Chemical pesticides target pest resurgence and secondary pests

Chemical pesticides are a key part of the armory of pest managers but they have to be used with care because population theory (see, in particular, Chapter 14) predicts some undesirable responses to the application of a pesticide. Below we discuss the range of chemical pesticides and herbicides before proceeding to consider some undesirable consequences of their use.

The use of inorganics goes back to the dawn of pest control and, along with the botanicals (below), they were the chemical weapons of the expanding army of insect pest managers of the 19th and early 20th century. They are usually metallic compounds or salts of copper, sulfur, arsenic or lead - and are primarily stomach poisons (i.e. they are ineffective as contact poisons) and they are therefore effective only against insects with chewing mouthparts. This, coupled with their legacy of persistent, broadly toxic metallic residues, has led now to their virtual abandonment (Horn, 1988).

Naturally occurring insecticidal plant products, or botanicals, such as nicotine from tobacco and pyrethrum from chrysanthemums, having run a course similar to the inorganics, have now also been largely superseded, particularly because of their instability on exposure to light and air. However, a range of synthetic pyrethroids, with much greater stability, such as per-methrin and deltamethrin, have replaced other types of organic insecticide (described below) because of their relative selectivity against pests as opposed to beneficial species (Pickett, 1988).

Chlorinated hydrocarbons are contact poisons that affect nerveimpulse transmission. They are insoluble in water but show a high affinity for fats, thus tending to become concentrated in animal fatty tissue. The most notorious is DDT: a Nobel Prize was awarded for its rediscovery in 1948, but it was suspended from all but emergency uses in the USA in 1973 (although it is still being used in poorer countries). Others in use are toxaphene, aldrin, dieldrin, lindane, methoxychlor and chlordane.

Organophosphates are also nerve poisons. They are much more toxic (to both insects and mammals) than the chlorinated hydrocarbons, but are generally less persistent in the environment. Examples are malathion, parathion and diazinon.

Carbamates have a mode of action similar to the organophos-phates, but some have a much lower mammalian toxicity. However, most are extremely toxic to bees (necessary for pollination) and parasitic wasps (the likely natural enemies of insect pests). The best-known carbamate is carbaryl.

Insect growth regulators are chemicals of various sorts that mimic natural insect hormones and enzymes, and hence interfere with normal insect growth and development. As such, they are generally harmless to vertebrates and plants, although they may be as effective against a pest's natural insect enemies as against the pest itself. The two main types that have been used effectively to date are: (i) chitin-synthesis inhibitors such as diflubenzuron, which prevent the formation of a proper exoskeleton when the insect molts; and (ii) juvenile hormone analogs such as methoprene, which prevent pest insects from molting into their adult stage, and hence reduce the population size in the next generation.

Semiochemicals are not toxins but chemicals that elicit a change in the behavior of the pest (literally 'chemical signs'). They are all based on naturally occurring substances, although in a number of cases it has been possible to synthesize either the semiochemicals themselves or analogs of them. Pheromones act on members of the same species; allelochemicals on members of another species. Sex-attractant pheromones are used commercially to control pest moth populations by interfering with mating (Reece, 1985), whilst the aphid alarm pheromone is used to enhance the effectiveness of a fungal pathogen against pest aphids in glasshouses in Great Britain by increasing the mobility of the aphids, and hence their rate of contact with fungal spores (Hockland et al., 1986). These semiochemicals, along with the insect growth regulators, are sometimes referred to as 'third-generation' insecticides (following the inorganics and the organic toxins). Their development is relatively recent (Forrester, 1993).

15.2.2.2 Herbicides

Here, too, inorganics were once important although they have mostly been replaced, largely owing to the combined problems of persistence and nonspecificity. However, for these very reasons, borates for example, absorbed by plant roots and translocated to above-ground parts, are still sometimes used to provide semipermanent sterility to areas where no vegetation

15.2.2.1 Insecticides insecticides and how they work the tool-kit of herbicides of any sort is wanted. Others include a range of arsenicals, ammonium sulfamate and sodium chlorate (Ware, 1983).

More widely used are the organic arsenicals, for instance disodium methylarsonate. These are usually applied as spot treatments (since they are nonselective) after which they are translocated to underground tubers and rhizomes where they disrupt growth.

By contrast, the highly successful phenoxy or hormone weed killers, translocated throughout the plant, tend to be very much more selective. For instance, 2,4-D is highly selective against broad-leaved weeds, whilst 2,4,5-trichlorophenoxyethanoic acid (2,4,5-T) is used mainly to control woody perennials. They appear to act by inhibiting the production of enzymes needed for coordinated plant growth, leading ultimately to plant death.

The substituted amides have diverse biological properties. For example, diphenamid is largely effective against seedlings rather than established plants, and is therefore applied to the soil around established plants as a 'pre-emergence' herbicide, preventing the subsequent appearance of weeds. Propanil, on the other hand, has been used extensively on rice fields as a selective post-emergence agent.

The nitroanilines (e.g. trifluralin) are another group of soil-incorporated pre-emergence herbicides in very widespread use. They act, selectively, by inhibiting the growth of both roots and shoots.

The substituted ureas (e.g. monuron) are mostly rather nonselective pre-emergence herbicides, although some have post-emergence uses. Their mode of action is to block electron transport.

The carbamates were described amongst the insecticides, but some are herbicides, killing plants by stopping cell division and plant tissue growth. They are primarily selective, pre-emergence weed killers. One example, asulam, is used mostly for grass control amongst crops, and is also effective in reforestation and Christmas tree plantings.

The thiocarbamates (e.g. S-ethyl dipropylthiocarbamate) are another group of soil-incorporated pre-emergence herbicides, selectively inhibiting the growth of roots and shoots that emerge from weed seeds.

Amongst the heterocyclic nitrogen herbicides, probably the most important are the triazines (e.g. metribuzin). These are effective blockers of electron transport, mostly used for their post-emergence activity.

The phenol derivatives, particularly the nitrophenols such as 2-methyl-4,6-dinitrophenol, are contact chemicals with broad-spectrum toxicity extending beyond plants to fungi, insects and mammals. They act by uncoupling oxidative phosphorylation.

The bipyridyliums contain two important herbicides, diquat and paraquat. These are powerful, very fast acting contact chemicals of widespread toxicity that act by the destruction of cell membranes.

Finally worth mentioning is glyphosate (a glyphosphate herbicide): a nonselective, nonresidual, translocated, foliar-applied chemical, popular for its activity at any stage of plant growth and at any time of the year.

15.2.2.3 Target pest resurgence

A pesticide gets a bad name if, as is usually the case, it kills more species than just the one at which it is aimed. However, in the context of the sustainability of agriculture, the bad name is especially justified if it kills the pests' natural enemies and so contributes to undoing what it was employed to do. Thus, the numbers of a pest sometimes increase rapidly some time after the application of a pesticide. This is known as 'target pest resurgence' and occurs when the treatment kills both large numbers of the pest and large numbers of its natural enemies (an example is presented below in Figure 15.2). Pest individuals that survive the pesticide or that migrate into the area later find themselves with a plentiful food resource but few, if any, natural enemies. The pest population may then explode. Populations of natural enemies will probably eventually reestablish but the timing depends both on the relative toxicity of the pesticide to target and nontarget species and the persistence of the pesticide in the environment, something that varies dramatically from one pesticide to another (Table 15.1).

the pest bounces back because its enemies are killed

Rat

Fish

Toxicity Bird

Honeybee

Persistence

Table 15.1 The toxicity to nontarget organisms, and the persistence, of selected insecticides. Possible ratings range from a minimum of 1 (which may, therefore,

Permethrin (pyrethroid)

2

4

2

5

2

include zero toxicity) to a maximum of 5.

DDT (organochlorine)

3

4

2

2

5

Most damage is done by insecticides that

Lindane (organochlorine)

3

3

2

4

4

combine persistence with acute toxicity to

Ethyl parathion (organophosphate)

5

2

5

5

2

nontarget organisms. This clearly applies,

Malathion (organophosphate)

2

2

1

4

1

to an extent, to each of the first six

Carbaryl (carbamate)

2

1

1

4

1

(broad-spectrum) insecticides. (After

Diflubenzuron (chitin-synthesis inhibitor)

1

1

1

1

4

Metcalf, 1982; Horn, 1988.)

Methoprene (juvenile hormone analogue)

1

1

1

2

2

Bacillus thuringiensis

1

1

1

1

1

Azodrin

23 30 Aug

B 13 20 27 5 Sep Oct

Bollworm population

40rr-r

Treatment

23 30 Aug

CL CO

500 400 300 200 100 0

Predator population

500 400 300 200 100 0

23 B 21 28 B Aug Sep Oct

23 30 B 13 20 27 5 Aug Sep Oct

Damaged bolls

B0 50 40 30 20 10

23 30 B 13 20 27 5 Aug Sep Oct

23 30 Aug

B 13 20 27 5 Sep Oct

23 B 21 28 B Aug Sep Oct jmJl

23 30 Aug

B 13 20 27 5 Sep Oct

Pest Resurgence
Jul 18 Jul 25 Aug 2 Aug 9 Aug 1B Aug 23 Aug 30

œ 1B0 » 140 § 120 r 100 (per 80 í§ B0 I 40 -1 20

] Control

Treatments with toxaphene-DDT Two treatments

ll H811 1

Jul B Jul 15 Jul 22 Jul 29 Aug 5 Aug 12

19BB

19B0 /.

19B8

/19B9

, . . i f

■ i i i i Iii

10.0

Figure 15.2 Pesticide problems amongst cotton pests in the San Joaquin Valley, California. (a) Target pest resurgence: cotton bollworms (Heliothis zea) resurged because the abundance of their natural predators was reduced - the number of damaged bolls was higher. (b) An increase in cabbage loopers (Trichoplusia ni) and (c) in beet army worms (Spodoptera exigua) were seen when plots were sprayed against the target lygus bugs (Lygus hesperus) - both are examples of secondary pest outbreaks. (d) Increasing resistance of lygus bugs to Azodrin®. (After van den Bosch et al., 1971.)

15.2.2.4 Secondary pests

The after-effects of a pesticide may involve even more subtle reactions. When a pesticide is applied, it may not be only the target pest that resurges. Alongside the target are likely to be a number of potential pest species that had been kept in check by their natural enemies (see Figure 15.1c). If the pesticide destroys these, the potential pests become real ones - and are called secondary pests. A dramatic example concerns the insect pests of cotton in the southern part of the USA. In 1950, when mass dissemination of organic insecticides began, there were two primary pests: the Alabama leafWorm and the boll weevil (Anthonomus grandis), an invader from Mexico (Smith, 1998). Organochlorine and organophosphate insecticides (see Section 15.2.2.1) were applied fewer than five times a year and initially had apparently miraculous results - cotton yields soared. By 1955, however, three secondary pests had emerged: the cotton bollworm, the cotton aphid and the false pink bollworm. The pesticide applications rose to 8-10 per year. This reduced the problem of the aphid and the false pink bollworm, but led to the emergence of five further secondary pests. By the 1960s, the original two pest species had become eight and there were, on average, an unsustainable 28 applications of insecticide per year. A study in the San Joaquin Valley, California, revealed target pest resurgence (in this case cotton bollworm was the target species; Figure 15.2a) and secondary pest outbreaks in action (cabbage loopers and beet army worms increased after insecticide application against another target species, the lygus bug; Figure 15.2b, c). Improved performance in pest management will depend on a thorough understanding of the interactions amongst pests and nonpests as well as detailed knowledge, through testing, of the action of potential pesticides against the various species.

Sometimes the unintended effects of pesticide application have been much less subtle than target pest or secondary pest resurgence. The potential for disaster is illustrated by the occasion when massive doses of the insecticide dieldrin were applied to large areas of Illinois farmland from 1954 to 1958 to 'eradicate' a grassland pest, the Japanese beetle. Cattle and sheep on the farms were poisoned, 90% of cats and a number of dogs were killed, and among the wildlife 12 species of mammals and 19 species of birds suffered losses (Luckman & Decker, 1960). Outcomes such as this argue for a precautionary approach in any pest management exercise. Coupled with much improved knowledge about the toxicity and persistence of pesticides, and the development of more specific and less persistent pesticides, such disasters should never occur again.

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Responses

  • mulu
    Why are choosing secondary pest and pest resurgence important?
    6 years ago
  • genevive
    What is target pest resurgence?
    3 years ago
  • gordon
    How to prevent a secondary pest resurgence?
    3 years ago
  • Sebastian Eisenberg
    What is pest resurgence in relation to insect pests?
    3 years ago
  • Pia
    What is the target pest for an insecticide?
    2 years ago
  • Mark Roberson
    What is resurgence and secondarypest outbreaks?
    2 years ago
  • EIJA-RIITT
    What is the difference between secondery pest and pest resurgence?
    2 years ago
  • Tewelde
    Can incorrect timing of a pesticide cause a secondary pest outbreak?
    2 years ago
  • adaldrida
    How to prevent secondary pest resurgence gcse?
    11 months ago
  • faizan
    Why did pesticide spraying lead to resurgence and secondarypest outbreak?
    7 months ago

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