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Synthetic fungicides are the primary means to control postharvest diseases of fruits. Biological control has emerged as one of the most promising alternatives to chemicals. During the last 20 years, several biological control agents have been widely investigated for use on different pathogens and fruit crops. Many biological control mechanisms have been suggested for use on fruit including antibiosis, parasitism, induced resistance in the host tissue and competition. With the aim of extending the use of the biofungicides, there have been many studies on the application of various combinations of control agents, and on the application integrated with chemical and physical means of protection. The formulation and application methods are key issues for the efficacy and successful outcome of the commercial product. Genetic engineering may provide a useful tool in the enhancement of the biological control efficacy. Since biofungicides are usually not as effective as pesticides, this approach should be viewed as an important component of an integrated disease management scheme given that a significant and permanent reduction of pesticide use is our goal.
Biological control studies require the ability to distinguish released pathogens from locally occurring isolates of the same species. We have developed a technique that differentiates genotypes using random amplified polymorphic DNA (RAPD) for the apomictic species Zoophthora radicans (Zygomycota: Entomophthorales), a pathogen of the potato leafhopper, Empoasca fabae (Homoptera: Cicadellidae). RAPD analysis was performed on Z. radicans isolates released in test plots in 1990 and 1991 for leafhopper control; isolates later recovered from the same plots and diverse other isolates were included in the analysis. RAPD fragment profiles of five recovered isolates proved very similar to those of the released isolates and different from all other isolates tested; they are probable descendants of the released isolates. One of the recovered isolates had RAPD profiles similar to isolates derived from aphids and probably represents a population endemic at the release site. In addition to verifying the successful establishment of our experimental releases, RAPD analysis revealed clear relationships among isolates derived from the same host taxon. We propose that this simple and relatively inexpensive method will be valuable in determining the establishment and spread of organisms released in biological control studies.
The ecological impact of introduced biological control agents on native species of arthropods is a matter of considerable debate. This study investigated the ability of the non-native predatory mite Neoseiulus californicus to feed on the native Typhlodromus pyri and vice versa, as both species now co-occur in UK orchards. Typhlodromips montdorensis is a candidate for introduction into the UK as a glasshouse biological control agent. The ability of T. montdorensis to feed on the widely used N. californicus was investigated to identify possible intraguild predation, which might influence the effectiveness of either or both species as predators of Tetranychus urticae. Both N. californicus and T. pyri consumed larval stages of each other, but in choice experiments both showed a preference for T. urticae. Both N. californicus and T. montdorensis also fed on each other, but whereas N. californicus again showed a preference for T. urticae, T. montdorensis fed equally on T. urticae and N. californicus. Interactions between N. californicus and T. pyri and N. californicus and T. montdorensis are discussed in relation to their effectiveness as biological control agents in the glasshouse and the natural control of spider mite in the field.
Biological control of insect pests is characterised by a persistent, strong reduction in the pest population following the introduction of a natural enemy. Analysis of mathematical models suggests that differential exploitation of patches of the pest in a spatially heterogeneous environment provides the most likely mechanism to account for known successes.
Biological control of exotic invasive plants using exotic insects is practiced under the assumption that biological control agents are safe if they do not directly attack non-target species. We tested this assumption by evaluating the potential for two host-specific biological control agents (Urophora spp.), widely established in North America for spotted knapweed (Centaurea maculosa) control, to indirectly elevate Sin Nombre hantavirus by providing food subsidies to populations of deer mice (Peromyscus maniculatus), the primary reservoir for the virus. We show that seropositive deer mice (mice testing positive for hantavirus) were over three times more abundant in the presence of the biocontrol food subsidy. Elevating densities of seropositive mice may increase risk of hantavirus infection in humans and significantly alter hantavirus ecology. Host specificity alone does not ensure safe biological control. To minimize indirect risks to non-target species, biological control agents must suppress pest populations enough to reduce their own numbers.
Biological control is not new, it is simply newly appreciated. This renewed appreciation stems from the widespread insecticide treadmill which is largely a product of insecticide disruption of the balance of insect communities. Biological control is a natural phenomenon; the regulation of plant and animal numbers by natural enemies. In this broad sense, biological control is vital to public health because it keeps the myriad insect species from out-competing us. It also has direct public health advantages as where natural enemies are manipulated to control disease vectoring insects. Insecticide distruption of biological control by insecticides and the resulting pesticide treadmill have serious public health implications. One is the increased pesticide load in the environment. The other is the acceleration of pesticide resistance in disease vectoring insects. The treadmill and its associated hazards will not abate so long as chemical control dominates our pest management strategy.
Biological control of insects that feed on our crops has become more practical in recent years by mass release of egg parasitoid microhymenoptera. Trichogramma species are now commercially reared and spread in commercial fields to control specific insect pests. Microhymenoptera species are, however, very small and morphologically indistinguishable within species, although strains of a given species differ in their efficiency to control specific insect pests. Traditional taxonomy is unable to differentiate microhymenoptera species at the strain level. It is becoming increasingly important to develop a reliable system to monitor genetic variations both within and between strains of commercially important microhymenoptera, to detect genetic drift occurring during several generations of multiplication, to protect patents, and to certify the lots of commercially released microhymenoptera. We have developed a system based on DNA markers to rapidly characterize individuals of five species of microhymenoptera from the genus Anaphes and Trichogramma including a new species of Anaphes not previously described. The main components of our system are a rapid and simple DNA micro-extraction method and fast DNA polymorphism analyses based on random amplified polymorphic DNA markers.
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Coccinellids have been widely used in biological control for over a century, and the methods for using these predators have remained virtually unchanged. The causes for the relatively low rates of establishment of coccinellids in importation biological control have not been examined for most species. Augmentative releases of several coccinellid species are well documented and effective; however, ineffective species continue to be used because of ease of collection. For most agricultural systems, conservation techniques for coccinellids are lacking, even though they are abundant in these habitats. Evaluation techniques are available, but quantitative assessments of the efficacy of coccinellids have not been done for most species in most agricultural crops. Greater emphasis is needed on evaluation, predator specificity, understanding colonization of new environments, and assessment of community-level interactions to maximize the use of coccinellids in biological control.
Fresh fruits are susceptible of be attacked by several pathogenic fungi after harvest due to both their high water and nutrients content and their loss of most of the intrinsic resistance that protected them over their development while attached to the plant. Most rot pathogens can be controlled by various methods such as refrigeration, controlled atmospheres and fungicides. Biological control strategies are emerging as promising alternatives to the use of synthetic fungicides. Several factors must be considered for the selection of biocontrol agents to be used against postharvest fruits diseases. Survivability of the antagonist is a major factor to determine its usefulness. Antagonists must survive and be effective after their exposure to both postharvest treatments and storage conditions. Several antagonistic microorganisms have been found that can effectively inhibit postharvest diseases. Just as there is a diversity among microorganisms, there is also a diversity of mechanisms by which they operate. Although in most cases these mechanisms have not been satisfactorily elucidated, they are likely to involve antibiosis, nutrient competition, stimulation of host defense, predation and parasitism. In many cases, probably more than one mechanism operate. The marketing of some of these antagonists may be feasible and they could be an alternative to synthetic pesticides.
Entomopathogenic nematodes have been used for biological control of certain insect pests. In these studies the nematodes were tested as a possible biological control agent for engorged female ticks. Five species of infective juveniles (IJs) were tested initially for their ability to penetrate and kill ticks, including Steinernema glaseri (SG), S. riobravus (SR), S. carpocapsae (DT), S. feltiae (SF) and Heterorhabiditis bacteriophora (HP88). Infective juveniles (IJs) of SRs and SFs appeared to be the most effective in killing ticks and invaded and killed 30 to 100% of replete females. These two nematode species were tested on several tick species including Amblyomma americanum, A. cajennense, A. maculatum, Dermacentor variabilis and Rhipicephalus sanguineus. Although the killing rate of each tick species varied, the nematodes did not appear to be host specific and were able to kill ticks of all species tested. Egg mass weights of exposed ticks of each species were significantly lower than those of the controls. Ticks were examined with microscopy to determine whether nematodes entered and multiplied inside ticks. Partially fed female Amblyomma americanum and Dermacentor variabilis exposed to 5000 IJs in petri dishes were collected at 8, 24, 48 and 96 hrs (Trial 1) and 1, 2, 3, 4, 7 and 9 days (Trial 2) post-exposure, and fixed, processed and embedded in resin for microscopy studies. Only a few nematodes were seen in the hemocoel and tissues and they were surrounded by a clear space. Bacteria, released from the nematodes, were present in the exposed ticks and appeared to increase daily causing a generalized infection. Degeneration of tick tissues and death of the ticks appeared to result from bacterial proliferation. Nematodes did not multiply within ticks as they do in insect larvae. In these controlled laboratory studies, exposure of ticks to nematodes resulted in tick mortality and reduced egg production. Entomopathogenic nematodes appear to have potential as a biological control agent of ticks, but future studies will be required to determine whether nematode/tick interactions will occur in the field.
We studied the population biology of the nematophagous fungus Hirsutella rhossiliensis to understand its potential as a biological control agent. Because the fungus is an infectious and transmissible parasite, we framed our study within an epidemiological context. Field observations, theory, and experiments demonstrated that (i) parasitism of nematodes by H. rhossiliensis is dependent on nematode density, (ii) local populations of the fungus will go extinct unless supplied with some minimum number of nematodes (the host threshold density), and (iii) natural epidemics of this fungus in populations of nematodes develop slowly and only after long periods of high host density. Additional in-depth research on population biology is needed to explain other biological control systems and to guide future research. The most effective research will combine field observation, theory, and experimentation.
During 1999-2001, ARS scientists published over 100 papers on more than 30 species of insect pest and 60 species of predator and parasitoid. These papers address issues crucial to the three strategies of biological control: conservation, augmentation and introduction. Conservation biological control includes both conserving extant populations of natural enemies by using relatively non-toxic pesticides and increasing the abundance of natural enemies in crops by providing or improving refuges for population growth and dispersal into crops. ARS scientists have been very active in determining the effects of pesticides on beneficial arthropods and in studying movement of natural enemies from refuges into crops. Augmentation involves repeated releases of natural enemies in the field, which can be inoculative or inundative. Inoculative releases are used to initiate self-propagating populations at times or in places where they would be slow to colonize. ARS scientists have studied augmentative biological control of a variety of pest insects. The targets are mostly pests in annual crops or other ephemeral habitats, where self-reproducing populations of natural enemies are not sufficiently abundant early enough to keep pest populations in check. ARS research in augmentative biological control centers on methods for rearing large numbers of healthy, effective natural enemies and for releasing them where and when they are needed at a cost less than the value of the reduction in damage to the crop. ARS scientists have researched various aspects of introductions of exotic biological control agents against a diversity of pest insects. The major issues in biological control introductions are accurate identification and adequate systematics of both natural enemies and target pests, exploration for natural enemies, predicting the success of candidates for introduction and the likelihood of non-target impacts, quarantine and rearing methods, and post-introduction evaluation of establishment, control and non-target impacts. ARS scientists have published research on several general issues in biological control. Among the most important are the mechanisms affecting mate- and host-finding and host specificity.
The public concern over the harmful effects of chemical pesticides on the environment and human health has enhanced the search for safer, environmentally friendly control alternatives. Control of plant pests by the application of biological agents holds great promise as an alternative to the use of chemicals. It is generally recognized that biological control agents are safer and more environmentally sound than is reliance on the use of high volumes of pesticides. Due to the importance of chitinolytic enzymes in insect, nematode, and fungal growth and development, they are receiving attention in regard to their development as biopesticides or chemical defense proteins in transgenic plants and microbial biocontrol agents. In this sense, biological control of some soil-borne fungal diseases has been correlated with chitinase production. Fungi- and bacteria-producing chitinases exhibit antagonism against fungi, and inhibition of fungal growth by plant chitinases has been demonstrated. Insect pathogenic fungi have considerable potential for the biological control of insect pests. Entomopathogenic fungi apparently overcome physical barriers of the host by producing multiple extracellular enzymes including chitinolytic enzymes, which help to penetrate the cuticle and facilitate infection. In this chapter, the role of chitinases in biological control and their potential use in the improvement of biocontrol agents and crop plants by genetic engineering is analyzed in view of recent findings.
Biological control is an important method of dealing with plant and insect pests. The control of rabbits by myxomatosis and the eradication of smallpox by vaccination are unusual examples of biological control, in that they involve a vertebrate and a viral pest respectively. Myxomatosis is a benign disease in Sylvilagus rabbits in South America which is transmitted mechanically by mosquitoes. In the European rabbit, Oryctolagus, which is a pest in Australia and England, the virus from Sylvilagus produces a generalized disease that is almost always lethal. Myxomatosis was deliberately introduced into Australia in 1950 and into Europe in 1952. It was at first spectacularly successful in controlling the rabbit pest, but biological adjustments occurred in the virulence of the virus and the genetic resistances of rabbits. After 30 years of interaction, natural selection has resulted in a balance at a fairly high level of viral virulence. Smallpox has been a major scourge of mankind for over 1500 years. It spread from Asia to Europe in the Middle ages and from Europe to Africa and the Americas in the 15th and 16th centuries. Jenner's cowpox vaccine provided a method of control that reduced the severity of the disease during the 19th century but failed to eliminate the disease from many countries before the 1930s. Thereafter it was eradicated from Europe and North America, but remained endemic in South America, Africa and Asia. In 1967 it was still endemic in 33 countries and W.H.O. established a programme for global eradication within 10 years. The goal was achieved in 1977. Problems of the eradication programme and reasons for its success will be described.
Classical biological control, i.e. the introduction and release of exotic insects, mites, or pathogens to give permanent control, is the predominant method in weed biocontrol. Inundative releases of predators and integrated pest management are less widely used. The United States, Australia, South Africa, Canada, and New Zealand use biocontrol the most. Weeds in natural ecosystems are increasingly becoming targets for biocontrol. Discussion continues on agent selection, but host-specificity testing is well developed and reliable. Post-release evaluation of impact is increasing, both on the target weed and on non-target plants. Control of aquatic weeds has been a notable success. Alien plant problems are increasing worldwide, and biocontrol offers the only safe, economic, and environmentally sustainable solution.
The availability of new genetic technologies has positioned the field of biological control as a test bed for theories in evolutionary biology and for understanding practical aspects of the release of genetically manipulated material. Purposeful introductions of pathogens, parasites, predators and herbivores, when considered as replicated semi-natural field experiments, show the unpredictable nature of biological colonization. The characteristics of organisms and their environments that determine this variation in the establishment and success of biological control can now be explored using genetic tools. Lessons from studies of classical biological control can help inform researchers and policy makers about the risks that are associated with the release of genetically modified organisms, particularly with respect to long-term evolutionary changes.