Genetic control strategies involve alterations of a targeted pest’s ability to reproduce or the insertion of some deleterious character into a pest population. Genetic control is a form of biological control of pest species which exploits the insect’s mate-seeking expertise to introduce genetic abnormalities (typically, but not necessarily, dominant lethal mutations) into the eggs of the wild population.
Since the genetic control methods utilise insect pests for their own destruction, these are also known as autocidal methods. Genetic approaches for managing arthropod pests have been considered by scientists for more than half a century. Yet, because of recent and current emerging technologies, genetic approaches are often regarded as novel and unproven.
Certainly, current advances and theory in this field of study remain to be implemented, but some applications of these methods, such as the sterile insect technique, are well known in their effectiveness and have great potential for managing arthropod pests. Research and development for genetic autocidal programme demand a long-term commitment before potential benefits can be realised.
Therefore, genetic approaches are usually developed only for the most economically important pest species. However, future costs of developing, implementing, and operating genetic control may be reduced substantially as a result of the technological gains from current programmes using genetic approaches and from recent developments in molecular genetic techniques.
The majority of the genetic control techniques have the unique property of becoming more effective as the target population is reduced in numbers. However, they tend to be less effective at high population densities. This contrasts sharply with the use of insecticides where net effectiveness decreases when populations become small.
Genetic control in most cases has to be viewed as an area-wide approach in which a crop or an animal or human population is protected from insect attack over a large geographical area. It is not suitable for a field by field or even a farm by farm approach as both the economics and the biology demand large-scale application.
This implies that effective genetic control programmes require considerable start-up funds and the large scale financial resources required is a major reason why these types of approaches do not find favour with the funding agencies. However, in the long run, area-wide approaches have a much better return on investment than do conventional farmer by farmer approaches.
A key element in area-wide economics is the mobilisation and organization of the beneficiaries. In the long term, genetic control techniques will only be successful, if they become commercially viable and are able to compete economically with other control methods.
Requirements for Genetic Control Techniques:
The detailed knowledge of the population dynamics, ecology and behaviour of the target pest is essential for the success of any genetic control technique. The level of knowledge required is much greater than for most other insect control strategies.
Some of the requirements are explained below:
1. Colonization and Mass Rearing:
All types of genetic control require the colonization and to some extent the mass rearing of the target species with individual species differing in the ease with which they accept these two processes. All developmental stages of the insect have to be provided with an environment that not only enables them to reproduce in a predictable and efficient manner, but also produces individuals with a certain level of quality at an acceptable economic price.
The effects of laboratory colonization on many aspects of insect behaviour are incremental, heterogeneous and to a certain degree unpredictable. Many quality parameters can be effectively monitored in the laboratory, e.g., size, survival, etc., but the assessment of behavioural parameters would seem to be of little value under these conditions.
As all genetic control techniques require the mating of the released insects with the wild population, any change in mating behavioural patterns will have an immediate detrimental effect on the efficiency of the technique. This aspect of quality has to be monitored in a representative and meaningful way, i.e., probably in the open field or in field cages.
2. Population Dynamics:
An understanding of the population dynamics, ecology and behaviour of a candidate species is a pre-requisite to the application of a genetic control method.
Information crucial to the economic assessment and development of a genetic control method includes the following parameters:
i. Time of the year when the pest population is at its lowest level, and methods of reducing the native pest population to minimal number.
ii. An estimate of the number of insects in the native population and how this number changes over time.
iii. Intrinsic rate of increase and long-range movement capabilities of a pest during various seasons and in different cropping systems.
iv. All the host plants capable of supporting a pest species, and the relative abundance and economic significance of each host plant.
v. Whether secondary pests would be benefitted or deterred by the reduction or absence of the target pest.
3. Post-Production Processes:
For any area-wide genetic control programme, a large number of insects have to be prepared for release. This involves marking the insects so that they can be recognized in the field, sterilizing them if necessary, transport to the field area, and then their dispersal over the treatment area. All these processes have to be carried out within a defined and generally short time frame, and have to be simple, economical and cause little damage to the insect.
In general, adult insects are released as they are mobile and less prone to the attack of predators. Being mobile, they can also aid in the dispersal process and they are usually released from aircraft for large scale programmes. Aerial release is often much cheaper than ground release and ensures a much better distribution of insects at a relatively low cost.
4. Field Monitoring:
A continuous evaluation of a field programme is essential both in terms of monitoring effectiveness and in making programme adjustments. Released insects must be clearly distinguishable from field insects in a rapid and secure way, and methods must be available to monitor the released and wild population. The current methods that rely on fluorescent dust are not optimal. The wrong classification of a single fly as wild as opposed to released can have a major impact on a programme where eradication is the goal.
The use of genetic transfer technology to introduce benign genetic markers will provide a high degree of security for the determination of the origin of the trapped insect. If any form of sterility technique is being used for control, a measure of the population fertility before and during the programme is highly informative. It is the only direct evidence that the released insects have interacted with the wild population.
5. Economic Analysis:
Before any genetic control programme is implemented, an in-depth economic analysis should be conducted. Such an analysis should take into consideration damage estimates by the pests, current cost of their control, cost estimate of a genetic control programme, and potential benefits to the environment. The advantages of various methods and combinations of methods of control should be analysed and compared for multiple years.
Knipling (1979) observed that reducing pest species to low populations may be difficult and costly by the use of insecticides or by a combination of several methods. However, once this is accomplished, continuous pest population management by genetic approaches may be most economical, effective and ecologically acceptable method.
The cost/benefit ratio is not a static figure, but it should continue to decrease with each passing year. Cost/benefit ratios, generated from increased production and the absence of conventional pest control, estimate that the screwworm programme saved $ 4 billion through 1987, and that melon fly programme saved more than $ 100 million per year.
Cytoplasmic Incompatibility Method:
In this case, a cross between two apparently conspecific populations results in only partial embryonation in some ova. Spermatozoa enter the egg cytoplasm, but no fusion occurs between the nuclei of spermatozoa and ova with the result, that zygote is not formed and the egg remains sterile.
This sterility appears to be due to a cytoplasmic factor and not to chromosomal incompatibilities. It has been found that crosses between individuals from certain populations of the same species of mosquito, Culex pipiens Linnaeus are sterile. Crosses between other populations are sometimes completely fertile or partially sterile. The phenomenon has also been discovered in a number of other mosquito species and has been used as a control measure in experimental releases.
The phenomenon is best known in the crosses between the subspecies of C. pipiens, collectively referred to as the C. pipiens-complex, viz. C. pipiens (temperate region), C. quenquefasciatus Say (tropical and sub-tropical) and C. p. australicus Dobrotworsky & Drummand (restricted to Australia and nearby regions). The same subspecies existing in a different part of the wild forms a strain reproductively incompatible with a strain of another country.
At least 20 reproductive types amongst these three subspecies are known which show following three types of incompatibilities:
(i) Partial incompatibility, where crossing of two strains produces low hatches of eggs.
(ii) Unidirectional incompatibility, where only one of the sexes of a particular strain is capable of producing sterile eggs.
(iii) Bidirectional incompatibility, where the reciprocal crosses are incompatible.
Field trials of this method were carried out in a village near Rangoon in Myanmar in 1971 and at Delhi in 1972 under a WHO/ICMR programme. The method involved release of incompatible strains of the mosquito, C. quinquefasciatus from other (temperate) regions.
Before actual release, suitable backcrosses in tropical and temperate strains were carried out to produce a variety that would survive tropical conditions without losing its incompatibility. This is possible in backcrosses between unidirectional incompatibilities. The trials at both the places demonstrated the efficacy of the method, but the invasion by mosquitoes from untreated regions was the obstacle preventing a complete eradication of the species.
Cytoplasmic incompatibility also, has been observed in the plum curculio, Conotrachelus nenuphar (Herbst); cherry fruit fly, Rhagoletis cerasi (Linnaeus); and in Ephestia cautella (Walker) leading to the hope that it might occur in many economically important insects. The use of sterility induced by cytoplasmic incompatibility has one serious drawback. Released strains of insects must be separated by sex before release, so that they might not interbreed and become established in the field.
Chromosome Translocation:
Translocation is the breakage of two non-homologous (dissimilar) chromosomes (as in a heterozygote) and the re-attachment of the broken parts to the wrong partners. During meiosis in gamete formation, the translocated chromosome segregates out into six types of gametes.
Of the six types of gametes, only two are balanced (orthoploid), having a full gene complement and, therefore, viable and four are unbalanced (aneuploid), having more genes of one chromosome and less of the other and, therefore, non-viable or lethal. Eggs produced by gametes having lethal genes will be sterile. This phenomenon can be utilised in the control of insects.
Translocations in insects can be produced by exposure to irradiation and to radiomimetic chemical (such as EMS, MMS, bleomycin, mitomycin-c, nitrogen mustard, etc.). Such translocated insects could be released into wild population. Mating between wild and translocated heterozygote individuals will result in the reduction of population of the wild type by a fall in the viable egg production. By persisting in the release of more and more translocated insects, the wild population (theoretically atleast) can be eliminated.
This method has been tried only in laboratory in cage experiments on mosquitoes, viz. Culex, Aedes and Anopheles; house fly, Musca domestica Linnaeus; lepido-pteran pests codling moth, cabbage looper, pink bollworm, etc. At first, translocated and wild population is kept in small cages under laboratory conditions. If there is a decline in the wild population, the same experiment is repeated with large population in large-sized cages kept under field conditions.
Sex-Ratio Distortion Method:
Some insects produce progeny of one sex more than the other-these can be male producing or female producing. Such insects when mass reared in the laboratory and released in nature, compete with wild population to mate and produce offsprings that are predominantly of one sex. In course of time, imbalance in the sex-ratio leads to a reduction in the population and ultimately to the extinction of the species itself.
The mechanism by which the above distortion is brought about is called meiotic drive where- (i) a factor called segregation distorter, present on a chromosome, drives only one of the two chromosomes of the XY (sex chromosome) pair to the poles to form gametes during meiosis, (ii) the driven chromosome undergoes supplementary replication with a concomitant loss of its homologue, or (iii) only one of the XY pair of chromosomes survives to end up in functional gametes, the other one, degenerating and being reabsorbed.
The sex-ratio distortion, it is believed, can also be caused by differential sperm behaviour. Depending upon the genetic constitution, one of the two kinds (X or Y) of sperms may compete better than the other to fertilize the ovum or even by a selective acceptance by the ovum of the two kinds of sperms.
In nature, meiotic drive sex-ratio distortion has been found in the mosquito, Aedes aegypti (Linnaeus). Some strains of this mosquito are predominantly male producing. Such males, when mass reared and released into wild populations, will produce mostly male progeny which, in course of time, will not only lead to their extinction but will also have an additional effect in reducing disease transmission, since it is only the female of this species that transmits the disease.
If the genetic constitution of an insect is known, it is also possible to produce sex distortion artificially by a selective breeding. A strain of Musca domestica Linnaeus has now been produced in the laboratory that is predominantly male producing. Thus, the method of sex-ratio distortion offers a great promise in insect pest control.
Integration of Genetic Control Methods:
The advantage of combining genetic control methods with other pest control methods has been recognized from the inception of genetic control. All the successful genetic control programmes explained above have non-genetic components that have served vital roles in the control or eradication of pests, as well as preventing re-infestation of pests.
Most often, genetic control methods have been integrated with insecticide applications, cultural controls, and quarantines. However, population models constructed to predict the potential advantages of combining genetic control methods with other methods, such as inundative releases of parasitoids, host plant resistance, pheromones for mating disruption, and insect pathogens, have suggested that these combinations would yield synergistic effects.
Because the ratio between irradiated and non-irradiated insects is the most critical factor in regulating the efficacy of SIT or sterility release programmes, any mortality agent (i.e., resistant host plants, insecticides) applied during a continuous release of genetically altered insects would benefit a release programme.
Although the mortality agent would reduce the number of both released and wild insects, it would not change the ratio. Therefore, subsequent to the application of the mortality agents, wild population would be lower and the ratio of released to wild insects would be increased by continual releases of genetically altered insects.
Integration of genetic techniques and inundative releases of parasitoids may be more complementary than most other pest control combinations because their optimal actions are at opposite ends of the host density spectrum and do not interfere with each other.
Although the use of parasitoids and sterile insect techniques have different modes of action, the effectiveness of the sterile insect technique increases the ratio of adult parasitoids to adult hosts, and the effectiveness of the parasitoids increases the ratio of sterile to fertile insects.
Greater pest suppression could be expected if parasitoid releases were combined with the F1 sterility technique. Not only is F1 sterility in lepidopterans more effective than full sterility in reducing population increases, the F1 sterility technique produces eggs and sterile F1 larvae that would provide an increased number of hosts for the parasitoids.
Several laboratory and field studies have been conducted to determine the compatibility and effectiveness of combining different types of genetic control techniques with other control techniques. The effects of F1 sterility and host plant resistance on Helicoverpa zea (Boddie) and Spodoptera frugiperda (J.E. Smith) development were investigated.
It was found that larvae resulting from irradiated male by non-irradiated female crosses were equally competitive with normal larvae for all measured parameters. Oppositional acceptance tests and parasitism studies with Glabromicroplitis croceipes (Cresson) in the laboratory and field using Heliothis virescens (Fabricius) and H. virescens- H. subflexa (Guenee) backcross larvae as hosts were conducted.
It was concluded that augmentative releases of G. croceipes during or following a sterile backcross release should not adversely affect the backcross release ratio, and that the two control techniques possibly could be used effectively together in an area-wide management programme for controlling H. virescens.
The effect of concurrent parasitoid and sterile fly releases on wild Ceratitis capitata (Wiedemann) populations in the Kula area of Maui, Hawaii was investigated. It was concluded that concurrent SIT and parasitoid augmentation programme may interact synergistically, producing a greater suppression in targeted insect population than either method used alone.