Know the enemy

Brian Callaghan discusses garden common pests

Most plant pests are tiny, and individually, can’t cause much damage. To compensate for this lack of size, however, many species have a prodigious rate of reproduction and a rapid rate of development once born. For example, an adult female aphid is capable of producing up to 100 offspring during its lifetime, each one taking around a week to reach an adult reproductive stage. Given these numbers, it’s logical to expect some pest damage in most years, and effective control has to be based on good preparation, early identification, and then rapid and thorough action. Ignore them at your peril.AphidsAphid damage is typified by curled, distorted leaves. Closer inspection will reveal colonies of bugs removing the sap from plant cells and causing them to collapse. Aphids also play an important role in the transmission of plant viruses which often effectively means the end of the plant’s useful life after losing vigour and productivity. To remove the plant sap, aphids insert a needle-like structure into the plant cells. This needle comprises two tubes. The aphid forces saliva down one tube into the plant, and this  forces the cell contents back up the other tube into the aphid’s digestive system. As aphids move between plants over the course of a season, viruses picked up from one plant will be spread to others. In some cases, the virus will lead to a gradual loss of vigour and premature end to the useful life of the plant. In others, the possibility of the virus spreading will necessitate immediate removal and destruction of the plant. Either way, the situation is to be avoided, especially with fruit crops which are more expensive to buy and establish than other crops such as vegetables.Most aphids over-winter as eggs laid in the bark of trees, shrubs and other undisturbed places. The eggs hatch in the spring and the emerging nymphs begin feeding almost immediately. Most often these nymphs develop into adult females which can produce wingless young without fertilisation. The decline of the local food supply triggers the production of winged offspring which can then fly off to establish new colonies on other plants. The whole reproductive process is so rapid and efficient, you need to act as soon as an infestation is spotted, both to limit the effects and also the cost of the control.CaterpillarsThese are the young of many different moths and butterflies and they can rapidly devastate crops. The adults lay eggs which hatch into larvae which are little more than feeding machines with the sole aim of acquiring and storing as much energy as possible for later development into the adult stage.The eggs are often laid on the underside of leaves to achieve some protection from bird predation, which makes them difficult to spot and extremely difficult to apply pesticides to. As a result, leaf damage is quite often the first sign of an infestation, although this can be confused with slug damage. A simple and effective way to differentiate between the two is to remember that caterpillars have jaw-like mouthparts and therefore, bite and chew their food, whereas slugs rasp their food in the same way a file wears away metal. This allows slugs to begin eating anywhere on a leaf, while caterpillars must begin at the edge of the leaf and eat inwards.Most summer caterpillar populations can be controlled by regularly and carefully inspecting the underside of leaves and crushing any clusters of eggs found. More serious infestations may demand the use of the biological control Bacillus thuringiensis (Bt) in common use on vegetable crops. This is sprayed onto the leaves of susceptible plants, to be consumed by caterpillars as they feed. The Bt proteins paralyse their digestive system so they cease feeding within hours and then die from starvation. It’s highly specific so Bt is considered extremely safe both for humans and beneficial insects.The winter moth caterpillars can be controlled by applying bands of grease, 100mm wide, to the trunks of trees. These will trap the females which crawl up the bark in winter to lay their eggs and prevent future generations developing. Blackcurrant gall mite I wasn’t sure whether to include this or not – although the bug can cause substantial damage, the most serious effect is disease transmission in the form of reversion virus. The gall mite breeds inside the buds of blackcurrant in late summer and autumn. After overwintering, the tiny mites emerge in spring to infect healthy buds and begin a further round of breeding. Infected bushes are most easily identified by enlarged buds in winter and early spring that should be removed and disposed of to limit spread. If the blackcurrant is contaminated with reversion virus, it will show in variations in leaf shape, enlargement of the main leaf vein, yellowing of leaves and an overall reduction in yield. It’s virtually impossible to cure virus-infected plants and spread is likely, so the best option is nearly always to remove and destroy the infected plant.EarwigsEarwigs are both friends and foes: friends in terms of predating on some plant pests but foes in that they can cause damage by boring into maturing fruit and feeding on flowers, vegetables, fruits and other plants. This leaves them looking ragged, covered with masses of small, irregular holes. Aside from the direct affects, the damaged tissue is left open to infection by wind and water-borne rots.Earwigs commonly feed at night and hide from predators during the day in moist, shady places beneath stones, and in debris.Effective control is achieved by removing debris and weed growth from the base of trees and bushes, and placing traps of containers stuffed with straw or old newspaper which are emptied and disposed of when full.Codling mothThis is a really annoying bug as often, the only visible sign of its presence is a tiny hole in the side or bottom of an otherwise perfect apple or pear. Most often the whole fruit is ruined by the attentions of this pest. Again, the juvenile stage is the most damaging because of the need to consume as much food as possible. The adults emerge from cocoons in late May and begin laying eggs on, or near, developing fruits during the early summer. A couple of weeks later, these eggs hatch small, white caterpillars with brown heads which bore into the fruit, feeding primarily in the area around the core of the fruit. The caterpillar’s exit hole is often visible in the side or base of the ripe fruit. When the fruit is cut open, you can see the frass-filled core and tunnel where the maggot had fed. After about four weeks’ feeding, the caterpillar is completely stuffed and leaves the fruit to form a cocoon where it spends its winter.  Thankfully, there is usually just one generation of codling moth each year, but in long summers, the earliest emerging caterpillars may produce a second generation of adults in August and September.Pheromone trapsOriginally intended to improve the timing of pesticide applications, pheromone traps consist of an open-sided structure, the bottom of which is coated with a very sticky substance. On this is a small pellet which releases a scent similar to that produced by unfertilised female codling moths. This attracts male moths which get trapped on the sticky material and become unavailable for mating. This won’t eradicate the codling moth problem, but it will help reduce infestations.When compared to other species on this planet, it’s sometimes difficult to believe that human beings still sit at the top of the food chain. Given the speed and efficiency of some bugs’ reproductive systems, it’s a wonder that we’ve not been replaced before now. Thankfully for us, one reason why the world isn’t dominated by, say, aphids, is the many natural checks on the potential population explosion of this organism. Extremes of weather, scarcity of suitable food, other predators etc all help keep aphids within controllable levels. It follows that anything which might undermine these natural checks – such as the effect of indiscriminate use of pesticides reducing colonies of natural predators – provides an important reason for avoiding or minimising the use of pesticides. In the long run, this can only make the problem worse by removing some of the natural checks on insect pest populations.

This article is from the May 2006 issue of Country Smallholding magazine. << To order back issues click the link to the left.