A key factor in protecting and conserving farmers’ friends is being able to tell friends from enemies. Various studies have shown that many farmers (and also many agricultural professionals) are not good at insect identification and know little about insect biology and behaviour. In vegetable pest management, if farmers cannot distinguish between pests and friends, they will be likely to use pesticides against any insect observed in the field. They do not want to risk having living things crawling over their crop and so resort to spraying.
Although proper identification of pests and beneficials is important it is not always easy. Take for instance ladybird beetles, which are one of the most talked about and valuable farmers’ friends because of their ability to eat large numbers of plant sucking aphids. But some ladybird species do eat plants and can cause crop damage! These species usually look dusty or slightly hairy – as compared with the very shiny species which are usually farmer’s friends. To make matters more difficult, most insects look very different at different stages in their development. For example, ladybird larvae look like tiny spotted crocodiles and are very different from the adults, but they also eat many aphids, scales and young caterpillars. This highlights the importance of being able to identify that “good and bad” on the crop no matter what life cycle stage is present.
Insect Life Cycles
Most crop pests are arthropods, which means they have external skeletons and jointed legs. Within this group, there are insects which have 6 legs as adults and mites which have 8 legs as adults (but 6 legs when younger). Arthropods are usually considered to be pests if they eat any part of the crop plant (chewing pests) or if they suck crop plant juices (sucking pests). The damage may be caused by adults or by the younger stages called larvae or in some cases nymphs. Other serious pests of vegetables include nematodes, tiny worms that live in and feed on plant roots (plant parasites) and larger animals such as rats, monkeys and even hippos.
Life stages of arthropods
Arthropods can look very different at different stages in their life cycle. It is useful to understand these different forms so that pests can be identified regardless of the stage they have reached in their life cycle. Arthropods go through a process called metamorphosis which means change of form. Some have complete metamorphosis where the young stages look completely different from the adults – see the butterfly in the left hand picture below. Others go through incomplete metamorphosis where the young look a little like small versions of the adult, but without fully developed wings – see the bug in the right hand picture below.
So how to make a correct identification? One good source of information is the local extension office which should have field guides and pest and disease identification guides on local problems. Sometimes such guides are also available at universities or colleges and more and more such information is available through the Internet. If you know of any good ones be sure and share the information with other stakeholders, particularly farmers.
It is also possible to find out whether an organism is a friend or a pest by setting up an insect zoo to investigate what the organism is doing on the crop. This involves putting the insect in a small jar together with leaves from that plant and any known pests which were also on that leaf. If it eats or sucks the leaves, it is a pest and if it eats the pests it is a natural enemy.