Handbook of Agricultural Entomology

Helmut F. van Emden

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118469590
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Entomology
• Index: Biological Sciences

Handbook of Agricultural Entomology by Helmut van Emden is a landmark publication for students and practitioners of entomology applied to agriculture and horticulture. It can be used as a reference and as a general textbook.

The book opens with a general introduction to entomology and includes coverage of the major insects (and mites) that cause harm to crops, livestock and humans. The important beneficial species are also included. Organisms are described in a classification of insect Orders and Families. The emphasis is on morphological characters of major taxonomic divisions, "spot characters" for the recognition of Families, and the life histories, damage symptoms and economic importance of the various pest species.

The book is beautifully illustrated in full colour with more than 400 figures showing both the organisms and the damage caused to plants with diagnostic characters indicated by arrows. Coverage is world-wide and includes much material stemming from the vast personal experience of the author. A companion website with additional resources is available at www.wiley.com/go/vanemden/agriculturalentomology

1

The World of Insects

1.1 The Diversity of Insects

The renowned 20th century geneticist, the late Professor J.B.S. Haldane was on a lecture tour in the USA when he was accosted by a female evangelist with the outburst ‘Professor Haldane, in your many years of study of the natural world, you must surely have formed a view of the nature of the Creator’. ‘Yes, Madam,’ replied Haldane tersely, ‘He is extraordinarily fond of beetles’. Figure 1.1 is an unusual visual representation of the living world, in which the size of the organism depicted is in proportion to the number of recorded species. You can see immediately that Professor Haldane had a point! Compare the size of the beetle with the icon for all the world’s mammals (the elephant) and that for the entire plant kingdom (the trees). Moreover, beetles are just one group of insects; many of the other insect groups (represented in total by the other insect) similarly put the number of mammal and plant species in the shade.

Fig. 1.1 The diversity of the living world with icons in proportion to the number of known species. For example, the elephant represents all mammals, the pine trees the whole plant kingdom, the shell all molluscs etc. The beetle and fly (which represents all groups other than the beetles) represent the number of species of insects.

(Modified from a cartoon by F.C. Fawcett and Q.D. Wheeler. www.coo.fieldofscience.com, with permission).

How many kinds of insects are there? This is an impossible question to answer, since there are many still awaiting discovery and we can only guess at how many. One of many estimates is that there are 1.75 million named organisms in the living world of which 1.5 million are species of insects, with as many of the latter still to be discovered. To understand the concept of so many insect species, think of me reading out their Latin names at the realistic rate of 33 species a minute. My agricultural entomology course at Reading was 25 lectures, each of 52 minutes, and I taught this course for 20 years. Had I done nothing but use all my lecture time over these 20 years just to read out the names of insects (I guess the students would soon have stopped turning up?), I would only have got through rather more than half of just the known species.

The only habitat insects have not conquered is the sea, though they are found right down to the shoreline (e.g. kelp flies). Otherwise you will find them up mountains, in caves, in and on animals and plants both living and dead, in the air to great heights and in rivers and lakes to great depths. Some egg-parasitoid wasps (fairy flies) are tiny, and less than 0.2 mm long and weigh only 0.004 mg; it is said they could fly through the eye of a needle (though surely it would have to be a darning needle?). The largest insect alive today is the Goliath beetle, which is some 12 cm in length and broad with it. It weighs about 50 g. Yet all, however small, are fully functional animals with brains and quite complex behaviours – they are miracles of miniaturisation.

1.2 The Impact of Insects on Us

We have to remember that the evolved diversity of insects when humans first appeared is not greatly different from what it is today. We date the origin of the human race to about 200,000 years ago, yet by then insects had already gone through well over 400 million years of evolution. Thus they have exploited the new resources created by the entry of the human race into history from that existing diversity far more than by evolving behaviours and properties they did not already possess.

Many people’s first reaction to insects is that they eat our crops, and this book on agricultural/horticultural entomology is only likely to reinforce that impression. However, herbivory is actually a rather unusual evolution among insects. We recognise over 30 different evolutionary lines (called ‘Orders’) of insects, and only nine of these have any herbivorous species. Herbivory presents insects with huge problems. Insects find it hard to sustain their high nitrogen : carbon ratio on such low nitrogen : carbon food as plants provide, and most can only benefit from cellulose if friendly fungi or bacteria do the digestion for them. The waxy surface of plant leaves and the verticality of stems make attachment difficult, especially in strong winds and heavy rain, and life in the open away from the soil surface brings the dangers of desiccation and greater apparency to predators.

However, in spite of the relatively low diversity of herbivorous insects (most are either grasshoppers, bugs, moths, flies or beetles), their impact on people is huge. There is an oft-quoted statistic to the effect that we would need to grow food on only two-thirds of the current acreage if insects did not take so much of what we grow either in the field or in storage – in spite of our efforts to control them. Numbers make up for a lack in diversity. A swarm of locusts may weigh more than a 100 tons and a hectare of sugar beet may host 200 million aphids.

Chinese cave paintings more than 6000 years old depict pests ravaging crops, and the book of Exodus in the Old Testament describes God visiting a plague of locusts on the errant Egyptians. The description here that the locusts ‘darkened the land’ (by the swarm blotting out the sun), ate ‘every herb of the land’ and that ‘there remained not any green thing in the trees, or in the herbs of the field’ is familiar to farmers suffering from locust swarms today. Insect plagues are certainly not just something from ancient history; famines in the Cameroon have occurred twice in the last 30 years as a result of plagues of armyworms (the caterpillars of a moth). Moreover, it is not only a question of what pests eat themselves; they can cause serious losses in other ways such as vectoring plant diseases, injecting saliva to which the plant may react badly and by fouling the plant with excreta. Figure 1.2 shows the 35 world crops that have the most pests recorded from them ranked in relation to the number of major and minor pests.

Fig. 1.2 Hierarchy of the 35 world crops with most pest species in relation to the number of major (black) and minor (grey) pests. Key to crops: 1, coffee; 2, rice; 3, citrus; 4, cotton; 5, pulses; 6, wheat; 7, maize; 8, apple; 9, sugar cane; 10, groundnut; 11, brassicas; 12, sorghum; 13, sweet potato; 14, tea; 15, potato; 16, coconut; 17, flowers; 18, cocoa; 19, tobacco; 20, tomato; 21, mango; 22, soybean; 23, banana; 24, castor; 25, guava; 26, pear; 27, fig; 28, peach; 29, strawberry; 30, macadamia; 31, oil palm; 32, sugar beet; 33, currants; 34, capsicum; 35, millet.

(Data from Hill 1974).

There are far more insect Orders with carnivores than with herbivores, though numerically carnivore populations tend to be smaller (often very much so) than those of herbivores. Carnivorous insects can be divided into predators, parasites and parasitoids.

Predators capture, kill and eat several (usually many) individual prey during their lifetime, though not all life stages necessarily share the habit. Thus many ladybird and hover fly species are important predators of aphids but adult hover flies are herbivorous and feed at flowers on pollen and nectar.

Parasites may utilise one or several individual hosts. Although their feeding may debilitate their host or infect it with a disease which may even be lethal, the feeding activity of a parasite is rarely directly lethal. Endoparasites feed on their host internally, and ectoparasites externally. The flea is an example of a parasitic insect.

Parasitoids resemble predators in that they kill their prey before emerging as an adult, but they differ in that they utilise just a single prey individual during their entire development, though one such individual may sustain several parasitoid individuals to maturity. Again parasitoids may be endoparasitoids or ectoparasitoids. The majority of insect endoparasitoids are within one group of the Order Hymenoptera.

Some carnivores are valuable predators and parasitoids of crop pests, but the first insects that humans will have encountered as problems, long before the relatively late development of agriculture in history, will have been parasites – biting and blood-sucking flies and bugs. The book of Exodus in the Bible records the plague of flies visited on the Egyptians by God to punish them.

The early hunter–gatherers will have been well aware of the pain and swellings inflicted by such insects, but it took until the late 19th century to establish that the bite of a mosquito could transmit malaria and incidentally also that insects could transmit diseases of plants. Malaria probably still kills about 2 million people each year, and was the reason for the Victorian explorers referring to West Africa as the ‘White man’s grave’. Biting flies and the diseases they transmit have had enormous influences on human history, including involvement in the fall of classical empires, and in the ability of people to live, raise livestock and even to wage war in different parts of the world.

The first insects, however, were probably scavengers on dead animal and plant material well before the oldest insect as yet found as a fossil appeared. This is an insect known as Rhyniognatha hirsti, which was found in about 400 million-year-old red sandstone in Scotland in 1926. It may even have had wings and perhaps fed on the spore-producing leaves of primitive plants allied to ferns.

Scavenging remains a widely distributed way of life in many Orders of modern insects. They remain important to us in breaking down the vast amounts of fallen leaf material in the tropics and in breaking down cattle dung and burying the corpses of small animals and birds. Insect scavengers are not just useful, they are essential to the cycle of life. However, they can also become pests if we put value on dead plant and animal materials. Thus scavengers such as clothes moths and some beetles attack our clothing and carpets, other beetles bore into the timber of our furniture and buildings; they even feed on the dead insects in museum collections.

Where would we be without bees and other insects to pollinate the many crops that would not produce fruits and seeds without insect activity? Although the pollen of some plants can be distributed by wind and water, many plants rely totally on insect pollination. Some (like snapdragons – Antirrhinum) have flowers designed so that only heavy insects like bees can ‘trip’ the opening of the flower to give access to the nectar and pollen. Cocoa is pollinated almost entirely by small midges. Plants that require insect pollination provide about 15% of the human diet in the USA.

One might suppose that most insects are economically neutral. The trouble is that the only way we are likely to find out is if an insect that seems of no importance to us disappears. For many years, an obscure moth called Swammerdamia, which lives on hawthorn, was regarded as economically neutral. However, by pure chance, the discovery was made in 1961 that its caterpillar is an essential overwintering host for an important natural enemy of a potentially serious pest of cabbages in the UK, the diamond-back moth (Plutella xylostella). Food webs can be very complex; the stability of an ecosystem depends on its herbivores, carnivores (including wasps!) and scavengers. Consequently, who can be sure that any insect is surplus to requirements?

1.3 The Impact We Have on Insects

1.3.1 World Distribution

Humans continue to move insects to new locations through travel and trade activities. Countries regularly pick up new pests in this way and most attempt to prevent, or at least delay, this by inspection and quarantining of potentially infested plants and other materials at points of entry such as docks and airports. The Colorado beetle (Leptinotarsa decemlineata) became a pest of potatoes when settlers started growing potatoes near the beetle’s weed host in the foothills of the Rocky Mountains in the 19th century. The beetle then spread westwards, causing famine in the USA, and was accidentally introduced into France during World War I (a previous introduction into Germany in 1877 had been eradicated). It spread across Europe, but as yet all appearances in the UK have been eradicated or otherwise failed to establish. In the last 25 years, the Russian wheat aphid (Diuraphis noxia) has become a feared invader. It reached the USA in 1986. It occurs in many eastern parts of Europe, and countries such as Germany and the UK are attempting to predict whether it is likely to survive in their country and what its economic impact may be. Further afield, Australia is facing a similar threat.

1.3.2 Climate Change

To the degree that humans are contributing to the pace of this phenomenon, they will be having an impact on all aspects of insect biology (especially fecundity and the number of generations per year) that are climate dependent. Also the distributions of insect species will change, either because of effects on their survival or because the distribution of the habitats and food they depend on change. Will malaria become a regular threat in the UK? Pests mentioned above, like the Colorado beetle and the Russian wheat aphid, may well find the UK a congenial environment!

1.3.3 Land Management Practices

The dramatic changes we make to natural habitats in order to fell trees, graze animals, grow crops and build houses have benefited some insects and disadvantaged others. Those that have benefited are of course insects that can exploit the riches of crop monocultures; most of these are quite rare in more natural habitats. Thus the frit fly (Oscinella frit), which can produce pest populations of 30 per m² from the stems and 110 per m² from the panicles of oat crops, ticks over in wild grasses at only 0.8 per m². Workers on cabbage root fly gave up a project to study the fly on wild brassicas because wild host plants were just too few. In another study, 60% of cages with carrot fly (Psila rosae) confined over ...