Rearing of Insects

The goal of organized insect rearing is to provide reliable, affordable sources of high-quality insects for their many important purposes. We are now able to rear literally thousands of insect species through multiple generations and many more for part of their life cycles. The greatest difficulty is to provide fresh host material or to develop a diet that is nutritionally complete and induces feeding, especially for predators or parasitoids. Precautions must be taken to start colonies with an adequate number of clean specimens and maintain them with very limited levels of mortality. These measures help to limit genetic bottlenecks caused by inbreeding, contamination with other species, and disease epizootics. As production levels and the number of species increase, rearing must be organized into systems with discrete operations or activities. These operations incorporate the rearing procedures necessary for each stage of the insect’s life cycle plus acquisition and maintenance of supplies, equipment, and facilities. Facilities must be designed and constructed to contain and maintain the insects under specified conditions. Problems encountered in established insect rearing systems invariably are caused by inattention to procedural details or lack of environmental control. For the foreseeable future, advances in insect rearing will be focused primarily on culturing new species, maintaining existing colonies more efficiently, supporting advances in research and pest management, and increasing the use of insects in people’s lives.


PURPOSES FOR REARING INSECTS

Insects are reared for many purposes that may not be obvious to the nonentomologist. Certainly, insects are aesthetically pleasing and, therefore, reared by those who appreciate their many shapes, colors, features, and behavior patterns. This appreciation may be shared by means of personal collections, static displays, insect zoos, butterfly houses, and even household pets. Many of these and similar experiences would not be possible without organized insect rearing.
Insects are reared for a wide range of primarily agricultural and public health research, esthetic, and utilitarian purposes, an increasingly popular example is the release of butterflies at social events. Chemical insecticides are developed by using laboratory colonies of insects to robotically screen massive numbers of candidates, more than 1 million insecticides per year in some instances. Similarly, plants are screened for resistance to insects or the disease organisms they transmit. Nontarget insects and plants are tested as possible hosts before nonindigenous natural enemies are released into nature. Insects have been reared, marked, and released to understand their orientation, dispersal, and migration. The cells of insects are used to study physiological processes, such as reproduction, ontogeny, growth, aging, and cold tolerance. The fruit fly, Drosophila melanogaster, is the “white rat” of geneticists and the nerves of large cockroaches are used by sensory physiologists. Students learn morphology by dissecting lubber grasshoppers, Romalea spp., and other insects reared in the laboratory. A variety of educational subjects involve insect colonies, including the process of insect identification, principles of insect taxonomy and systematics, and engineering aspects of insect flight. A somewhat gruesome but effective practice is the postoperative surgical use of the blow fly maggots, Lucilia seri-cata and Phormia regina, to maintain clean wounds.
The major use for laboratory-reared insects is in biologically based methods of integrated pest management. One of these methods, biological control, is the rearing and release of parasitoids, predators, and pathogens to suppress pest insects. A global industry has developed to rear and sell these natural enemies (www.anbp.org, www.ibma .ch/). Autocidal control is accomplished using the sterile-insect technique or inherited sterility. Male insects are reproductively sterilized with gamma-irradiation or chemicals and released by the thousands per hectare to mate with wild females. This ensures that most of the wild females mate with sterile males and do not produce offspring. Inherited sterility is a variation using partially sterile males to induce sterility in subsequent generations. Genetic control takes advantage of altered genes to disrupt the insect’s physiology and behavior. In the near future, we may use mass-reared insects to widely distribute manipulated endosymbionts via paratransgenic strains.
Insects have been reared to enhance wild and domestic populations, particularly for improving their products and benefits. Silk and honey production are obvious examples but pollination and pest management are equally important. Elaborate strain development methods have been used for silkworms, honey bees, screwworms (autocidal control), parasitic wasps (biological control), and others. Field insectary populations of expensive, showy butterflies and beetles have been used to augment natural populations that have become depleted. Eventually, the use of reared insects in conservation may include protection of rare and endangered species and repopulation, as is currently practiced with birds and predators.


KINDS OF INSECTS THAT ARE REARED

Insect rearing may be partial (egg to larva or nymph and adult) or complete (egg to egg). Virtually any free-living insect can be collected as an egg, larva, or nymph and maintained through stages in metamorphoses until it becomes an adult. However, this often requires considerable knowledge about the species’ habitat and natural food. Re-creation of soil or aquatic environments, symbiotic relationships and specialized foods can make rearing difficult. Some insects undergo temperature- and photoperiod-dependent diapause or require host plant cues to terminate multiyear cycles. Trophallactic feeding may necessitate maintenance of an entire colony, as in termites, ants, and other social insects. Because of these and other peculiar life history characteristics, the easiest insects to rear are relatively small, multivoltine (more than one generation per year), plant-feeding, terrestrial species with wide host ranges and no unusual environmental requirements. Species that infest common crops, landscape plants, or stored products are particularly suited to artificial rearing.
Complete rearing of an insect for one or more generations is often complicated by the mating and oviposition requirements of the adults. Species-specific requirements for temperature, humidity, amounts of space, light characteristics, photoperiod, population size, food, oviposition stimulants and substrates, and other environmental conditions all must be understood and provided. Insects may swarm and couple in flight, form mating aggregations on host plants, orient to each other by means of pheromones or auditory signals, transfer spermatophores (sperm packets), engage in postmating female guarding to protect their paternity, and perform other unimaginable actions to produce another generation. Fortunately, most of the species we rear for multiple generations have less complicated requirements; these are primarily butterflies and moths (Lepidoptera, >300 species have been reared), beetles (Coleoptera, >200 species), flies (Diptera, nearly 200 species), bugs (Heteroptera, <100 species), and bees and wasps (Hymenoptera, <100 species). Grasshoppers and katydids (Orthoptera), lacewings (Neuroptera), cockroaches (Blattodea), termites (Isoptera), and fleas (Siphonaptera) are also reared but in reduced numbers (roughly 10-20 species). Many more species undoubtedly could be reared using the techniques developed for their close relatives.

NATURAL AND ARTIFICIAL DIETS

Immature herbivorous (plant-feeding) insects can often be reared by feeding them clean, fresh-cut, or potted versions of the plant material on which they feed in nature. The roots, stems, leaves, flowers, or fruit must be readily available or grown in sufficient quantities. Examples are larval silkworms raised on mulberry leaves, boll weevils on cotton squares and bolls, tropical fruit flies on papaya fruit, and monarch butterflies on milkweed leaves. Similarly, medical and veterinary insects, e.g., adult mosquitoes and biting flies, are fed on their hosts or suitable surrogates, such as rodents, sheep, goats, or pigs.
Substitute plants can also be used to maintain herbivorous insects that are adaptable, usually readily available human and animal food. Green beans can be used for plant bugs, lettuce for grasshoppers, dry dog food for cockroaches, and cow manure for house flies. Rearing natural enemies of plant- or animal-feeding insects is considerably more difficult because three trophic levels must be synchronized: the plant or animal, the insect host, and the natural enemy.
Artificial diets have been developed to simplify and improve the rearing of both plant- and animal-feeding insects. Henry Richardson’s development in 1932 of a bran, alfalfa meal, yeast, and diamalt formula for rearing house flies eliminated the objectionable mess and odor of cow manure. A commercial diet made of wheat bran (33.3%), alfalfa meal (26.7%), and brewer’s grain (40%) is now available for rearing house fly larvae (CSMA medium; Chemical Specialties Manufacturer’s Association, Ralston-Purina, St. Louis, MO). Another historical advancement was M. H. Haydak’s 1936 grain flower, milk powder, honey, and glycerin diet for stored-grain insects, such as mealworms and flower moths. Gelled diets were developed for rearing insect larvae that require large quantities of contained water in their diets. The first was Pearl’s 1926 diet for Drosophila spp., followed by Botger’s 1942 larval medium for the European corn borer, Ostrinia nubilalis. Interestingly, these and subsequent diets have been gelled with agar, a polysaccharide derived from seaweed, that was previously developed for use in bacteriology by Robert Koch in the late 1800s. Agar remains the standard gelling agent for culturing both microorganisms and insects; however, its cost and requirement for heating have led to the development of alternative materials: polysaccharides (industrial gums, cellulose, pectin, plant starches), heteropolysaccharides (carrageenan, sodium and calcium alginate), scleroproteins (gelatins, animal albuminoids), starch polymers (polyacrylonitrile graft copolymers), crude fibrous plant products, and waxes. Ground plant fibers, such as sugarcane bagasse (pulp remaining after the sugar is extracted), corncobs, and carrot powder are used to rear tropical fruit fly larvae, e.g. the Mediterranean fruit fly, Ceratitis capitata. The awful stench of using
a mixture of dried blood, milk, and yeast in water for rearing screw-worm larvae, Cochliomyia hominivorax, was virtually eliminated by a starch polymer-gelled diet developed primarily by David Taylor of the USDA, Agricultural Research Service (ARS), in 1988. A practical artificial diet for predaceous insects has recently been perfected and patented by Allen Cohen, USDA, ARS, retired.

INITIATION AND MAINTENANCE OF COLONIES

Insect colonies are initiated from field-collected specimens or samples from previously established colonies. Any developmental stage can be used to start a colony, but surface-sterilized eggs are generally preferred because they are durable, easy to ship, and less likely than other stages to carry a pathogen. However, eggs may be difficult to find in nature and often larvae suffer high levels of mortality as first instars because they are not yet adapted to the laboratory. It is generally advisable to use late instars, hold them in individual containers for parasitoid and pathogen screening, combine the adults in mating cages with a suitable oviposition substrate, and collect and treat the eggs before colonization. From source, field, or insectary, the degree of success achieved over multiple generations will depend on the quality of the colonized insects and the skill with which they are reared. Many species that can be colonized and reared for multiple generations are much larger, healthier (free of malnutrition, pathogens, parasitoids, and predators), uniform in growth, and more active and fertile than those in nature. Special consideration must be given to rearing insects that are required to behave normally, particularly those destined for release to suppress wild populations. Fruit flies (ca. 1 billion per week), screwworms (ca. 500 million per week), and other species that have been mass reared for 20 or more generations in isolation may no longer interact and mate with their target populations in nature. To avoid this so-called strain deterioration, insectary populations can be recolonized or infused periodically using specimens collected from the targeted wild population. Yields will be relatively low for several generations of a new colony destined for mass production, unless a previously isolated strain has been adapted to the insectary in anticipation of its use for colonization. Infusion is accomplished best by holding field-collected larvae, obtaining adults, and combining their eggs with those of the mass-reared colony, as explained earlier. A large number of the new insects must survive to ensure that the colony has been infused. Also, genetic bottlenecks (sources of selective mortality) must be avoided because these will hasten the selection of insects that no longer behave normally.
There are several options for starting a colony from one that already exists. Colony starts can be obtained by contacting entomologists who publish on species of interest or are involved in large-scale, biologically based pest management programs. In the 1980s, the Entomological Society of America published lists of Arthropod Species in Culture and specialized directories still exist for Drosophila strains and other species. Arthropod Species in Culture listed colonies of the following taxonomic orders (number of species, colonies): Acari
(41, 77), Anoplura (1, 1), Coleoptera (78, 266), Diptera (168, 301),
Heteroptera (90, 206), Hymenoptera (119, 169), Lepidoptera (101,
308), Mallophaga (3, 3), Neuroptera (2, 2), Orthoptera (64, 203),
Siphonaptera (2, 7), and Thysanura (4, 11). Suppliers of Beneficial Organisms in North America is maintained by Charles Hunter of the California Environmental Protection Agency (www.cdpr.ca.gov). This publication lists more than 130 species of beneficial organisms available from 142 suppliers. The most popular species offered for sale were the green lacewing, Chrysoperla carnea (65 suppliers); brown lacewing, C. rufilabris (54); mealybug destroyer, Cryptolaemus mon-trouzieri (52); whitefly parasitoid, Encarsia formosa (54); convergent ladybeetle, Hippodamia convergens (56); predaceous mite, Phytoseiulus persimilis (54); and egg parasitoid, Trichogramma pretiosum (77). Peter Ebling recently established the Insect Producer Database to provide a comprehensive worldwide listing of producers who are willing to sell or donate live insects (www.insect.glfc.cfs.nrcan.gc.ca/). There are many commercial sources of insects, including Carolina Biological Supply and Combined Scientific Supplies. Pioneers in supplying diets for research are BioServe and Southland Products.

CATEGORIES OF INSECT REARING

There are three distinct categories of insect rearing: single species, multiple species, and mass rearing. In single-species rearing, immature stages are usually fed on host plants or animals, although artificial diets may be substituted. Seminatural environments and oviposition substrates are duplicated from nature and all rearing operations can be performed by a single individual. Multiple-species rearing is usually accomplished in centralized facilities to support research. There is economy of scale in the rearing operations, i.e., diet preparation, egg treatment, larval rearing, harvesting of pupae, and adult colony maintenance can be combined for similar species. Multiple-species rearing is common in research laboratories, such as those used for insecticide or transgenic crop development, and is typically performed by a small staff. Mass rearing involves a single species produced for biologically based pest management that is reared in factory-like facilities with controlled environments, artificial diets and oviposition substrates, mechanized equipment, and operations performed by work units. The largest facilities are used to rear the screwworm and Mediterranean fruit fly. Although based on single-species rearing, mass rearing systems are distinct in design and implementation, not merely multiplications of scale.

BASIC INSECT REARING OPERATIONS

Regardless of the size of the colony, insect rearing is organized into operations based primarily on the species’ life history. At some level of complexity, insect rearing operations include inventory, acquisition, and storage of supplies; diet preparation and containerization; egg collection and treatment; larval or nymphal development; pupal or adult recovery and distribution; adult colony maintenance; quality control; and facility and equipment maintenance. Depending on the species, these operations are subdivided into procedures for which space is allocated and personnel are trained and assigned. “Traffic patterns” are established in the facility that flow from relatively clean areas to those that are potentially contaminated, i.e., diet preparation through larval development and diet disposal. Diet and eggs are usually handled more carefully until they are sealed in clean rearing containers. Isolating the larvae, pupae, and adults in containers protects them from contamination and prevents workers from being exposed to potentially harmful microorganisms and allergens. Rearing operations are performed in synchronized sequences, so they can be interfaced at the most appropriate times.

REQUIREMENTS FOR INSECT REARING FACILITIES

Insect rearing facilities have evolved from field insectaries with outdoor temperature and humidity to environmentally controlled laboratories that contain insects under high security. Field insectaries provide adequate environments for rearing insects under seminatural conditions on host plants or animals. However, pathogens, parasitoids, and predators are not controlled, and workers are exposed to potentially dangerous pathogens and allergens. Conversely, temperature, humidity, air quality and quantity, light quality and photoperiod, sanitation, and security are closely regulated in laboratory insectaries. Insects and supplies are carefully screened for contaminants before they are admitted and human access and exposure are limited. All openings to a laboratory insectary are sealed or filtered to prevent insects from entering or leaving, particularly in quarantine facilities. Quarantine and containment facilities have strict construction requirements and operational protocols. Laboratory insectaries must be well insulated and have highly filtered, recirculated air to be efficient and cost effective.

PROBLEMS ENCOUNTERED IN MAINTAINING INSECT COLONIES

Virtually all severe insect rearing problems result from failure to perform standard operating procedures or defective environmental controls. Once established, rearing operations become routine and individual procedures are easy to forget. For example, a dietary ingredient may be omitted or destroyed by overheating, eggs may be accidentally desiccated following surface sterilization (dechorionation), closely related species may be mixed unintentionally, or larval densities may be more or less than required per container. Additionally, dietary ingredients can deteriorate after prolonged storage. These kinds of problems can be detected and corrected before a colony is lost. However, temporary loss of environmental control can destroy all of the insects. Pathogens can also be devastating, although diseased insects are usually confined to certain containers that can be eliminated before others are contaminated. Parasitoids and predators must be detected and similarly discarded. Genetic deterioration from inbreeding and genetic drift has been blamed for declines in insect colonies, but this is not a typical problem in large colonies. Insect rearing is generally safe for humans unless they are hypersensitive to insect proteins, react to the physical irritation of insects or their body parts, work with insects that sting or bite, or expose themselves to toxic substances used in the operations.

THE FUTURE OF INSECT REARING

Insect rearing is in transition, along with the science and technology it supports. Appreciation of the natural history of insects and their rearing is increasing steadily as people enjoy ecotourism, butterfly houses and gardens, insect zoos, butterflies in ceremonies, and educational products based on insects. Insects are commonly used as baits for fishing and, in certain countries, have become popular pets. Rearing is becoming more important as field collection of insects is restricted to preserve habitat biodiversity and protect germplasm ownership. This is analogous to the collection of orchids and other showy organisms. Insects are no longer major sources of natural products, such as silk, wax, or dyes, and their use as human food is very limited. However, bird-watching and exotic pet ownership have become extremely popular in affluent countries, causing a significant increase in the use of insects as animal food. Rearing insects to produce bioactive substances remains a research support activity in the fields of biology and medicine. Living as well as dead insects are used increasingly in classroom education.
Agricultural uses for insects have expanded dramatically during the past 40-50 years in the development and support of new pest management technologies. Every major company that produces chemical insecticides or pest-resistant plants maintains a multispecies insect rearing facility. Although insecticides have provided effective insect control at individual farms and residences, overall losses to insects have not declined and we need new options for sustainable pest management. Large federal and state entomological research laboratories and university entomology departments also have laboratory insec-taries, although the current trend is toward decentralization and outsourcing. The sterile-insect technique, pioneered by E. F. Knipling, USDA, ARS, has proven too expensive, unless used on an area-wide basis with stringent regulatory controls, as in the screwworm and Mediterranean fruit fly eradication programs. Advances in insect rearing have enabled the eradication of these species and others from vast geographical areas. As a result, area-wide approaches to pest management are increasing, along with new methods for using genetically modified organisms. Reliable, cost-effective rearing will be required for these technologies to be successful.
Major advances in insect rearing are currently being made in support of biological control. Natural enemies are reared and released to prevent rather than cure pest problems, and they rarely have unacceptable nontarget effects. Another major advantage in using natural enemies is that they do not induce the pest resistance that eventually makes insecticides ineffective. We now have efficient host-rearing systems for at least 40 parasitoids and predators. Advancements are also being made in the mechanization of rearing operations, large-scale release technology, and rearing of newly discovered natural enemies. Global markets are increasing in organic food production, ornamental and vegetable greenhouse crops, urban pest management, filth fly control, home gardening, and other specialized areas. Biological control will continue its expansion as insect resistance to chemical insecticides increases, worker protection and food safety regulations are enforced, people avoid real or perceived environmental contamination, and the quality and efficacy of natural enemies improves.
Insect rearing will play a critical role in the future of entomology. Insects will always be appreciated for their intrinsic value, used as a source of useful materials, and produced as food for wildlife. Moreover, they will become increasingly important for pest prevention in natural areas, crops, and buildings and in human and animal wastes. Other uses will include transgenic biological control and autocidal methods (sterile-insect technique, paratransgenesis). Insects will be reared to restore and supplement insect populations in nature; control pests over vast, low value-per-acre lands; and eliminate chemical insecticides in specialized cropping systems.
Insect rearing can be enhanced most by learning more about the natural history of insects, emphasizing their ecology, behavior, and systematics. This knowledge can be used to create artificial diets and environments that separate species from limiting factors, biotic (pathogens, parasitoids, and predators) and abiotic (temperature, humidity, and light). As knowledge about insects is gained, it must be published, communicated at meetings, and widely distributed to help advance the field. The Entomological Society of America and International Organization for Biological Control have been particularly helpful in disseminating insect rearing information. Insect rearing is often considered a support activity to be described in the methods sections of articles on other subjects. This makes the information difficult to retrieve and has led to relatively obscure publications on insect rearing. Another major limitation has been a general lack of formal education and training in insect rearing. However, Frank Davis conducts a popular annual workshop on the subject at Mississippi State University, Allen Cohen has a continuous educational program at Insect Diet and Rearing Research, LLC (www.insectdiets.com/), and David Dame covers the subject in his biannual FAO, International
Atomic Energy Agency short course at the University of Florida. This is the best time in history to discover and disseminate insect rearing knowledge and obtain training in this essential field.

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