Our gardens are home to many natural predators, from the familiar ladybugs and spiders to the less-familiar rove beetles and parasitic wasps. These beneficial garden insects, or “beneficials,” are your garden’s first line of defense.
Natural predators are important in all habitats, natural and artificial. In nature, the populations of wasps, ladybugs, spiders, and birds have long regulated the abundance of pests. For example, spiders control Douglas-fir tussock moth caterpillars, and birds prey on spongy moths. Under normal conditions, predators can prevent outbreaks when pest numbers are low to moderate, but problems can arise when a change in the weather or a natural disaster occurs, such as a fire, flood, or drought, which all disrupt the balance between prey and predator.
The Predator-Pest Balance
Much research has been carried out on predator-pest relationships, and it’s clear that systems with low biodiversity are particularly vulnerable to pest outbreaks. This is what we must try to avoid in our gardens. In my own growing space, I’m aiming for a mosaic of habitats to attract a wide range of beneficials to prey on different types of pests. Over the past 10 years, I’ve seen a steady increase in biodiversity, and now, it’s not unusual for me to wander around the garden and spot a variety of predators. I still have pests, of course. For these predators to thrive, there must be some prey animals, so I’ve had to learn to live with low levels of pests to keep my beneficials fed.
A key challenge, though, is to ensure enough predators are around early in the growing season to control the aphids, whiteflies, red spider mites, and various larvae so their numbers don’t spiral out of control. Peak season, as far as garden pests are concerned, is early summer, so you’ll need beneficials to multiply rapidly to take advantage of the food bonanza through midsummer. You can achieve this by understanding and identifying each predator’s life-cycle stages and dietary requirements as well as by providing the right plants and overwintering habitats to enable them to thrive.
Don’t Overreact
It’s important not to overreact when you see pests. For example, you might notice that your broad beans are smothered by black flies and decide to do something quickly to prevent serious damage to the crop. But this is where you have to be careful. If lots of prey is around, you likely already have increasing numbers of predators, as the predator population will lag behind the prey population. If you’re tempted to use a chemical spray, you may well take out both prey and predator, and, in doing so, you’ll destroy the natural cycle and won’t have any chance to get back some natural control. If you can, be patient, let nature take its course, and wait for the predators to “kick in.” If you can’t wait, check carefully for any predators on the plants before using an organic product – or simply your fingers – to remove or squash the pests.
Parasitic Predators
I once read an article that described the actions of the parasitoid wasps as being far worse than any science-fiction movie. The scene of the creature bursting out of its living host in the film Alien may seem far-fetched, but it’s happening every day in your garden, albeit on a much smaller scale. A vast range of parasitic predators, or “parasitoids,” dwell in the garden. Adult parasitoids live normal, independent lives, but their larvae hatch inside another living organism, consuming it and growing. This benefits our gardens if the host is a pest species. Usually, the host dies before or as the parasitoid emerges to live its adult life. Among the most common parasitoids in the garden are parasitic flies and wasps. Some infest a range of hosts, while others seek out specific hosts. The larvae feed on the host and then pupate, either inside or out, before emerging as an adult. The adults generally feed on nectar and pollen.
Parasitic Flies
These aren’t the annoying houseflies and bluebottles you may know, but bristly flies that are mostly gray-black in color with short antennae and huge eyes but no biting mouthparts. They lay their eggs on leaves, so the host will either eat the eggs or the newly hatched larvae as it feeds. Parasitic flies parasitize hosts, such as caterpillars, sawfly larvae, and leatherjackets. Once the host dies, the larvae will continue to eat the remains before pupating and overwintering. One of the United Kingdom’s largest flies is Tachina grossa, which parasitizes caterpillars; it’s so large that it’s often mistaken for a black bumblebee.
Parasitic Wasps
Wasps and bees belong to the order Hymenoptera: insects with two pairs of wings and a distinctive narrow waist. Included in that order are honeybees, bumblebees, solitary bees, and social wasps – remarkably, of the U.K.’s 7,761 species of Hymenoptera, around 6,500 are parasitoid. Parasitic wasps vary enormously in size, ranging from 1 to 30 millimeters (1-1/4 inches) long. Many females have an ovipositor, a stinglike attachment at the end of the abdomen, which is used to pierce the body of the host and lay eggs. The parasitic wasp’s hosts are mostly butterfly and moth caterpillars and pupae and sawfly larvae.
It’s easy to spot the larger Ichneumonid wasps, but the small wasps tend to go unnoticed. Both inhabit our gardens. Like parasitic flies, the parasitic wasps can attack the egg, larval, and adult stages of the host. Once the parasitoid egg hatches, the larvae will feed on the host’s body tissues, taking the fat stores and nonessential organs first, as they don’t want the host to die too soon. Once their larval stage is complete, they’ll pupate. Some will pupate outside the dead host, others inside. Among those most commonly found in gardens is the tiny braconid parasitoid Cotesia glomerata that looks like a flying ant and parasitizes the caterpillars of the large white butterfly.
Chalcid wasps (pronounced “kal-sid”) are just 3 to 9 millimeters (1/8 to 1/3 inch) in length. You can identify them by the swollen femurs on their hind pair of legs and by their glossy, metallic colors. Because of their size, the number of chalcid wasps is often underestimated, but they’re incredibly useful. They lay their eggs in the eggs and larvae of flies, beetles, moths, butterflies, leafhoppers, thrips, and scale insects. Occasionally, you may spot one tapping leaf surfaces with its antennae in search of their host’s “scent,” but the presence of sick or dead hosts is a sure sign you have a healthy population of chalcids. Many of the smaller parasitic wasps are used as biocontrol in greenhouses.
Social wasps aren’t the most popular of insects, but we need to encourage their existence, because they’re such a useful predator! There are eight species of social wasps in the U.K., and they’re both pollinators and predators. Their life cycle will start in autumn, when a mated queen hibernates in holes while the rest of the colony dies. She’ll become active in spring, find a nest site, and start building her nest and laying eggs. Four weeks later, the first generation of workers will emerge. The workers will hunt for insects, especially caterpillars, to feed to the larvae, while the workers themselves will feed on the sugar-rich secretions of the larvae. Social wasps are valuable predators in the garden, as they help control the numbers of insect pests. But by late summer, the number of wasp larvae will decline and the food supply for the workers will dry up. Then, they’ll have to seek out sugar, which will result in damaged fruits and conflict with people.
Other Beneficial Predators
Parasitic flies and wasps aren’t the only creatures that can rid unwanted pests from your garden. Here are a few more predators that can create a diverse defense for your plants.
Spiders
Spiders are generalist hunters and have a varied diet. One important fact is that they tend to kill more prey than they eat, so they’re incredibly useful in spring when they can limit their prey’s early population growth and generally exert a stabilizing effect. A garden can contain a surprising diversity of spiders, which is important, since the failure of one species won’t affect pest control if there are others to take its place. You can boost spider diversity by covering the soil with mulch, especially straw, and building log piles and bug hotels.
Centipedes
The fast-moving centipede makes its home in leaf litter, under pots, in logs, and in the compost heap. Centipedes use their jaws to inject a paralyzing venom into their prey. It’s easy to distinguish between the centipede and the closely related millipede: Centipedes have a single pair of legs per segment, while millipedes have two. Even millipedes aren’t harmful; they mostly eat dead and decaying matter, which is why they’re found under the bark of rotting logs and in the compost heap, so they also have a useful role to play. Only occasionally will they be tempted to feed on seedlings.
Earwigs
Many gardeners trap earwigs with upturned clay pots stuffed with straw and set on sticks, especially around dahlias, because they can damage the flower buds, but these insects’ benefits far outweigh their disadvantages. Earwigs are scavengers, predators, and pollinators. They feed on dead and decaying matter for the most part, but they’ll also prey on aphids, snails, and other small pests and they’ll help control codling moths on fruit trees. If earwigs are causing a problem around your young crops and flower beds, use live traps and move them to a log pile.
Amphibians
Toads prey on slugs and snails, grasshoppers, ants, flies, and other invertebrate animals. They live away from water as adults, digging out shallow burrows in deep leaf litter and log piles in which to shelter and overwinter. You’ll also find toads in compost heaps, so do a toad check before you stick your fork in it. Frogs are similar in many respects to toads, eating a similar range of prey, but they’re more agile. They hibernate in pond mud or under piles of logs, stones, and leaf litter. Both frogs and toads need water to breed, so if you want to encourage frogs and toads, then building a small pool is the answer.
Reptiles
If you live in a more rural area, it’s not uncommon to find grass snakes, slow worms, and lizards in the garden. I’ve found grass snakes under piles of compost in the hoop house, and gardeners frequently report seeing them under sheets of metal and finding their eggs in compost heaps. Grass snakes are useful predators, eating insects and small mammals, but they also consume frogs and toads, which make up the bulk of their diet. Slow worms emerge from their shelters at dusk or after rain to prey on insects, slugs, snails, worms, and spiders.
To encourage reptiles, provide an open, sunny spot where they can bask. I put down corrugated metal sheets and broken pieces of slate, under which grass snakes and slow worms can sun themselves. Lizards enjoy a sunny wall or pile of rocks. The heat of a compost heap will attract the reptiles too. Think about hibernation places as well, such as an undisturbed pile of rocks, a log pile, or a pile of leaves.
Don’t shy away from creating an inviting habitat for natural predators so they can take care of your garden pests. With just a few inexpensive adjustments, you can watch them protect your plants and bolster the diversity of your space – a symbiotic relationship for both gardener and garden-dweller.
Sally Morgan is the editor of Soil Association’s Organic Farming magazine, writes regularly for gardening and smallholding magazines, and is a member of the Garden Media Guild. Sally also runs smallholder courses on her organic farm in Somerset, U.K. The following excerpt is from her new book The Healthy Vegetable Garden: A Natural, Chemical-Free Approach to Soil, Biodiversity and Managing Pests and Diseases (Chelsea Green Publishing, September 2021), available below, and is reprinted with permission from the publisher.
