Beneficial insects come in many shapes and sizes, but their numbers can be curtailed by misguided pesticide applications or as a result of inaccurate identification. Many are essential pollinators while others are predators feeding on plant pests and other invertebrate animals. These include ladybugs, some hoverfly larvae, and lacewing larvae, while ground beetles feed on a wide range of soil-dwelling pests.
This is the use of nature’s own armory of predators, parasites, and pathogens to control pests. They are of most use where they can be released into a protected environment, such as a greenhouse. Some nematodes, as used to control slugs and vine weevil, can also be used outdoors, but only when soil temperatures are at or above 41 degrees F. Accurate and early identification is essential to ensure the correct control is used and at the right time of year. If in any doubt, seek advice. In some cases, chemical control may be the only option when you have an infestation, but only apply according to the instructions, especially when spraying edible crops. Be aware that the use of insecticides is likely to kill biological controls and beneficial insects.
Barriers are a very useful way of preventing pests from getting to your prized herbs, let alone damaging them. Most are designed to counteract the pest by being uncomfortable to cross or impossible to jump or climb over. Copper tape supposedly gives slugs a small but harmless electric shock, while bands of vaseline or grease make movement difficult for slimy creatures. Take care to remove fallen vegetation as this can create a bridge that will soon be discovered and exploited to the full. Sharp sands, gravels, and ground-up eggshells are used relatively successfully to restrict snail damage. Used on their own, each provides a modicum of protection, but if used in conjunction with biological control and considered use of slug pellets, any damage will be minimal.
Slugs and Snails
These cause massive damage to seedlings and developing leaves. Organic control can be hopeless, but reducing the population using pet-friendly pellets from early spring may enable less drastic methods to be used in future.
Adults leave notches in leaf margins and larvae devour roots. Use insecticidal drenches on non-edible container plants, but biological controls are also available. The adults are active at night when they can be seen by flashlight and destroyed.
Watch for signs of a growing infestation, including ants rushing around or stems and leaves sticky with honeydew. Biological and chemical controls are available and can be effective, but the former is best reserved for indoor crops.
Bay Sucker Nymphs
These insects suck sap, causing leaf curling and discoloration. As adults they may be visible, resting on young shoot tips, and should be picked off. Insecticides are unlikely to be effective, but removing affected leaves can help.
They look beautiful, but are a serious pest of lavender and rosemary. Control by removing adults and larvae by hand, or spray ornamental plants with insecticides and treat culinary herbs with pyrethrum or thiacloprid.
Rabbits and Hares
Such pests can cause severe damage and may even kill young trees and other plants. Tree guards or fencing can be effective, but keep a close watch for gaps. Live trapping can also be used where fencing is not practical.