CLASSIFICATION OF BIOLOGICAL PEST CONTROL AGENTS
Biological pest control agents are naturally
occurring or genetically modified agents that are
distinguished from conventional pesticides by their
unique modes of action, low use volume and target
species-specificity.
Two major catagories may be distinguished:
* Biochemical pest control agents, such as
hormones and growth regulators.
* Microbial pest control agents, such as
bacteria, viruses and fungi.
1. Biochemical pest control agents
A chemical must meet the following two criteria
in order to be classified as a biotechnical pest
control agent and meet the data requirements for this
class of compounds:
* The chemical must exhibit a
mode of action other than direct toxicity in the target
pest (e.g. growth regulation, mating disruption).
Botanical insecticides such as nicotine and pyrethrum
kill and thus are not considered biochemical pest
control agents.
* A biochemical must be naturally
occurring, or if the chemical is synthesized by man,
then it must be structurally identical to a naturally
occurring chemical. Identical in this sense means the
molecular structure of the major component of the
synthetic chemical must be the same as the molecular
structure of the naturally occurring analog.
Minor differences among stereochemical isomers
are acceptable unless an isomer is found to have
significantly different toxicological properties than
another isomer.
In cases of doubt, such as if the exact
molecular structure of the naturally occurring compound
is unknown, or if the mode of action is different in
the target compared with non-target organisms, a
country's regulatory authority should rule on a
case-by-case basis whether such chemicals should be
classified as a bio- chemical pest control agent or as
a conventional pesticide.
Biochemical pest control agents fall into four
general biologically functional classes:
a. Semiochemicals
These are chemicals emitted by plants
or animals which modify the behaviour of receptor
organisms of like or different kind. They include
pheromones, allomones and kairomones.
* Pheromones are substances emitted by
members of one species which modify the behaviour of
others within the same species. Even at extremely low
concentrations, these chemical messages induce insects
to aggregate, help them to find a partner, provide an
alarm signal or lead them to sources of food. The most
common phero- mones are sex attractants excreted by
abdominal glands of the female to attract males for the
purpose of mating; and the aggregation pheromones which
are produced by one or both sexes of an insect species
which bring both sexes together for feeding and
reproduction. Sex hormones are common among moths and
butterflies; the aggregation hormones, among beetles.
* Allomones are chemicals emitted by
one species which modify the behaviour of different
species to the benefit of the emitting species. Many
plants produce secondary substances that repulse
insects and prevent them from feeding; these are
classified as allomones. The oil of Citronella grass
has long been used by man as an insect- repellent
applied to the skin.
* Kairomones are chemicals emitted by
an animal organism which, at very low concentration,
modify the behaviour of individuals of a different
species to the disadvantage of the emitting and the
benefit of the receptor species. For instance, an
animal parasitoid may be guided by it in finding a
host. Kairomones, like pheromones, can be used to
attract insects to traps for the purpose of monitoring
or catching them.
b. Hormones
Hormones are biochemical agents that
are synthesized in one part of an organism and
translocated to another part, where they have
controlling, regulating or behavioural effects. Two
major groups of hormones have been identified; they
are:
* Molting hormones or ecdysteroids.
These comprise a group of closely related water-soluble
steroids in insects, of which several active analogs
are found in plants. As yet, no effective control of
insects through feeding or topical application of
either natural or plant-derived ecdysteroid analogs
could be obtained. Moreover, as their synthesis appears
to be prohibitively expensive, the commercial
production of ecdysteroids still remains in the
research phase.
* Juvenile hormones. Four very similar
juvenile hormones are produced by insects in the
process of their immature development, which maintain
the insects in their nymphal or larval form. These
juvenile hormones, as well as many hundreds of
synthesized active analogs -- the so-called juvenoids
-- are able to affect insect development almost equally
well by any mode of application. Conforming to their
mode of action, these juvenile hormones and juvenoids
are also designated as insect growth regulators (IGRs).
The natural hormones do not have enough
environmental stability and are too costly to be
synthesized; thus, they were not developed into
commercial products. But the synthesis of their
analogs, the juvenoids, turned out to be very
productive and yielded many active compounds of often
enhanced selectivity. To date, only five juvenoids have
been registered for use against insect pests; it
appears that the listed juvenoids are used in protected
environments; only fenoxycarb can be used in a field
crop situation. This is one of several drawbacks of
juvenoids; others can be summarized as follows:
The instability of these compounds,
when exposed to sunlight and wind as a foliar residue.
Fenoxycarb is the most stable of the five compounds in
field application.
Except for kinoprene, the juvenoids do
not cause direct mortality in the target insects.
The juvenoids are able to affect the
target insects only during two distinct but short
periods of their life. These brief periods, the
so-called windows of sensitivity, co-incide with the
last larval instar and with early embryonic development
in the egg-phase. Application of juvenoids in the last
larval instar inhibits the process of metamorphosis. An
excess of juvenile hormone or juvenoid results in the
development of intermediate forms between nymphs and
adults which, if they survive the molt, are incapable
of reproduction. Application to the egg stadium causes
an ovicidal effect that may result in abnormal embryo
development, failure in hatching or death after
emergence.
The advantages of juvenoids as chemical
insect control agents are found in the following
characteristics:
Juvenoids demonstrate a rather extreme
selectivity for insects, in which they affect receptor
systems that do not exist in other forms of life and an
often very high but selective activity on sensitive
target species.
These compounds have an extremely low
toxicity to non-target organisms and mammals.
Little cross-resistance is as yet
expected to exist against these juvenoids.
Juvenoids which show neither a
repelling nor direct killing action, as most
conventional insecticides do, have proven to be very
effective against social insects such as fire ants and
pharao ants. Baiting with fenoxycarb and methoprene
generally gives good control since juvenoids brought
into the nest by returning workers affect the brood and
the social interactions.
* Diflubenzuron is a synthetic
compound with contact and stomach activity that
interferes with chitin deposition at the time of
molting and prevents the shedding of the old skin. This
leads to the death of the larvae and pupae, or to the
development of non-viable adults. It also prevents
hatching of insect eggs. Diflubenzuron is effective
against lepidopterous caterpillars, grasshoppers, fly
and mosquito larvae and weevils. It has no systemic
activity and thus sucking insects are in general not
affected. This forms the basis of its selectivity in
favour of many insect predators. The trade name is
Dimilin. Another promising IGR showing similar activity
is teflubenzuron (Nomolt).
* Kinoprene is a juvenile
hormone mimic that, when applied to larval stages,
prevents their metamorphosis into viable adults. Uses
include the control of stored product pests and indoor
insect pests, but it is particularly effective against
insects of the order Homoptera such as white fly,
aphids, mealybugs and scale insects in greenhouses.
Although this compound had proved highly effective for
pest control on ornamentals in greenhouses, it appears
to have been withdrawn from the market as its market
share was too small to justify the cost of keeping it
available.
* Methoprene is also a juvenile
hormone mimic with similar activity; it was registered
in 1975 as a commercial IGR for use against mosquito
larvae. Other uses include control of storage pests,
homopterous fleas, leafminers and pharao ants.
Juvenoids such as hydroprene and methoprene are very
useful in confined environments such as greenhouses,
houses and storage buildings.
c. Natural plant regulators
Natural plant regulators are chemicals
produced by plants that have inhibitory, stimulatory,
or other modifying effects on the same or other species
of plants. Some of these are termed "plant hormones" or
"phytohormones".
Some examples of plant growth
regulators (PGRs) are as follows:
* Auxins-compounds induce elongation in
the cells of shoots. In 1934, it was discovered that
auxins enhance the rooting of cuttings. Further uses of
this PGR are for the thinning of fruits and the
increasing of flowering. Auxins are formed within the
plants from auxin precursors. Manufactured products
with similar activity are indoleacetic acid,
B-naphthaleneacetic acid and 2,4-D.
* Gibberellins or gibberellic acid
derivatives are substances which stimulate cell
division and cell elongation through the induction of
increased enzyme production in the cell. They occur
naturally in all plants. Gibberellins are used to
increase stalk length, delay yellowing, break dormancy
in seed potatoes, increase fruit set and yields etc.
* Cytokinins are naturally occurring or
manufactured substances that induce cell division and
the regulation of differentiation in plant parts. These
compounds were discovered in 1955 and their usefulness
lies in prolonging the storage life of green
vegetables, cut flowers and mushrooms. Zeatin is a
natural substance, whereas kinetin and adenine are
synthetic cytokenins.
* Inhibitors are substances that
interfere with a physiological process in plants
regulating growth, germination of seeds, or the action
of hormones, gibberellins and auxines. Inhibitors that
occur naturally in plants are usually hormones.
Substances with inhibiting or retarding action are used
to prevent sprouting of bulbs and tubers, prevent
sucker growth on tobacco plants and induce shorter stem
growth in grains. Naturally occurring inhibitors are
benzoic acid, gallic acid, cinnamic acid and abscisic
acid. There are also synthetic inhibitors such as
maleic hydrazide.
d. Enzymes
In this context, enzymes are defined as
protein molecules that (1) are the instruments for the
expression of gene action and that (2) catalyze
chemical reactions.
2. Microbial pest control agents
The pesticides referred to as microbial pest
control agents include formulations either of naturally
occurring, infective micro-organisms, such as bacteria,
fungi, protozoa and mycoplasmas, and viruses, or
genetically modified micro-organisms that are used to
achieve natural control of pests.
These pathogens and parasites are isolated and
mass produced for use as a commercial pesticide. The
basic general principle required for their registration
is that the product should demonstrate effectiveness
and not present unacceptable hazard to users, consumers
of treated foods and the environment.
Microbial insecticides are preparations based
on disease-inducing organisms (entemopathogens) or
parasites which normally infect or poison an insect or
mite and ultimately cause its death. Microbial
insecticides are usually rather specific to certain
orders of insects or mites and non-toxic to man, other
mammals and plants.
Microbial insecticides have already been used
for many years among a wide range of crops, forests and
mosquito habitats where they are applied using
conventional spraying techniques. They are often
applied in combination with chemical insecticides and
other means of control, within an integrated pest
management approach.
Their main advantages are their environmental
safety and highly specific action, but a major drawback
is their lack of biological persistency under field
conditions. Therefore, these microbial insecticides
could not compete on the commercial market with more
persistent and effective chemical pesticides, such as
the successful synthetic pyrethroids. However, recent
developments in the field of biotechnology, in
particular in the areas of protein and genetic
modification, have sparked a renewed interest in all
groups of biological control agents.
Biotechnology appears to create new
opportunities to overcome some of the long-standing
disadvantages of microbial insecticides, such as their
rapid breakdown, inconsistency of activity and costly
production. In 1988, the world market for microbial
insecticides was estimated at $US 70 million, or 1 per
cent of the total insecticide market. The new
developments seem to offer a possibility for slow
expansion of the market share of microbially based
products to an estimated $US 300 million by 1992
(Cannon, 1989). In addition to the microbial
insecticides, other microbial pesticides have already
been developed that demonstrate a fungicidal,
nematicidal or herbicidal activity.
Bacteria are the most researched and most
widely exploited pathogens for pest control, mainly
because of the high efficacy of certain strains of the
infectious bacterium Bacillus thurigiensis (Bt),
against susceptible species of insects of the orders
Lepidoptera, Diptera, Homoptera and (very recently)
also Coleoptera.
The bacterium Bt forms within its cell an oval
endospore and at the same time a proteinaceous
inclusion commonly called a "crystal". This crystal,
which represents up to 40 per cent of the dry weight of
the bacterium cell at the time of sporulation, contains
an endotoxin. Upon ingestion by an insect, this
endotoxin may break down in an alkaline, mid-gut
environment under the action of enzymes to release a
smaller, active protoxin. This lethal protoxin damages
the lining of the mid-gut wall leading to membrane
rupture and lysis. The bacterial spores are thus
released in the abdominal cavity of the insect where
they multiply, eventually killing the insect.
Furthermore, a cessation of feeding occurs usually
within hours of ingestion owing to paralysis of the
mouthparts.
The selectivity of Bt strains appears to depend
on the acidity (pH-value) of the mid-gut content.
Insects having an alkaline mid-gut content of pH 8-10
are susceptible (e.g. lepidopteran larvae). But the
desert locust, Schistocerca gregaria, having a more
acidic mid-gut content of pH 6.2-7.1 is non-susceptible
as the known Bt endotoxins remain insoluble and hence
inactive under such acidic conditions.
The microbial insecticides that have found
widespread use against caterpillars and beetle grubs
are produced from B. thurigiensis var. kurstaki under
trade names such as Bactospeine, Thuricide and Dipel.
Those microbials produced from Bt var. israelensis are
most effective against larvae of mosquitoes and black
fly; brand names are Bactimos and Teknar.
Vigorous research over recent years has yielded
many new varieties of Bt which are effective against a
number of different insect species. Some of the new
discoveries are more than 10 times as effective as
those used in traditional Bt formulations. A serious
drawback of traditional Bt formulations has been their
instability under field conditions; they break down
quickly when exposed to ultra-violet light. By means of
genetic manipulation, an ingenious solution to this
problem of short persistency has been recently
developed and patented under the tradename "Cellcap" .
Bacillus sphaericus also produces a crystalline
toxin, and several strains have proven to be effective
against larvae of mosquitoes. No appearance of
resistance in larvae against these toxins has as yet
been observed either in the field or during laboratory
tests at sub-lethal doses.
Fungal microbial insecticides contain parasitic
fungi which are capable of penetrating the insect
cuticle directly. After piercing the cuticle -- often
by means of enzymes that break down chitin and protein
-- the fungus multiplies inside the insect, producing
lethal metabolites. Peak mortality normally occurs 5-10
days after application, depending on dose and insect
size.
Microbial insecticides are formulated, for
instance, with the use of the following fungi:
Beauveria bassiana, Verticillium lecanii, Entemophaga
asiatica, E. grylli, Metharizium anisopliae, Zoophthora
radicans. Parasitic fungi, generally, have
host-specific strains and are not hazardous to
non-target organisms. They are cheap to produce and may
be formulated and applied in a way similar to chemical
insecticides. Since these fungi broach directly through
the insect cuticle, they have a contact action without
having to await ingestion by the target insects. By
this property, a fungal microbial avoids a prolonged
exposure to ultra-violet light, low relative humidity
and other adverse climatic conditions which may
inactivate it. Thus, waiting for the pest to ingest the
pathogen is a critical limitation for the use of
microbials based on viruses, bacteria and protozoa.
Close observation, however, has revealed that
under adverse conditions a pathogen such as Beauveria
will kill a target insect only if it either hits the
insect directly or contacts it quickly after
application, but before it is inactivated. Past
attempts to induce epidemics from single applications
were not successful, as the disease did not
automatically increase to epidemic levels. A high
relative humidity is required for the production of
spores. When low humidity prevails, the disease cannot
spread and a high kill would only be obtained through a
highly effective initial application. Constraints which
apply to pathogen use are similar to those limiting
effective chemical pesticide use; formulation and
application technology are critical.
Recently, it has been found that the use of
oil-based fungal pathogen formulations hold much
greater promise than water-based formulations that
evaporate very rapidly under dry conditions. In
dose-response studies on a weevil pest, Beauveria
bassiana was over 30 times more effective by topical
application when formulated in a vegetable oil (Prior
et al., 1988). The suggested reason is that insect
cuticles are lipophilic (fat-loving), so that the oil
spreads out over the cuticle and penetrates into the
thin membranes at the articulating surfaces of the
joints, carrying the spores to the most vulnerable part
of the cuticle. By contrast, water-based fungal
formulations run off and the spores are lost. Thus,
much interest exists in the development of oil-based
fungal formulations for low and ultra-low volume
application. Further examples of fungal insecticides
are:
Hirsutella thompsonii, a parasitic fungus from
which a spore-formulation is produced that kills the
citrus rust-mite and also infects spider mites.
Formulations of Verticillium lecanii control aphids,
scales, thrips and red spider mites in greenhouse
crops. Furthermore, a number of other fungi are
currently being investigated to determine their
potential microbial value: e.g. Nomuraea spp. and
Paecilomyces spp.
Viral microbial insecticides are formulated on
the basis of nuclear polyhedrosis viruses.
A virus is a sub-microscopic infectious agent
existing as a nucleo-protein entity that is composed of
protein and a nucleic acid, RNA or DNA. The protein
surrounds, as a crystalline shell (capsid), the nucleic
acid component which transfers the genetic information.
Owing to their lack of organ structures, viruses are
able to replicate only within the living cells of a
host. Since viruses contain only DNA or RNA and lack
their own metabolism, they are not recognized as being
organisms.
Nuclear polyhedrosis viruses infect the
destructive Heliothis spp. caterpillars, such as the
cotton bollworm and tobacco budworm, which damage many
types of crops. Nuclear polyhedrosis viruses are very
specific and highly virulent and can survive for years
because of their protective capsid. Following ingestion
by an insect, the protein capsid dissolves in the
insect's alkaline mid-gut juices. The infectious,
individual, mature virus particles, the virions,
permeate the gut lining and eventually enter into the
bloodstream where they multiply rapidly and take over
control of susceptible cells. This results in the death
of the insect within two to nine days, depending on the
stage of development of the insect.
The baculoviruses include the group known as
polyhedrosis viruses. Production of these viruses
involves rearing them on living insects, which is a
costly process.
Nematodes, which occur naturally in the soil
and parasitize soil-insects, are excellent material for
the formulation of microbial insecticides. Once
released into the soil, they are able to disperse over
short distances and they are attracted to their insect
hosts. The infective juvenile stages of genera such as
Steinernema and Heterorhabditis are usually ingested by
the insects, but they may also enter via the anus or
spiracles. From the gut of the insect they pass through
the gut wall into the circulatory system, where they
release bacteria. These released bacteria reproduce
rapidly and cause the insect's death within one to two
days.
Active nematodes need to be surrounded all the
time by a water film; only some genera are able to
survive drought conditions in a particular cyst-form.
Thus, the use of nematodes as microbials has been
mainly restricted to the soil environment, or protected
habitats where they are able to survive. Above the soil
surface they are easily exposed to ultra-violet light
and desiccation, which prohibits their use against
foliar-feeding insect pests.
Neoplectana carpocapsae is a nematode that acts
as a vector in transmitting a pathogenic bacterium
lethal to termites. Commercial microbial formulations
are already on the market.
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