09:50 - 04 March 2011
As seedling start to sprout, cuttings burst into growth and the greenhouse comes alive, the greenhouse gardener has to start looking out for greenhouse pests. One of the biggest pest problems in the early spring greenhouse is greenfly. These are a type of aphid that are very partial to soft, fresh new plant growth and seedlings. As the weather warms slightly in spring, any overwintering eggs quickly hatch and the offspring take advantage of the warmer greenhouse conditions and start breeding.
Know your enemy
Be vigilant, keep a check on everything so that you know when a problem is emerging, that’s the time to successfully attack and control it. Normally aphids coincide their lifecycle and time of attack with the period when their host plant is at its most vulnerable. So bursting buds and fresh spring growth in the greenhouse are all susceptible to early emerging pests such as aphids.
Unlike plants in the garden, the greenhouse is such a protected environment that predators such as birds are unlikely to be much help. Instead the greenhouse gardener needs to rely on other methods.
Manual methods
One of the most effective ways of keeping a population explosion down is to physically remove the bugs when you see them. First signs of aphids are fat bugs clinging to new stems and shoots. These can be picked off or even squashed.
Keep an eye out in the garden for ladybirds and collect a few and move to the greenhouse. Adult ladybirds and ladybird larvae feed on aphids and will make short work of low-level populations. This bug eat bug philosophy is also employed by biological control products that can be released into a greenhouse to eat or destroy greenhouse pests. If you intend to harness the power of nature in this way then it is important to understand that you cannot then use insecticides in the greenhouse. If the aphid epidemic is large then it may be better to knock the pest back using an organic contact control before introducing your predators. Don’t be tempted to use any other chemicals, especially systemic pesticides as these will persist in the greenhouse and kill your beneficial insects.
A greenhouse is a good place to use predatory controls as they are fairly confined to the greenhouse environment where they will feast on or parasitise the pests. You do need to identify your problem properly as each biological control is very selective in its host or prey, so if you get the identity wrong, your pest control won’t work.
At the first sign of an aphid problem you can introduce the aphid parasite Aphidius which will parasitise and kill the pest. An aphid-eating midge called Aphidoletes can control larger populations.
Damage limitation
They might be very small but aphids can cause a range of problems. In optimum conditions the adult female can actually give birth to live young, missing out the egg stage and increasing the population very rapidly. It is perhaps the aphid’s ability to breed and multiply so prolifically that makes it such a persistent problem.
Their main form of attack is to suck the sap from fresh young shoots. The range of plants affected is immense and the initial results are stunted growth, and weaker plants. Heavily infested plants can die. Aphids can also spread disease. Just as mosquitoes can pass malaria to humans in areas where this disease is prevalent, aphids can spread plant viruses. These can be debilitating for many greenhouse and garden crops such as tomatoes, potatoes, cucumbers, peas, marrows and squashes and many more besides.
Sooty Mould
Some plants can suffer from another problem related to the presence of aphids; sooty mould. Sometimes this is the first obvious sign of aphid presence. The bugs secrete sweet sticky honeydew, which covers leaf surfaces and sooty moulds grow on this residue, creating unsightly blackened leaves. Although the mould is not considered to be harmful, if there’s enough of it, it can reduce the amount of light that reaches the leaves and so reduce the plant’s abilities to make its own food. Some greenhouse plants such as citrus are particularly susceptible to sooty mould problems.
