Corn Borer Rearing for Integrated Pest Management.
Corn stalk and stem borers such as the European corn borer (Pyrausta nubilalis) and the lesser corn stalk borer (Elasmopalpus lignosellus) are common local (North American) economically important pests in cultivated corn production. These types of pests are common on many of the food staples around the world such borers which feed upon rice and corn. Knowledge of the insects' biology is helpful in developing management practices for that particular pest. Lepidopteran borers with damaging larval stages are difficult to manage with many common practices such as cultural or chemical initiatives alone. Therefore, host plant resistance is the preferred methodology in controlling stem/stalk borer pests in corn production (Songa, J. M., et al 2001).
The unpredictability and seasonal dynamics of field populations of insects necessitate the use of artificial infestations for targeted stalk borer resistance screening studies (Songa, J. M., et al 2001). The following paper will discuss the mass rearing of maize stem borers (Chilo partellus, Busseola fusca, Sesamia calamistris, Chilo orichalcociliellus and Eldana saccharina) at a large Kenyan (Africa) facility. The facility itself can perfect its practices; yet the novel scientific insight will largely come from new and existing corn borer research.
Lepidopteran stem borers are one of the major constraints to maize production in Kenya with the aforementioned species comprising the most destructive respective insects (Songa, J. M., et al 2001). The mass rearing facility is located in the Kenya Agricultural Research Institute (KARI) in collaboration with the International Maize and Wheat Improvement Center (CIMMYT). The facilities include: two insectaries for larval and pupal development and for egg incubation, one room for diet preparation and subsequent insect infestation, one room for pupal harvesting, one room for adult emergence and oviposition, and one washing room, and a storage room for diet ingredients (Songa, J. M., et al 2001).
The founder stem borer colony was procured from the insectaries at the International Centre of Physiology and Ecology (ICIPE) in order to ensure that it was free from major parasites or pathogen infections (Songa, J. M., et al 2001). In order to avoid genetic decay and maintain the original behavioral characteristics of the species seasonal gene infusion is used as the main colony maintenance strategy (Songa, J. M., et al 2001). At the conclusion of every season field populations of stem borer larvae and pupae are collected from various maize growing regions in Kenya (Songa, J. M., et al 2001). Otherwise, continuous rearing of generations of insects in a laboratory setting will effectively select the population for laboratory fit insects instead of a population representative of the field variety.
The fundamental environmental disparity between field and laboratory populations is that of the temperature regime. More research is needed in this facet of insect rearing efforts, but a summary of such researched observations is included in Table A (Sibly, R. M. & Atkinson, D. 1994).
Under normal field conditions increased growth rates usually selects for larger adults, but under reproductive isolation the effects depend on the details of trade-off curves relating fecundity to development period (Sibly, R. M. & Atkinson, D. 1994). Field observations of the lesser cornstalk borers concur that the respective life cycle is shortest under dry conditions and when summer temperatures are the highest (Leuck, D. B. 1966). Conversely, when field temperatures become lower the insect cycle is lengthened and pupae result as the overwintering stage (Leuck, D. B. 1966).
Controlled lighting conditions could also adversely affect research findings (Conway, S. C. & Harding, J. A 1967). The average emergence times and weight of larvae increased with the length of light per day (Conway, S. C. & Harding, J. A 1967). Table B illustrates the effect of light regimes noted with European corn borer rearing (Conway, S. C. & Harding, J. A 1967). C:\Users\Travis\Desktop\Masters Project\Table B.jpgThe relatively small weight discrepancies become more apparent as the larvae enters into the pupation stage (Conway, S. C. & Harding, J. A 1967). Corn borer rearing should be performed properly, and monitored for quality assurance. Small but important differences could go unnoticed and eventually confound statistical studies often performed (Conway, S. C. & Harding, J. A 1967).
Much of the undesirable temperature effects are inevitable; however, through research and trial and error efforts optimal temperature regimes can be established for consistent production. This particular rearing facility has one standard rearing condition (28 ̊C, 60-70% RH, 12-12 light/dark photoperiod) room, and another room designated for delayed development populations hosting the following conditions: (20 ̊C, 60-70% RH, 12-12 light/dark photoperiod) (Songa, J. M., et al 2001). The rate of development of the larvae or pupae may need to be altered in order to better coordinate the larval supplies or egg supplies with the generation cycles and research needs (Songa, J. M., et al 2001).
The mass production of insects on artificial diets during the last decade has greatly accelerated research on control of pests (Vanderzant, E. S. 1974). Diets can play an essential role in egg production and in sustaining longevity of females (Simmons, A. M. & Lynch, R. E. 1990). Diet preparation at the Kenyan facility is exact and uniform in preparation, distribution, and infestation procedures. Fractions A, B and C from Table C (Songa, J. M., et al 2001) are prepared as procedures stipulate and the fractions are then blended together to yield the final diet product.
Many species of phytophagous lepidopterans have been reared successfully on artificial dietary media (Beck, S. D. et al. 1968). In a few cases, chemically defined (holidic) diets have been sufficiently complete to permit effective nutritional requirements (Beck, S. D. et al. 1968). However, holidic media have not proved very successful for the continuous rearing of a laboratory culture of any lepidopterous species, because of suboptimal growth, a loss of viability, and an attenuation of fecundity over the span of several generations (Beck, S. D. et al. 1968). The addition of crude plant materials is believed to bestow: an unidentified growth factor (such as a trace element or vitamin), a behavioral factor (such as a feeding stimulant), or optimum concentration ratios among specific nutrients or classes of nutrients (Beck, S. D. et al. 1968). Ascorbic acid was found to be the feeding stimulant for the growth of locusts (Vanderzant, E. S. 1974). Therefore, plant feeding insects require the presence of microorganisms must be provided to avoid the loss of ascorbic acid (Vanderzant, E. S. 1974).
A fair amount of knowledge is surfacing as to the nutritional requirements of economically important insects. However, little is known concerning the effects of nutrition on growth and development patterns (Beck, S. D. 1950). Debates about such topics date back to the late nineteenth century with postulates by Dyar (1890) (Beck, S. D. 1950). “Dyar's Law,” states that arthropod growth is geometric in C:\Users\Travis\Desktop\Masters Project\Table D.jpgnature and remains proportional with regards to weight and linear dimensions, and is associated with instar progression (Beck, S. D. 1950). Critics of Dyar's Law claim such findings were centered about research with gradual metamorphosis insects; specifically the Hemipteran Order of insects (Beck, S. D. 1950). With the Class Insecta (insects) there are three types of metamorphosis, and broad growth extrapolations should be made cautiously. It was found that the influence of diet on head width was the same as its influence on weight and that the two effects could not be separated (Beck, S. D. 1950). Table D from recent research on European corn borers clearly does not support Dyar's overall geometric size increase regime (Beck, S. D. 1950). Progression factors varied significantly from instar to instar and from diet regime to diet regime, which varies significantly from Dyar's ratios Table E depicts weight as the determining factor of head capsule length; not the number or particular instar status, which follows more of an allometric growth pattern (Beck, S. D. 1950). Allometric growth formulas have been applied to several other animal forms as well (Beck, S. D. 1950). It is also known that molting is not associated with absolute growth, but with physiology (Beck, S. D. 1950). Furthermore, the act of molting will not in it of itself add significant growth size or girth to the developing larvae, pupae or adult. This fact has been understood for over a century; ecdyses results in no appreciable growth (Riley 1883).
The combination of specific research enriched with mass production insect monitoring has yielded delineated species biology's. Table F illustrates the general timelines for the five species reared at the Katumani insectary (Songa, J. M., et al 2001). Such a timeline allows for targeted monitoring of the populations in field or in this case in the laboratory (targeting infestation, harvesting, and cleaning and sorting dates) (Songa, J. M., et al 2001). The cleaned pupae are transferred in batches of 100, per petri dish to a ventilated plastic emergence container (Songa, J. M., et al 2001). The emergence cages are monitored at the beginning of each day, and the emerged moths are transferred to an aluminum-frame oviposition cage (capacity of 200 pairs of adults) (Songa, J. M., et al 2001).
All of the aforementioned equipment, tasks, and timing are all taken with care and are standardized and monitored for consistency. Although, to efficiently mass produce five different corn borer species on a limited budget individual insects cannot be manipulated at each step: pupal harvesting is done only once to save time and money and is conducted when at least 40% of the larvae had pupated to ensure the greatest overall safety for all insects(Songa, J. M., et al 2001). Precautions are taken primarily at critical control points identified for each respective corn borer species. According to the rearing procedures recommendation by ICIPE and which we had adopted, Busseola fusca larvae should be reared singly due to their cannibalistic nature (Odindo & Onyango 1998). However, because of the laborious nature of this activity, we found it necessary to explore ways of reducing this problem: supply adequate food in a larger container (Songa, J. M., et al 2001). The aforementioned solution coincides with such behavioral findings of corn borers: crowded larvae tended to disperse and establish themselves uniformly in areas of lesser competition (Ramsey, T. A. & Brown, G. C. 1984).
The Katumani insectary performed an independent experiment guided at determining the survival rate disparity between the glass vial rearing method and that of the adapted larger container method. The survival rate results were 85.2% and 80.5% respectively (Songa, J. M., et al 2001). Considering the cost of time incurred during infestation of the glass vials with single larvae, the higher risk of breakage of the vials (during handling and washing of the vials) and their relative higher cost, it was decided that rearing B. frusca in the jars would be a better method to use, and is what has been used in the insectary recently (Songa, J. M., et al 2001).
Density dependent and or intra-specific competition has been noted for the European corn borer in both field and laboratory setting (Ramsey, T. A. & Brown, G. C. 1984). Table G illustrates the mean larval survival rate of borers reared on artificial diets in a laboratory setting (Ramsey, T. A. & Brown, G. C. 1984). Density may affect an insect's growth and development by reducing the availability of resources due to interference or exploration competition, and metabolic products produced by conspecifics may prolong larval growth (Breden, F. & Chippendale, M. G. 1989). In addition, high density may increase both the opportunity for cannibalism and the nutritional importance of cannibalism (Breden, F. & Chippendale, M. G. 1989). These effects are believed to occur in the field as well, but are mediated by ease of migration relative to that of captive insects.
Integrated Pest Management
The Katumari insectary's main objective is to rear and supply insects that have traits and also have the behavioral characteristics typical of field insects (Songa, J. M., et al 2001). The key performance traits that have been the focus of the insectary are: fecundity of mated females, fertility of eggs laid, accurate development periods, and host acceptability similar to field populations (Songa, J. M., et al 2001). Such qualities are difficult to study in a field setting and impossible to quantify. However, such knowledge obtained in a controlled laboratory environment permits the establishment of appropriate control measures (Freitas, R. M. et al 2007).
Integrated pest management (IPM) is the ultimate objective of the insectary. The intent of IPM is to lower pest populations below economically important levels; eradication is not the mission (Barfield, C. S. & O'Neil, R. J. 1984). The term “integrated” implies that indeed many fronts are needed for such a management approach. These disciplines include plant pathology, entomology, plant breeding, agronomy, biochemistry, physiology, genetics, ecology, systematics, biogeography, and evolution (Harris, M. K. & Frederiksen, R. A. 1984). Rational usage of pesticides is also a proponent of IPM programs (Huang, X. & Mack, T. P. 2001). Pesticides are used sparingly due to of the inefficiencies of broad band application < 0.1% of pesticides applied to crops reaches the target pest (Huang, X. & Mack, T. P. 2001), and because of the associated environmental/ecological concerns, and finally the pest population's resistance would be expedited (Casida, J. E. & Quistad, G. B. 1998).
Host plant resistance plays a major role in integrates pest management strategies on many crops (Harris, M. K. & Frederiksen, R. A. 1984). This management approach is compatible with other control methods (Harris, M. K. & Frederiksen, R. A. 1984). Hence, the emphasis placed on host plant resistance by the entomologist working at the Katumani insectary. Measured progress in identification of host plant resistance requires carefully designed experiments to systematically screen for the resistance. The insectary can then expedite such a methodical and calculated approach to pest management.
Since the insectary began rearing in 1999; gradual justified changes in procedures have been documented over the 10 year span. Regular tests are conducted to monitor and compare the behavior and qualities of the laboratory reared insects versus the local wild population (Songa, J. M., et al 2001). The result is quality standardized protocol for each respective borer species reared at the insectary. Ultimately, a consistent and representative insect population will be paramount to a sector of the integrated pest management research.
Two standard sets of records are kept for record management and quality assurance protocols: infestation data and production data (Songa, J. M., et al 2001). Both data records are detailed records of empirical evidence regarding the rearing itself. Infestation data denotes details about the respective diets: diet preparation, date of infestation, and number of insects placed into each specific container. The production data denotes units of production: number of eggs harvested, number of pupae harvested, dates, and physical characteristics of the insects themselves.
Human Health Concerns
The quality and consistently of production and infestation is strictly monitored at the insectary as well as the health of the humans and insects at the facility. To prevent microbial contamination: floors are mopped daily, equipment is properly sterilized, personal protection equipment is by personnel, and a strict quarantine regulation is maintained when using field material for gene infusions (Songa, J. M., et al 2001).
The three main issues to human health concerns of the workers at the insectary are: moth scales, toxic fumes, and microbial contaminants (Songa, J. M., et al 2001). These three ailments can be averted with proper personal protective equipment (primarily facemasks, gloves, and a lab coat), use of fume hoods near toxic chemicals, and maintaining a clean laboratory and a fresh insect diet.
Research in a controlled environment is important to agriculture production. The controlled setting allows for populations to cycle more quickly than that of naturally occurring field populations. Also, population density manipulations can be made to fully research the host-plant interaction at the heart of the research. Interactions can be observed more directly and designed in a scientific research manner. All of the assertions and discoveries in the laboratory research should be verified in the field both qualitatively and quantitatively. The consecutive research and testing will complement each other with economic pest relieve as the outcome.
Rapidly growing human populations are the overriding driving force necessitating the efficient production of food staples. However, this force is amplified in poor and developing countries such as Kenyan Africa as covered in this review. This review was just a sampling of the many respective insect species reared/or to be reared and many generations to quantify the insect and the crop/insect interaction for similar purposes in agriculture. The next century of agronomic Entomological research will perfect the mass rearing of economically important insects. Integrated pest management will require expertise from a number of academic and professional angles: genetic and transgenetic discoveries will also be intimately associated with such integrated pest management high throughput research and testing. The insect species should also be researched on the individual strain of the species as factors such as resistance is a function of a particular strain of insect species. The research fronts are endless, but this is the model for success antidote of the Kenyan corn borers but also for similar research in the future.
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