As Nebel points out, pests have been a major component of the environmental resistance to human population growth
it is our success in controlling macro and micro pests that has allowed the exponential growth in human population over the past few hundred years
What is a pest?
microbes that cause human or other animal or plant diseases (viruses through worms, etc)
nuisance insects
insect disease vectors
insects and other animals (eg rodents) that destroy crops and/or food
predators that prey on domestic animals
macro (eg termites) and micro (eg mold) decomposers that decompose wood and other building materials, fabrics etc
plants that hinder desirable land uses (ie weeds)
For food production, the important control measures in the last 100 or so years have been chemical pesticides (used against animal pests) and herbicides (used against plant pests)
food production is reduced by more than one third by pests; without pest control agents the damage would be much worse
the chemical agents have undesirable side effects; an alternative approach is to alter the environment in some way so that damage to crops from crops is minimized
biological agents or chemical agents with specific properties are used to alter some aspect of the ecosystem
The Chemical Biocides
going back 60 years or more, inorganic toxins like lead arsenate were used to kill just about everything (including children and farm workers)
in the late 30s and 40s organic compounds began to be used to kill insects and other pests
dichlorodiphenyltrichloroethane (DDT) was introduced as an insecticide in 1938
this compound is effective against a broad spectrum of insect pests but is relatively nontoxic to mammals
its cheap
it has been very effective in reducing the incidence of insect-spread malaria, sleeping sickness and typhus (and therefore has saved and continues to save millions of human lives)
changed the face of agriculture
Mueller was awarded the Nobel Prize for the discovery that DDT is an insecticide
DDT was soon joined by a host of other chlorinated hydrocarbons
these are absorbed by the fatty tissue around bug nerves then cause convulsions and death
in the 50s, it became apparent to scientists that these chemicals were having a destructive effect on birds
the effects are due in large part to the bioaccumulation of persistent pesticides
DDT is not readily decomposed or digested in the food chain, though it does break down to DDE
DDE disrupts Ca+2 metabolism in birds, causing eggs to be thin shelled and easily broken
although it is not known to be toxic, DDT also accumulates in the fatty tissues of humans and other animals
in 1962, Rachel Carson summarized these scientific findings in the bestseller Silent Spring
rallied public sentiment against DDT and other chemical pesticides
fired up the ecology movement of the 60s and 70s that led to the creation of the EPA
the EPA banned DDT along with a number of other chlorinated pesticides (chlordane is still used against termites)
the chemical pesticides in use today are not persistent like the chlorinated hydrocarbons; these compounds are degraded within a few weeks or months
organophosphates and carbamates have mostly replaced organochlorine compounds for killing bugs in the US
organophosphates are essentially nerve gas; they inhibit the insects’ acetylcholinesterase; this enzyme is critical in shutting off messages from nerves to muscles; again, convulsions and death
carbamates are relatively new as pesticides; they work by a similar mechanism to that of organophosphates; they degrade quickly
on the other hand, the compounds (organophosphates, carbamates) are quite toxic to mammals and other animals
the manufacture of these compounds can also be quite dangerous as the citizens of Bhopal can attest
these substances probably do not pose a hazard to the general public but do cause acute poisoning of 1000s of farm workers every year
these compounds are quite wide spectrum, and thus kill beneficial insects (including bees) along with pests
This effect on birds (including the near extinction of the bald eagle and other fish eating birds) is one example of the negative side effects of chemical pesticides
for humans, the effects of pesticide residues is widely disputed; some blame pesticide residues for neurological and other health effects, including cancer; others claim that the real world effects are negligible in comparison with background
there are natural (botanical) products used as biocides, including nicotine, pyrethrin and rotenone
these are biodegradeable and tend to have less ecological impact and human toxicity (except for nicotine)
two other effects include the breeding of resistant pests and the outbreak of secondary pests
a chemical pesticide puts a powerful selection pressure and the rapidly reproducing insect community; the individuals with the physiological or morphological abilities to withstand an applied chemical continue to reproduce; their offspring do quite well because the competition has all been killed off (as have their predators)
hundreds of insect species have become resistant to common pesticides (I guess this is good business for the chemical companies)
mosquitoes worldwide have become resistant enough to DDT so that malaria is again on the rise
remember predator-prey relationships? Reduce the number of prey and the predator declines in numbers as well (even more severely it turns out)
with the predators decimated, the original pest or some other species is left unchecked; a population explosion may occur (either a resurgence of the original pest or a secondary infestation by some other)
the predator species may be cute little songbirds (or any ecologically important species)
these problems of resistance and resurgence (and acute toxicity) occur even with nonpersistent pesticides
Alternative Methods of Pest Control
if we understand the life cycle of a pest and its ecological connections, we may be able to protect crops, etc, from damage without risking poisoning farm workers; a natural pest control measure might also be genuinely effective, without resurgence and resistance
Nebel gives four general types of natural controls:
cultural controls = nonchemical manipulations of the environment that hinder pest proliferation or exclude pests
the classic example is window screens
other sanitary engineering processes (protecting watersheds, etc) serve as cultural controls against microbial and other parasitic pests
draining swamps is a cultural control against mosquitoes
crop rotation serves as a cultural control, since many pests need a specific host who is rotated out; the timing of plantings can also have an effect
quarantine of possible carrier materials
agricultural inspection stations
zebra mussel inspections and quarantines
chemical agents might be effective in this situation
natural enemies = predation maintains the "balance of nature"; the presence of a predator may maintain the population of a pest around some lower level; as long as this level is below that at which crops are damaged to the point of non-profitability (the so-called economic threshold) then the natural control can be considered effective
ladybugs are good to have in your garden (and spiders are good to have in the house)
some of the worst pests are those that have been introduced to a foriegn habitat => introduce a species the feeds on the first invader and a population balance may be reestablished
this can be the ideal pest control technique because it reproduces itself
as mentioned above, the use of chemical pesticides may be counterproductive in this respect since the broad spectrum toxins may kill the control species as well as the pest (or may induce a sudden famine for the control species)
the potential for biological controls has been barely tapped
genetic controls = pests species have evolved to exploit some host or other food source; if that target species can be genetically altered in some way, it may no longer suit the pest
a large goal of plant breeding efforts has been the generation of parasite (eg fungus) resistant strains
breeding may also be effective against insect pests
many plants produce chemicals that are noxious or toxic to insects (eg, nicotine is a powerful toxin to insects and humans alike)
other plants have physical characteristics that repel or destroy insects (eg, turpentine glues down bugs that would seek to feed on injured pines)
breeders may be able to select for some natural chemical or physical attribute to increase the resistance of a plant against some pest
genetic engineering has the potential to introduce a myriad of traits not already present in a plant (or other organism)
over the course of evolution, insects evolved around many defensive measures that plants came up with; likewise, insects can become resistant to the human-bred anti-insect traits
natural chemical controls = the use of an insects own hormones or pheremones to control its population
a hormone can sidetrack the life cycle of a pest
a pheremone can be used as bait to trap or poison males or simply as a diversion to reduce the number of successful matings
another biological control is the use of sterile males = large numbers of these are raised in the lab, irradiated and released; these outcompete the wild males and can greatly reduce the number of pests born in the next generation
Integrated Pest Management is the use of the entire spectrum of pest control measures, natural and artificial, based on the application of scientific knowledge in conjunction with thorough surveillance of pests (see p 423)
this approach can greatly reduce the need for chemical pesticides and the associated dollar, health, and environmental costs