of fatty acids. They work by clogging up the vital parts of the pests. However, you can only apply them two or three times, as they also clog up the plants’ pores. Sadly, I do not find them very effective in killing pests.

Natural Predators. There are a few natural predators that you can introduce to your greenhouse to attack the pests, but there are limitations to their effectiveness.  

Aphids. They are a major pest in the greenhouse, feeding on the sap of plants, spreading viruses, and their sticky droppings can lead to moulds. As they breed at a prodigious rate, you must spray as soon as you see them. There are several different types of Aphid, but most usually green coloured ones are found in the greenhouse.

Crops affected. All crops in the greenhouse can be affected.

Treatment. Spray with permethrin type insecticides or the organic ones based on fatty acids, or soap solutions. I do not find the organic ones particularly effective.

Red Spider Mite. This is the one pest that I really fear in the greenhouse, as by the time that you notice it, you will have already have a severe infestation with much damage to leaves. The first that you usually notice is some marks, the size of a pinhead, on leaves. You may see very fine webs at the edges of leaves, and if you look even closer, you may see very small red-brown spider mites. If you blow on the leaves, it will disturb the mites and you may see them move. If you still have any doubts, look at the leaf through a magnifying glass. The largest concentration of the mites will be on the underside of the leaf. Click on the adjacent photo for an enlarged image.

Crops  affected. Expect to find it attacking any plant in a greenhouse.

Treatment. It is becoming difficult to treat due to pesticide resistance. Consider changing the active chemical ingredient of your spray after several treatments. Spray with bifenthrin, or Provado, paying special attention to the underside of the leaves. As it thrives in hot, dry conditions, you could try spraying with tepid water, but that can lead to other problems of moulds in a greenhouse.

Slugs and Snails. These pests can easily crawl and climb their way into your greenhouse, and feast on your plants and tender seedlings.

Crops affected. Any plants can be affected.

Treatment. Make sure that you  have sprinkled slug pellets on the floor and on staging and trays. My staging is made of a heavy wire mesh and makes it more difficult, but not impossible, for slugs and snails to cross the mesh to reach the pots of plants and seedlings. They will do anything to eat a tender seedling!

Whitefly. I am happy to say that I have never seen this pest in my greenhouse, but other people are plagued with it. Be very careful not to bring it into your greenhouse with plants already infected with it, such as ones brought in from commercial suppliers. Whitefly suck the sap of plants, severely weakening them.

Crops affected. Expect to find it attacking any plant in a greenhouse.

Treatment. Difficult to control with chemical means and will need repeated spraying. As an alternative, try hanging up the bright yellow sticky cards to attract them and catch them as they fly past.

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