Insects and Humans
This chapter will focus on those insects that humans describe, in their economically minded way, as bene?cial or harmful, though it should be appreciated from the outset that
these constitute only a very small fraction of the total number of species. It must also be realized that the ecological principles governing the interactions between insects and humans are no different from those between insects and any other living species, even though humans with their modern technology can modify considerably the nature of these interactions.
Of an estimated 5–10 million species of insects, probably not more than a fraction of 1% interact, directly or indirectly, with humans. Perhaps some 10,000 constitute pests that, either alone or in conjunction with microorganisms, cause signi?cant damage or death to
humans, agricultural or forest products, and manufactured goods. Worldwide food and ?ber
losses caused by pests (principally insects, plant pathogens, weeds, and birds) are generally estimated at about 40%, of which 12%are attributable to insects and mites. These ?gures do
not include postharvest losses, estimated to be about 20%, and occur despite the application of about 3 million tonnes of pesticide (worth more than US$31 billion, including about US$9 billion of insecticide). In the United States alone, crop losses related to insect damage rose from 7% to 13% in the period 1945–1989, despite a tenfold increase in the amount of
insecticide used (>120,000 tonnes each year).
On the other hand, the value of bene?ts derived from insects is several fold that of losses
as a result of their pollinating activity, their role in biological control, and their importance as honey, silk, and wax producers. That insects do more good than harm probably would come as a surprise to laypersons whose familiarity with insects is normally limited to mosquitoes, house?ies, cockroaches, various garden pests, etc., and to farmers who must protect their livestock and crops against a variety of pests. If asked to prepare a list of useful insects, many people most likely would not get further than the honey bee and, perhaps, the silkmoth, and would entirely overlook the enormous number of species that act as pollinating agents or prey on harmful insects that might otherwise reach pest proportions.
Humans have long recognized the importance of insects in their well-being. Insects and/or their products have been eaten by humans for thousands of years. Production of silk from silkmoth pupae has been carried out for almost 5000 years. Locust swarms, which originally
may have been an important seasonal food for humans, took on new signi?cance as humans
turned to a farming rather than a hunting existence. However, with rare exceptions, for example, the honey bee and silkmoth whose management is relatively simple and labor
intensive, until recently humans neither desired nor were able, because of a lack of basic knowledge as well as technology, to attempt large-scale modi?cation of the environment of
insects, either to increase the number of bene?cial insects or to decrease the number of those
designated as pests.
2. Bene?cial Insects
Insects may bene?t humans in various ways, both directly and indirectly. The most
obvious of the bene?cial species are those whose products are commercially valuable.
Considerably more important, however, are the insects that pollinate crop plants. Other
bene?cial insects are those that are used as food, for biological control of pest insects and
plants, in medicine and in research. For some of these useful species, humans modify their environment so as to increase their distribution and abundance in order to gain the bene?ts.
2.1. Insects as Pollinators
An intimate, mutualistic relationship has evolved between many species of insects and plants, in which plants produce nectar and pollen for use by insects, while the latter provide a transport system to ensure effective cross-pollination. Though some crop plants are wind-pollinated, for example, cereals, a large number, including fruits, vegetables, and ?eld
crops such as clovers, rape, and sun?ower, require the service of insects. In addition, ornamental ?owers are almost all insect-pollinated.
The best known, though by no means the only, important insect pollinator is the honey bee, and it is standard practice in many parts of the world for fruit, seed, and vegetable producers either to set up their own beehives in their orchard sand?elds or to contract this job out to
beekeepers. For example, in California about 1.4 million hives are rented annually to augment natural pollination of almonds (about 50% of the hives), alfalfa, melons, and other fruits and vegetables. Under such conditions, the value of bees as pollinators may be up to140 times their value as honey producers. Using this factor, it is estimated that the increased value of crops attributable to honeybee pollination in the United States is about US$15 billion each year. This estimate does not take into account the value of other, natural pollinators of crops, nor obviously has a value been placed on the importance of all pollinators of non-crop plants, which are vital to species diversity and as food for wildlife.
2.2. Insects as Agents of Biological Control
It is only relatively recently that humans have gained an appreciation of the importance of insects in the regulation of populations of potentially harmful species of insects and plants. In many instances, this appreciation was gained only when, as a result of human activity, the
natural regulators were absent, a situation that was rapidly exploited by these species whose status was soon elevated to that of pest. In the ?rst one example given below, none of the
organisms is a pest in its country of origin because of the occurrence there of various insect regulators. The discovery of these regulators, followed by their successful culture and release inthe area where the pest occurs, constitutes biological control.
The classical example of an insect pest brought under biological control is the cottony cushion scale, Icerya purchasi, which was introduced into California, probably from Australia, in the l860s. Within 20 years, the scale had virtually destroyed the recently established, citrus-fruit industry in southern California. As a result of correspondence between American and Australian entomologists and of a visit to Australia by an American entomologist, Albert
United States as biological control Koebele, two insect species were introduced into the
agents for the scale. The ?rst, in 1887, was Cryptochaetum iceryae, a parasitic ?y, about
which little is heard, though DeBach and Rosen consider that it had excellent potential for control of the scale had it alone been imported. However, the abilities of this species appear to have been largely ignored with the discovery by Koebele of the vedalia beetle, Rodolia
cardinalis, feeding on the scale. In total, only 514 vedalia were brought into the United States, between November 1888 and March 1889, to be cultured on caged trees infested with scale. By the end of July 1889, the vedalia had reproduced to such an extent that one orchardist, on whose trees about 150 of the imported beetles had been placed for culture, reported having distributed 63,000 of their descendants since June 1! By 1890, the scale was virtually wiped out. Similar successes in controlling scale by means of vedaliaor Cryptochaetum have been reported from more than 60 countries.
The one example described above indicate one method whereby the importance of biological control can be demonstrated, namely, by introduction of potential pests into areas where natural regulators are absent. Another way of demonstrating the same phenomenon is to destroy the natural regulators in the original habitat, which enables potential pests to undergo a population explosion. This has been achieved frequently through the use of non-selective insecticides. For example, the use of DDT against the codling moth, Cydia
pomonella, in the walnut orchards of California, led to outbreaks of native frosted scale, Lecanium pruinosum, which was unaffected by DDT, whereas its main predator, an encyrtid, Metaphycus californicus, suffered high mortality. Another Lecanium scale, L. coryli,
introduced from Europe in the 1600s, is a potentially serious pest of apple orchards in Nova Scotia but is normally regulated by various natural parasitoids and predators (especially
mirid bugs). Experimentally it was clearly demonstrated in the 1960s that application of DDT destroyed a large proportion of the Blastothrix and mirid population, and this was followed in the next two years by medium to heavy scale infestations. Recovery of the parasite and predators was rapid, however, and by the third year after spraying the scale population density had been reduced to its original value.
2.3.Insectsas Human Food
As noted in the previous chapter, insects play a key role in energy ?ow through the
ecosystem, principally as herbivores but also as predators or parasites, which may themselves be consumed by higher-level insectivorous vertebrates. In turn, some of these vertebrates, notably freshwater ?sh and game birds, are eaten by humans. Moreover, in many parts of the
world, insects (including grasshoppers and locusts, beetle larvae, caterpillars, brood of ants, wasps and bees, termites, cicadas, and various aquatic species) historically played, and continue to have, an important part as a normal component of the human diet.
Aboriginal people of the Great Basin region in the southwestern United States traditionally spent much time and effort harvesting a variety of insects, principally crickets, grasshoppers, shore ?ies (Ephydridae) (especially the pupae), caterpillars, and ants (adults and pupae) though bees, wasps, stone?ies, aphids, lice, and beetles were also consumed on an
opportunistic basis. Some of the insects were eaten raw though most were baked or roasted prior to being consumed; further, large quantities, especially of grasshoppers and crickets, were dried and ground to produce a ?our that was stored for winter use.
In parts of southeastern Australia the aboriginals would seasonally gorge themselves on bogong moths (Agrotis infusa ) which estivate from December through February in vast
numbers in high-altitude caves and rocky outcrops in the Southern Tablelands. Some tribes would make an annual trek over a considerable distance (up to 200 km) to take advantage of this seasonal food source, returning each year to the same area.
There has been some increase in interest in the potential of insects as food, including discussion of the subject at international conferences. However, most North Americans and Europeans have not yet been educated to the delights of insects, despite the efforts of authors such as Taylor and Carter, DeFoliart, and Berenbaum to increase the popularity of insects as food. The western world’s bias against eating insects has two negative impacts. First, it may
be seen as a missed opportunity. Compared to livestock, insects are much more ef?cient at converting plant material into animal material with high nutritional value. With relatively little research, industrial-scale mass production of food insects should be possible. Second, as
less-developed areas of the world become increasingly westernized, their populations may be expected to eat fewer insects. This could lead to nutritional problems in areas where the economy is already marginal.
2.4. Soil-Dwelling and Scavenging Insects
By their very habit the majority of soil-dwelling insects are ignored by humans. Only those that adversely affect our well-being, for example, termites, wireworms, and cutworms, normally “merit” our attention. When placed in perspective, however, it seems probable that
the damage done by such pests is greatly outweighed by the bene?ts that soil-dwelling insects
as a group confer. The bene?ts include aeration, drainage, and turnover of soil as a result of
burrowing activity. Many species carry animal and plant material underground for nesting, feeding, and/or reproduction, which has been compared to ploughing in a cover crop. Many insects, including a large number of soil-dwelling species, are scavengers; that is, they feed on decaying animal or plant tissues, including dung, and thus accelerate the return of elements to food chains. In addition, through their activity they may prevent use of the decaying material by other, pest insects, for example, ?ies. Perhaps of special interest are the dung beetles
(Scarabaeidae), most species of which bury pieces of fresh dung for use as egg-laying sites. Generally, the beetles are suf?ciently abundant that a pat of fresh dung may completely
disappear within a few hours, thus reducing the number of dung-breeding ?ies that can locate
it. Furthermore, the chances of ?y eggs or larvae surviving within the dung are very low
because the dung is ground into a ?ne paste as the beetles or their larvae feed. Likewise, the
survival of the eggs of tapeworms, roundworms, etc., present in the dung producer, is severely reduced by this activity.
3. Pest Insects
Since humans evolved, insects have fed on them, competed with them for food and other resources, and acted as vectors of microorganisms that cause diseases in them or in the organisms that they value. However, as was noted in the Introduction, the impact of such insects increased considerably as the human population grew and became more urbanized. Urbanization presented easy opportunities for the dissemination of insect parasites on humans and the diseases they carry. Large-scale and long-term cultivation of the same crop over an area facilitated rapid population increases in certain plant-feeding species and the spread of plant diseases. Modern transportation, too, encourages the spread of pest insects and insect-borne diseases. Further, as described in Section 2.3, some of the attempts at pest eradication have back?red, resulting in even greater economic damage.
3.1. Insects That Affect Humans Directly
A large number of insect species may be external, or temporary internal, parasites of humans. Some of these are speci?c to humans, for example, the body louse (Pediculus
humanus) and pubic louse (Phthirus pubis), but most have a varied number of alternate hosts
which compoundsthe problemof their eradication.With rare exceptions, for example, some myiasis-causing ?ies, insect parasites are not fatal to humans. In large numbers, insect
parasites may generally weaken their hosts, making them more susceptible to the attacks of disease-causing organisms. Or the parasites, as a result of feeding, may cause irritation or sores which may then become infected.
But by far the greatest importance of insects that parasitize humans is their role as vectors of pathogenic microorganisms (including various “worms”) some well-known examples of
which are given in Table 24.1. The pathogen is picked up when a parasitic insect feeds and may or may not go through speci?c stages of its life cycle in the insect. Bacteria and viruses
are directly transmitted to new hosts, an insect serving as a mechanical vector, whereas for protozoa, tapeworms and nematodes, an insect serves as an intermediate host in which an essential part of the parasites’ lifecycle occurs. In the latter arrangement the insect is known as
a biological vector.
3.2. Pests of Cultivated Plants
Damage to crops and other cultivated plants by insects is enormous; in the United States alone losses in potential production are estimated at 13% and have a value of about US$30 billion annually, despite the application of more than 100,000 tonnes of insecticide. Remarkably, about three quarters of the insecticide used is applied to 5% of the total agricultural land, especially that growing row crops such as cotton, corn, and soybean. Damage is caused either directly by insects as they feed (by chewing or sucking)or oviposit, or by viral, bacterial, or fungal diseases, for which insects serve as vectors. Especially important as “direct damagers” of plants are Orthoptera, Lepidoptera, Coleoptera, and
Hemiptera (see the chapters that deal with these orders for speci?c examples of suchpests).
Several hundred diseases of plants are known to be transmitted by insects including about 300 that are caused by viruses . Especially important in disease transmission are Hemiptera,
particularly leafhoppers and aphids. Three aspects of the behavior of these insects facilitate their role as disease vectors: (1) they make brief but frequent probes with their mouthparts into host plants; (2) as the population density reaches a critical level, winged migratory individuals are produced; and (3) in many species, winged females deposit a few progeny on
each of many plants, from which new colonies develop. On the basis of their method of transmission and viability (persistence in the vector), viruses may be arranged in three
categories. The non-persistent (stylet-borne) viruses are those believed to be transmitted as contaminants of the mouthparts. Such viruses remain infective in a vector for only a very short time, usually an hour or less. Semipersistent viruses are carried in the anterior regions of the gut of a vector, where they may multiply to a certain extent. Vectors do not normally remain infective after a molt, presumably because the viruses are lost when the foregut intima is shed. Persistent (circulative or circulative-propagative) viruses are those that, when acquired by a vector, pass through the midgut wall to the salivary glands from where they can infect new hosts. Such viruses may multiply within tissues of a vector, which retains the ability to transmit the virus for a considerable time, in some instances for the rest of its life. Persistent viruses, in contrast to those in the ?rst two categories, maybe quite speci?c with
respect to the vectors capable of transmitting them .
3.3. Insect Pests of Stored Products
Almost any stored material, whether of plant or animal origin, maybe subject to attack by insects, especially species of Coleoptera (larvae and adults) and Lepidoptera (larvae only). Among the products that are frequently damaged are grains and their derivatives, beans, peas, nuts, fruit, meat, dairy products, leather, and woolen goods. In addition, wood and its products may be spoiled by termites or ants. Again, readers should refer to the appropriate chapters describing these groups for speci?c examples.
Estimates of the worldwide postharvest losses of foodstuffs (especially stored grains) may be as high as 20%, of which about one-half is attributable to insects and the rest to microorganisms, rodents, and birds. Even in well-developed countries such as the United States, Canada, and Australia where storage conditions are more adequate and pesticide treatment is available, losses of 5% to 10% are estimated for stored grains. Given a world-wide estimate for the production of wheat, coarse grains, and rice as about 1.5 billion tonnes in 1981–1982, perhaps as much as 150 million tons may have been lost as a result of insect damage. The nature of the damage caused by stored products pests varies. Grain and other seed pests not only eat economically valuable quantities of food, but cause spoilage by contamination with feces, odors, webbing, corpses, and shed skins, and by creating heat and moisture damage that permits the growth of microorganisms .Pests of household goods such as clothing and furniture principally cause damage by spoilage, for example, by tunneling, defecating, and creating odors.