Vegetarian insects, arthropods and mollusks can ruin plants in all sorts of ways and are the bane of the gardener’s life. Sometimes you need to take urgent action, yet at other times you don’t need to bother doing anything at all. This is why it pays to know your enemy.
Identifying Absent Pests
Plant pests work in lots of different ways. Very often you will never spot what is causing the damage. Many pests are at their most active at night, so unless you venture out with a torch, all you will normally see is the results of their work. In some cases this is enough to identify the culprit and take the necessary action.
If leaves have been ‘rasped’, leaving a skeleton of ribs, with perhaps a few slime trails, you can safely blame it on snails. Nowadays they are dab hands at climbing – they will even shin up trees and walls, often going to an amazing height to reach something tasty.
Brassica seedlings with lacy holes in the leaves have almost certainly been attacked by flea beetle. Rhododendron leaves with scallop-shaped bites out of the edges of the leaves will have been nibbled by vine weevil. If vine weevil adults are around, look out for the larvae that may be attacking susceptible plants, too.
Plants With Particular Pest Problems
Some plants attract particular pests. Tomatoes and fuchsias are martyrs to whitefly; cacti are often victims of mealy bug; orchids and citrus trees, in common with many conservatory shrubs, suffer from scale insects. There are also lily beetles, cabbage white caterpillars, and carrot root fly. At least if you know what to expect, you know what to do about it. Alternatively, you might decide not to grow the sort of plants that are susceptible to problems. If you don’t grow fuchsias in your greenhouse, for instance, you are likely to keep the rest of the plants relatively clear of whitefly.
You don’t always need to know precisely which creature is responsible for causing damage, as most chemical sprays eradicate most common pests. If aphids are the problem, you can get specific aphicides that kill only aphids and don’t harm beneficial insects, so use these wherever possible.
Some pests, however, are not easy to get rid of. Large caterpillars and beetles such as the lily beetle are resilient creatures that take so long to absorb a lethal dose of pesticide that, since they continue feeding on your plants in the meantime, it is better to pick them off by hand.
Don’t be in too much of a hurry to reach for the pesticide: you do not always need to do anything about pests. If an infestation is small, or likely to disappear on its own, there is no need to do anything at all. Aphids on roses in spring are a good example – there are plenty of baby Bluetits and hungry adult birds that should take care of this problem for you.
A low level of insect population is actually a good thing in a garden, as it ensures there is always a thriving population of predators ready to increase at a moment’s notice if pests start to get out of hand. That is what you call natural balance, and it is well worth having. Keep the pesticide for serious outbreaks of an infestation that really threaten a plant and then use it selectively – there is no need to smother the whole garden with it; just treat the affected plants.