Weeds can be looked at as useful because of their ability to thrive in adverse conditions, their hardiness and their value as green manures -particularly because of their accumulation of scarce minerals. But more importantly they are essential for maintaining populations of beneficial insects. In this article I am now going to ask you to broaden your outlook even further and to appreciate pests as extremely valuable allies to us gardeners.
Much as weeds are only plants in the wrong place pests are really only creatures whose use we have not yet realised. Many common pests act as pollinators; flies and wasps can be just as effective as bees and butterflies if not as pretty, even pollen beetles aid flowers as they never consume it all.
In some cases pests are useful indicators of another problem or imbalance. It is usually the unhealthy or over-lush growth that suffers from a pest invasion. For example aphids do not actually suck sap; aphids pierce the plant's cell walls and the plant's internal pressure forces sap through the aphid. Thus aphids prefer and congregate on plants with the highest pressure and will actually leave plants with lowered sap pressure. High sap pressures are no more a sign of good health than high blood pressure and often indicate the plant is struggling against an excessively fertilised soil or suffering from water stress. Sudden increases in pest numbers may also indicate the dearth or death of their controlling predators and parasites and warn us to take corrective action. More slugs and snails chomping away in our garden should teach us to encourage more songbirds and hedgehogs and not to reach for the poisonous slug pellets which probably caused the demise of the natural predators in the first case.
In some ways we should regard most pests as nature's way of controlling sickly and diseased plants. Much the same as wolves keep herd animals healthy by removing the sick, undernourished and unfit before they spread their diseases or poor genes to the others. For example you only have to plant out lettuce or cabbage to soon observe how slugs and snails prefer wilting leaves to plump. Likewise I have often noticed how the cabbage white caterpillars seem satisfied with nibbling the outer leaves of my Brassicas but descend in hordes to decimate the hearts and all of the sickly crops of others who drench their garden with fertilisers and pesticides!
A more surprising benefit pests can confer is saving us work by thinning and pruning. It is usual for the apple trees to thin themselves with the June drop but you may observe that there is a later thinning of apples infested by sawfly maggots, which redden and fall weeks before the rest of the crop which then each swell to a greater size. Many gardeners have noticed that redcurrants suffer apparently appalling attacks from their leaf blistering aphid and have then been pleasantly rewarded with large and heavy yields of fruit. The aphids are merely tip pruning the redcurrants, damaging the topmost leaves allowing more light and air to penetrate to the ripening fruit, reducing the sap pressure thus changing vegetative buds into fruit buds to ensure even larger crops the following year. This is substantiated by our actions when we summer prune the redcurrants and remove the blistered foliage, and more besides, with exactly the same intentional result the aphids have already initiated. The same goes for sweet cherries; most years their leaves are curled and the tips are withered by black aphis attacks. This again saves us the effort of summer pruning and ensures a good supply of fruit buds for the following year; so if you wish to increase the cherry crop forget aphid damage and find a way to keep the blackbirds off!!!
Obviously if we wish to have a viable population of predators and parasites in our gardens then we have to keep a minimal level of pests alive to feed them. There will be no song thrushes in the garden with no snails! However there is a difference in the degree of pest populations needed to maintain predator and parasite populations from the optimum levels for increasing them. If we wish to have more songbirds we need endure more snails -but they need not all be living on our lettuce and Hostas. Instead by creating wild areas well away from our most susceptible crops we can encourage greater pest populations where they can do little harm while the songbirds will fly around and pick off the pests in the more exposed areas such as our vegetable plot. Likewise the sweet cherries not only have great numbers of black aphis but breed up vast numbers of ladybirds which can then control other aphids on other plants. Honeysuckles, vetches and lupins are all excellent sacrificial plants that will suffer immense aphid attacks so acting as resource units for predator production to guard the rest of our garden. To prevent these sacrificials being too unsightly they can be situated at the back of borders or in a wilder area.
Pests occupying a niche may equally prevent other pests from becoming more problematical; the annoying wood louse which eats holes in leaves and chomps off seedlings is also partial to small caterpillars as part of their diet. Even the much damned wasp is really a friend as all spring and early summer the brood needs protein food and the adults continually scavenge for caterpillars and other pests to feed to them. It is only when the brood matures that wasps become a threat to our fruit! In much the same way I have recently started to employ ants. After watching a documentary on how some species of ants protect their host plant in return for nectar I tried bribing the ant colony in my cold frame with some jam. Every year previously they had bothered me by farming aphids and my sweet peppers were often badly infested. This year I have tried a daily dollop of jam on each pepper's stem; aphids do appear but then promptly disappear, I presume the ants are now eating them as a protein balance to their excessive supply of sugar. I have already discovered that when an ant colony gets large I can thin them out by putting an earthenware flowerpot on top their nest, they bring their 'eggs' up to 'hatch' in the heat of the day and I can then easily steal these to feed to my fish and fatten my baby chicks transferring the ant's hard work into my food.
Pests are thus of course often recyclers; they turn plant material into themselves but also into more accessible or enriched forms; wood lice chew up tough woody material turning it into a finely ground state activated with micro-organisms, wasps do likewise and many of their papery nests are already buried underground. Snails accumulate calcium ready for songbird's eggshells and they can often be seen cleaning the algae off glass and other flat surfaces turning this problematical resource into fertile droppings. And although you may loathe spiders I have encouraged them in my greenhouse for their pest control and then been surprised by how many droppings they create which soon goes to feed my plants.
In fact this is the most important value of pests; they create fertility. For example suppose by foregoing slug pellets you encourage just one more family of birds to live in your garden. Forget about the pest control they do, about their beauty or songs, even disregard the carbon dioxide they contribute to the plants they live in and think about the fertility they create. Add up the eggshells, the fledgling that doesn't make it, the feathers they all moult and the droppings they each make. This last is no small sum; birds have rapid metabolisms, they eat a lot and so excrete a lot. Two adults and five young will each drop many grams of manure a day, if we say four grams a day apiece this adds up to an ounce a week, that is over three pounds of excellent fertiliser per bird every year. One small family of birds thus annually creates the equivalent of a very large bucket of fish, blood and bone meal all spread around in small rapidly biodegradable packages -and totally for free.
Indeed if in any garden you can maintain larger numbers of pests then you will have larger numbers of predators and parasites living on them as the chains of life increase. Our soil and plants then benefit from all their dead bodies and droppings -which are never supplied in a chemically sterilised garden. As with the birds the total weight of insect droppings is not insignificant and their fertility value is high. Chitin is the main component of insect exoskeletons and when incorporated in the soil is believed to create especially important compounds that ensure health in plants. The honeydew from aphids is particularly useful as many other insects, most of them beneficial, use it as a carbohydrate source; the aphids do not use all the sugars but extract predominantly minerals and proteins from the sap passing through them. Honeydew falling on the ground encourages several varieties of nitrogen fixing soil organisms and thus creates even more fertility. At first glance flies may seem to be an unmitigated pest but this last winter an unseen unfound dead rat provided a vast number of flies living in my Orangery (a rather posh name for my polytunnel), they spent the winter there and cleared all the honeydew off my citrus leaves leaving them cleaner than I have ever managed to achieve before.