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Trends affecting research strategies in plant resistance to insects
Agriculture, Ecosystems and Environment, 18 (1986) l-7 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TRENDS AFFECTING RESEARCH RESISTANCE TO INSECTS’
C. MICHAEL
STRATEGIES
1
IN PLANT
SMITH
Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803 (U.S.A.) (Accepted
for publication
27 November
1985)
ABSTRACT Smith, C.M., 1986. Trends affecting research Agric. Ecosystems Environ., 18: l-7.
strategies
in plant
resistance
to insects.
Varieties of crop plants showing resistance to insect pests have been in use in modern agricultural production systems for nearly 40 years. The need for their continued use is especially critical in developing countries today, since both food and food production costs have increased greatly. The expanding populations of some developing countries currently endanger the habitats of the germplasm of several major crop plants. For this reason, major efforts are now necessary to collect, preserve and maintain as much wild germplasm of major crop plants as possible. These efforts may lead to the exchange of advanced biotechnology techniques from developed countries for exotic crop plant germplasm from developing countries. Many varieties of insect-resistant crop plants rely on allelochemicals to convey antibiotic effects on pest insects. However, high levels of allelochemicals can have detrimental effects on beneficial insects, and induce insecticide tolerance in non-target insect pests. Therefore, a need for caution exists in the development of crop varieties which utilize allelochemicals in their resistance. Accurate, reproducible bioassay techniques are essential to the continued development of insect-resistant varieties. This is especially important today, since groups of scientists at different locations may be involved cooperatively in identifying and categorizing plant resistance to insects. Insect, plant and environmental variables should be standardized, since each can greatly affect the validity of bioassay results.
INTRODUCTION
In order to feed the predicted world population, world food production must double by the beginning of the 21st century. We need look no further than the drought ravaged areas of famine in sub-Saharan Africa for graphic evidence of the need for increased agricultural productivity. In developing countries, crops which are resistant to insects must obviously play an integral role in solving these types of food production problems, since the capital ‘Presented at a symposium on Plant Resistance to Insects: Research Strategies for the 21st Century. Annual Meeting of the Entomological Society of America, 9-13 December 1984, San Antonio, Texas. 0167-8809/86/$03.50
0 1986 Elsevier
Science
Publishers
B.V.
West Africa
Europe Greece Y:zz
Fig. I. Approximate percentage countries in the world,
of net income
spent
on food
by populations
of various
for chemical insect control is normally spent on the cost of food itself (Fig. 1). The following discussion is aimed at illustrating some of the trends which will affect research strategies to develop insect-resistant crop plants in the future. FROM THE GREEN
REVOLUTION
TO THE GENE REVOLUTION
Just as the green revolution brought high yielding varieties to tropical Asia and Latin America in the 1960’s, current research in biotechnology suggests that we are on the verge of a “genetic revolution” which will improve crop productivity by increased stress-resistance to insect-, diseaseand environmental-stresses. In the mean time, a fundamental need exists to maintain varietal selection and development by conventional plant breeding. As Borlaug (1983) stated, “there are no assurances that insect-resistant varieties developed by genetic engineering will be more long-lasting than those developed to date”. Research efforts should also continue to collect, preserve and maintain germplasm of the major world food crops with as much genetic diversity as possible. These efforts are currently in serious jeopardy due to the slash and bum agricultural practices in much of the humid tropics of the world, and to population expansion in areas such as the Brazilian Amazon basin (Walsh, 1981). There is currently concern by some members of the International Board of Plant Germplasm Resources that rice and wheat collections may be especially inadequate in their content of wild species. At present, wild genes from land races and exotic crop plant species are collected mainly by germplasm centers in developing countries, and new genes or genetic expression are being sought in the genetic engineering laboratories in many more technologically advanced countries. As the concept of a world economy rapidly becomes a reality, more emphasis should be placed on the exchange of germplasm for genetic engineering technology to accomplish future world food production goals.
3
RATIONAL DEVELOPMENT OF ALLELOCHEMICALLY-BASED PLANT RESISTANCE TO INSECTS Research involving chemically-based plant resistance to insects has increased dramatically in the past 10 years, with interest focusing on which chemicals make resistant varieties resistant and which make susceptible varieties susceptible (Fig. 2). From a biogenetic standpoint, different types of natural products (alkaloids, phenolics and terpenes) have all been shown to have allelochemical activities (Table I). In some cases, selection has been for high levels of allelochemical in resistant varieties. Recent research, however, suggests the need for a more moderate approach to the integration of plant allelochemicals with existing chemical and biological control measures. In their work with tomato cultivars with high levels of cu-tomatine, Campbell and Duffey (1979) found that although toxic to Heliothis zea Boddie, cw-tomatine was also toxic to of the parasite Hyposoter exigue, suggesting a need for the development varieties with only moderate e-tomatine content. Numerous examples exist in cotton, potatoes, rice and soybean which indicate that high levels of resistance, whether allelochemically-based or not, reduce pest insect populations to the extent of having negative effects on beneficial insect populations. Kennedy (1984) has recently demonstrated that 2-tridecanone, a toxin of the wild tomato, Lycopersicon hirsutum f. glabratum C.H. Mull, mediates resistance to the tobacco homworm, Manduca sexta (L.), and the Colorado potato beetle, Leptinotarsa decimlineata (Say), but also induces enhanced tolerance to carbaryl in Heliothis zea. At the metabolic level, Dowd et al. (1983) demonstrated that the soybean looper, Pseudoplusia includens (Walker), and the cabbage looper, Trichoplusia ni Hiibner, larvae fed on diets which are rich in phenolics from an insect-resistant Asian IO-
1
I
I
I
Fig. 2. Annual research productivity on plant allelochemics affecting insect behavior, 1960-1980. (A-A) repellents; (o-o) attractants; (*+) feeding stimulants; (0-o) feeding deterrents and growth inhibitors.
4 TABLE I Distribution
of plant chemicals
Attractants Alcohols Alkaloids Amino acids Disulfides Isothiocvanates Phenol& Terpenes
affecting
insect behavior by chemical
(%) 25 3 5 3 5 28 31
n = 61 Feeding stimulants Amino acids Falvonoids Glucosides Nucleic acids Sterols Sugars
(%) 20 20 15 5 15 20
class, 1955-1980
Repellents Amino acids Phenolics Terpenes
(%) 10 45 45
n=9 Feeding deterrents and inhibitors Alkaloids Amino acids Glucoides Phenolics Sterols - Sesquiterpene lactones Tannins Terpenes
(%) 24 5 7 10 2 16 1 35
n = 119
n = 54
soybean variety have opposite hydrolytic esterase levels. Activity in P. includens is depressed, indicating a partial resistance effect via allelochemicals, but activity in 2’. ni is enhanced, suggesting the possibility of increased tolerance to pesticides degraded by hydrolytic esterases. Both findings indicate the negative effects of high levels of chemically-based insect resistance which are possible in crop management systems, and demonstrate the need for caution in varietal development. DEVELOPMENT
AND USE OF ACCURATE
BIOASSAYS
Biological variation in the test insect and plant material can affect the outcome and reproducibility of plant-resistance bioassays. Numerous articles (Chapman, 1974; Cook, 1976; Kennedy, 1977; Maxwell, 1977; Smith, 1978) have pointed out the need for the refinement of insect-plant bioassay methodology and technique. It is important, therefore, to recognize and understand the ability of each of these variables to influence a bioassay of plant resistance to insects. In order to conduct cooperative research between groups of scientists at several locations, it is also important to attempt to standardize each of these variables to ensure comparable results. The age, sex and peak activity-period (feeding or oviposition) of the test insect, and how the insect is handled prior to testing all affect the outcome of the bioassay. In several instances, female insects have been shown to feed more on both resistant and susceptible plant foliage than males. Such
3
is the case with the Colorado potato beetle, Leptinotursu decemlineatu (Say) @chalk and Stoner, 1976), and the Mexican bean beetle, Epilachna vurivestis Mulsant (Smith et al., 1979). The effects of environmental factors on the expression of plant resistance to insects have been reviewed by Tingey and Singh (1980). As such, temperature, relative humidity, light intensity, soil fertility and soil moisture have all been shown to affect the expression of insect resistance in plants. Several plant variables affect the validity of bioassay data. In addition to obvious factors such as the plant part being bioassayed and the concentration of biologically active materials in that plant, plant age is a major factor in the study of insect resistance. Resistance may either increase or decrease in older plant tissues, depending on the presence or absence of the chemicals mediating resistance. Thus, in the leaves of mature plants, resistance increases in soybean (Reynolds and Smith, 1985), sweetclover (Beland et al., 1970) and tobacco (Abernathy and Thurston, 1969), but decreases in corn (Klun and Robinson, 1969) and sorghum (Woodhead and Bernays, 1977). Natural products which impart insect resistance can be related to the photosynthetic process in plants and, as a result of this, can vary greatly in content during the course of one 24-h period. Shade et al. (1975) demonstrated that the fluid droplet output from trichomes of a Medicugo species which is resistant to alfalfa weevil, Hyperu posticu Gyllenhal, larvae was greatest during mid-day periods of peak photosynthesis. Thompson et al. (1971) determined that cotton plant volatile concentration was greatest during late afternoon hours. From a practical standpoint, the most efficient collection of plant volatiles or plant foliage for phytochemical analysis should be conducted during the peak production period. Kogan and Paxton (1983) cite numerous instances of increased levels of plant resistance to insects which are induced by insect feeding. In several cases, the concentration of plant phenolic compounds increases, in what appears to be a general plant defensive response similar to plant response to pathogen attack. The response can occur in as little as 4 h (Westphal et al., 1981), and can occur in both insect-resistant and insect-susceptible plant varieties (Smith, 1985). It is very important, therefore, that prior feeding damage or wounding to test plants be determined before attempting comparisons of insect resistance evaluations from different locations. SUMMARY
Until biotechnological methods are more fully developed for application in agricultural production systems, conventional plant breeding will continue to be the primary method for production of insect-resistant crop plants. This method may be adversely affected, however, by current human population expansion into the native habitats of wild crop plant germplasm. One method of avoiding this problem is the continued enhancement
of germplasm collections at International Agricultural Research Centers throughout the world. Facilities for receiving, processing and storing germplasm accessions exist at these centers, and efforts to collect wild germplasm should receive additional support. The effects of plant allelochemicals on beneficial insects and secondary pests, cited previously, are similar to the monogenic vertical insect resistance effects of some varieties of wheat, rice and sorghum. In either instance, high levels of antibiosis resistance can or has eliminated large segments of the insect pest population, but has also prompted the development of resistance-breaking biotypes. Where possible, future plant breeding efforts should attempt to include tolerance resistance to lessen the chance for biotype development. Many research programs which are aimed at identifying insect resistance and incorporating it into crop plants spend a great deal of time and resources developing methods and techniques to measure resistance. Variable factors in test plants and insects, in addition to environmental factors, all have very real effects on determining the occurrence and degree of insect resistance in plants. Careful documentation of the effects of each of these factors will allow the development of baseline conditions for plant evaluations of insect resistance. Determination of such baseline conditions will greatly expedite progress in developing insect-resistant crop plants and will make valid comparisons between different research locations possible.
REFERENCES Abernathy, C.O. and Thurston, R., 1969. Plant age in relation to the resistance of Nicotianato the green peach aphid. J. Econ. Entomol., 62: 1356-1359. Beland, G.L., Akeson, W.R. and Manglitz, G.R., 1970. Influence of plant maturity and plant part on nitrate content of the sweetclover weevil-resistant species Melilotus infesta. J. Econ. Entomol., 63: 1037-1039. Borlaug, N.E., 1983. Contribution of conventional plant breeding to food production. Science, 219: 689-693. Campbell, B.C. and Duffey, S.S., 1979. Tomatine and parasitic wasps: Potential incompatibility of plant antibiosis with biological control. Science, 205: 700-702. Chapman, R.F., 1974. The chemical inhibition of feeding by phytophagous insects: a review. Bull. Entomol. Res., 64: 339-363. Cook, A.G., 1976. A critical review of the methodology and interpretation of experiments designed to assay the phagostimulator activity of chemicals to phytophagous insects. In: T. Jermy (Editor), The Host Plant in Relation to Insect Behavior and Reproduction. Plenum Press, New York, pp. 47-54. Dowd, P.F., Smith, C.M. and Sparks, T.C., 1983. Influence of soybean leaf extracts on ester cleavage in cabbage and soybean loopers (Lepodoptera: Noctuidae). J. Econ. Entomol., 76: 700-703. Kennedy, G.G., 1984. 2-tridecanone, tomatoes and Heliothis zea: potential incompatibility of plant antibiosis with insecticidal control. Entomol. Exp. Appl., 35: 305311. Kennedy, J.S., 1977. Behaviorally discriminating assays of attractants and repellents. In: H.H. Shorey and J.J. McKelvey (Editors), Chemical Control of Insect Behavior. Wiley, London, pp. 215-229.
7 Klun, J.A. and Robinson, J.F., 1969. Concentration of two 1,4-benzoxazinones in dent corn at various stages of development of the plant and its relation to resiShCe of the host plant to the European corn borer. J. Econ. Entomol., 62: 214-226. Kogan, M. and Paxton, J., 1983. Natural inducers of plant resistance to insects. In: P.A. Hedin (Editor), Plant Resistance to Insects. American Chemical Society, Washington, DC, pp. 154-171. Maxwell, F.F., 1977. Host-plant resistance to insect - chemical relationships. In: H.H. Shorey and J.J. McKelvey, Jr., (Editors), Chemical Control of Insect Behavior. Wiley, New York, pp. 299-304. Reynolds, G.W. and Smith, C.M., 1985. Effects of leaf position, leaf wounding and plant age of two soybean genotypes on soybean looper (Lepidoptera: Noctuidae) growth. Environ. Entomol., 14: 475-478. Schalk, J.M. and Stoner, A.K., 1976. A bioassay differentiates resistance to the Colorado potato beetle and tomatoes. J. Am. Sot. Hortic. Sci., 101: 74-76. Shade, R.E., Thompson, T.E. and Campbell, W.R., 1975. An alfalfa weevil larval mechanism detected in Medicago. J. Econ. Entomol., 68: 399-404. Smith, C.M., 1978. Factors for consideration in designing short-term insect-host plant bioassays. Bull. Entomol. Sot. Am., 24: 393-395. Smith, C.M., 1985. Expression, mechanisms and chemistry of resistance in soybean, Glycine max L. (Merr.) to the soybean looper, Pseudoplusia includens (Walker). Insect Sci. Appl., 6: 243-248. Smith, C.M., Brim, C.A. and Wilson, R.F., 1979. Feeding behavior of Mexican bean beetle on leaf extracts of resistant and susceptible soybean genotypes. J. Econ. Entomol., 72: 374-377. Thompson, A.C., Baker, D.N., Gueldner, R.C. and Hedin, P.A., 1971. Identification and quantitative analysis of volatile substances emitted by maturing cotton in the field. Plant Physiol., 48: 50-52. Tingey, W.M. and Singh, S.R., 1980. Environmental factors influencing the magnitude and expression of resistance. In: F.G. Maxwell and P.R. Jennings (Editors), Breeding Plants Resistant to Insects. Wiley, New York, pp. 87-113. Walsh, J., 1981. Germplasm resources are losing ground. Science, 214: 421-423. Westphal, E., Bonner, R. and LeRet, M., 1981. Changes in leaves of susceptible and resistant Solanum dulcamara infested by the gall mite Eriophyes cladophthirus (Acarino, Eriophyoidea). Can. J. Bot., 59: 875-882. Woodhead, S. and Bernays, E.A., 1977. Changes in release of cyanide in relation to palatability of sorghum to insects. Nature, 270: 235-236.
TRENDS AFFECTING RESEARCH RESISTANCE TO INSECTS’
C. MICHAEL
STRATEGIES
1
IN PLANT
SMITH
Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803 (U.S.A.) (Accepted
for publication
27 November
1985)
ABSTRACT Smith, C.M., 1986. Trends affecting research Agric. Ecosystems Environ., 18: l-7.
strategies
in plant
resistance
to insects.
Varieties of crop plants showing resistance to insect pests have been in use in modern agricultural production systems for nearly 40 years. The need for their continued use is especially critical in developing countries today, since both food and food production costs have increased greatly. The expanding populations of some developing countries currently endanger the habitats of the germplasm of several major crop plants. For this reason, major efforts are now necessary to collect, preserve and maintain as much wild germplasm of major crop plants as possible. These efforts may lead to the exchange of advanced biotechnology techniques from developed countries for exotic crop plant germplasm from developing countries. Many varieties of insect-resistant crop plants rely on allelochemicals to convey antibiotic effects on pest insects. However, high levels of allelochemicals can have detrimental effects on beneficial insects, and induce insecticide tolerance in non-target insect pests. Therefore, a need for caution exists in the development of crop varieties which utilize allelochemicals in their resistance. Accurate, reproducible bioassay techniques are essential to the continued development of insect-resistant varieties. This is especially important today, since groups of scientists at different locations may be involved cooperatively in identifying and categorizing plant resistance to insects. Insect, plant and environmental variables should be standardized, since each can greatly affect the validity of bioassay results.
INTRODUCTION
In order to feed the predicted world population, world food production must double by the beginning of the 21st century. We need look no further than the drought ravaged areas of famine in sub-Saharan Africa for graphic evidence of the need for increased agricultural productivity. In developing countries, crops which are resistant to insects must obviously play an integral role in solving these types of food production problems, since the capital ‘Presented at a symposium on Plant Resistance to Insects: Research Strategies for the 21st Century. Annual Meeting of the Entomological Society of America, 9-13 December 1984, San Antonio, Texas. 0167-8809/86/$03.50
0 1986 Elsevier
Science
Publishers
B.V.
West Africa
Europe Greece Y:zz
Fig. I. Approximate percentage countries in the world,
of net income
spent
on food
by populations
of various
for chemical insect control is normally spent on the cost of food itself (Fig. 1). The following discussion is aimed at illustrating some of the trends which will affect research strategies to develop insect-resistant crop plants in the future. FROM THE GREEN
REVOLUTION
TO THE GENE REVOLUTION
Just as the green revolution brought high yielding varieties to tropical Asia and Latin America in the 1960’s, current research in biotechnology suggests that we are on the verge of a “genetic revolution” which will improve crop productivity by increased stress-resistance to insect-, diseaseand environmental-stresses. In the mean time, a fundamental need exists to maintain varietal selection and development by conventional plant breeding. As Borlaug (1983) stated, “there are no assurances that insect-resistant varieties developed by genetic engineering will be more long-lasting than those developed to date”. Research efforts should also continue to collect, preserve and maintain germplasm of the major world food crops with as much genetic diversity as possible. These efforts are currently in serious jeopardy due to the slash and bum agricultural practices in much of the humid tropics of the world, and to population expansion in areas such as the Brazilian Amazon basin (Walsh, 1981). There is currently concern by some members of the International Board of Plant Germplasm Resources that rice and wheat collections may be especially inadequate in their content of wild species. At present, wild genes from land races and exotic crop plant species are collected mainly by germplasm centers in developing countries, and new genes or genetic expression are being sought in the genetic engineering laboratories in many more technologically advanced countries. As the concept of a world economy rapidly becomes a reality, more emphasis should be placed on the exchange of germplasm for genetic engineering technology to accomplish future world food production goals.
3
RATIONAL DEVELOPMENT OF ALLELOCHEMICALLY-BASED PLANT RESISTANCE TO INSECTS Research involving chemically-based plant resistance to insects has increased dramatically in the past 10 years, with interest focusing on which chemicals make resistant varieties resistant and which make susceptible varieties susceptible (Fig. 2). From a biogenetic standpoint, different types of natural products (alkaloids, phenolics and terpenes) have all been shown to have allelochemical activities (Table I). In some cases, selection has been for high levels of allelochemical in resistant varieties. Recent research, however, suggests the need for a more moderate approach to the integration of plant allelochemicals with existing chemical and biological control measures. In their work with tomato cultivars with high levels of cu-tomatine, Campbell and Duffey (1979) found that although toxic to Heliothis zea Boddie, cw-tomatine was also toxic to of the parasite Hyposoter exigue, suggesting a need for the development varieties with only moderate e-tomatine content. Numerous examples exist in cotton, potatoes, rice and soybean which indicate that high levels of resistance, whether allelochemically-based or not, reduce pest insect populations to the extent of having negative effects on beneficial insect populations. Kennedy (1984) has recently demonstrated that 2-tridecanone, a toxin of the wild tomato, Lycopersicon hirsutum f. glabratum C.H. Mull, mediates resistance to the tobacco homworm, Manduca sexta (L.), and the Colorado potato beetle, Leptinotarsa decimlineata (Say), but also induces enhanced tolerance to carbaryl in Heliothis zea. At the metabolic level, Dowd et al. (1983) demonstrated that the soybean looper, Pseudoplusia includens (Walker), and the cabbage looper, Trichoplusia ni Hiibner, larvae fed on diets which are rich in phenolics from an insect-resistant Asian IO-
1
I
I
I
Fig. 2. Annual research productivity on plant allelochemics affecting insect behavior, 1960-1980. (A-A) repellents; (o-o) attractants; (*+) feeding stimulants; (0-o) feeding deterrents and growth inhibitors.
4 TABLE I Distribution
of plant chemicals
Attractants Alcohols Alkaloids Amino acids Disulfides Isothiocvanates Phenol& Terpenes
affecting
insect behavior by chemical
(%) 25 3 5 3 5 28 31
n = 61 Feeding stimulants Amino acids Falvonoids Glucosides Nucleic acids Sterols Sugars
(%) 20 20 15 5 15 20
class, 1955-1980
Repellents Amino acids Phenolics Terpenes
(%) 10 45 45
n=9 Feeding deterrents and inhibitors Alkaloids Amino acids Glucoides Phenolics Sterols - Sesquiterpene lactones Tannins Terpenes
(%) 24 5 7 10 2 16 1 35
n = 119
n = 54
soybean variety have opposite hydrolytic esterase levels. Activity in P. includens is depressed, indicating a partial resistance effect via allelochemicals, but activity in 2’. ni is enhanced, suggesting the possibility of increased tolerance to pesticides degraded by hydrolytic esterases. Both findings indicate the negative effects of high levels of chemically-based insect resistance which are possible in crop management systems, and demonstrate the need for caution in varietal development. DEVELOPMENT
AND USE OF ACCURATE
BIOASSAYS
Biological variation in the test insect and plant material can affect the outcome and reproducibility of plant-resistance bioassays. Numerous articles (Chapman, 1974; Cook, 1976; Kennedy, 1977; Maxwell, 1977; Smith, 1978) have pointed out the need for the refinement of insect-plant bioassay methodology and technique. It is important, therefore, to recognize and understand the ability of each of these variables to influence a bioassay of plant resistance to insects. In order to conduct cooperative research between groups of scientists at several locations, it is also important to attempt to standardize each of these variables to ensure comparable results. The age, sex and peak activity-period (feeding or oviposition) of the test insect, and how the insect is handled prior to testing all affect the outcome of the bioassay. In several instances, female insects have been shown to feed more on both resistant and susceptible plant foliage than males. Such
3
is the case with the Colorado potato beetle, Leptinotursu decemlineatu (Say) @chalk and Stoner, 1976), and the Mexican bean beetle, Epilachna vurivestis Mulsant (Smith et al., 1979). The effects of environmental factors on the expression of plant resistance to insects have been reviewed by Tingey and Singh (1980). As such, temperature, relative humidity, light intensity, soil fertility and soil moisture have all been shown to affect the expression of insect resistance in plants. Several plant variables affect the validity of bioassay data. In addition to obvious factors such as the plant part being bioassayed and the concentration of biologically active materials in that plant, plant age is a major factor in the study of insect resistance. Resistance may either increase or decrease in older plant tissues, depending on the presence or absence of the chemicals mediating resistance. Thus, in the leaves of mature plants, resistance increases in soybean (Reynolds and Smith, 1985), sweetclover (Beland et al., 1970) and tobacco (Abernathy and Thurston, 1969), but decreases in corn (Klun and Robinson, 1969) and sorghum (Woodhead and Bernays, 1977). Natural products which impart insect resistance can be related to the photosynthetic process in plants and, as a result of this, can vary greatly in content during the course of one 24-h period. Shade et al. (1975) demonstrated that the fluid droplet output from trichomes of a Medicugo species which is resistant to alfalfa weevil, Hyperu posticu Gyllenhal, larvae was greatest during mid-day periods of peak photosynthesis. Thompson et al. (1971) determined that cotton plant volatile concentration was greatest during late afternoon hours. From a practical standpoint, the most efficient collection of plant volatiles or plant foliage for phytochemical analysis should be conducted during the peak production period. Kogan and Paxton (1983) cite numerous instances of increased levels of plant resistance to insects which are induced by insect feeding. In several cases, the concentration of plant phenolic compounds increases, in what appears to be a general plant defensive response similar to plant response to pathogen attack. The response can occur in as little as 4 h (Westphal et al., 1981), and can occur in both insect-resistant and insect-susceptible plant varieties (Smith, 1985). It is very important, therefore, that prior feeding damage or wounding to test plants be determined before attempting comparisons of insect resistance evaluations from different locations. SUMMARY
Until biotechnological methods are more fully developed for application in agricultural production systems, conventional plant breeding will continue to be the primary method for production of insect-resistant crop plants. This method may be adversely affected, however, by current human population expansion into the native habitats of wild crop plant germplasm. One method of avoiding this problem is the continued enhancement
of germplasm collections at International Agricultural Research Centers throughout the world. Facilities for receiving, processing and storing germplasm accessions exist at these centers, and efforts to collect wild germplasm should receive additional support. The effects of plant allelochemicals on beneficial insects and secondary pests, cited previously, are similar to the monogenic vertical insect resistance effects of some varieties of wheat, rice and sorghum. In either instance, high levels of antibiosis resistance can or has eliminated large segments of the insect pest population, but has also prompted the development of resistance-breaking biotypes. Where possible, future plant breeding efforts should attempt to include tolerance resistance to lessen the chance for biotype development. Many research programs which are aimed at identifying insect resistance and incorporating it into crop plants spend a great deal of time and resources developing methods and techniques to measure resistance. Variable factors in test plants and insects, in addition to environmental factors, all have very real effects on determining the occurrence and degree of insect resistance in plants. Careful documentation of the effects of each of these factors will allow the development of baseline conditions for plant evaluations of insect resistance. Determination of such baseline conditions will greatly expedite progress in developing insect-resistant crop plants and will make valid comparisons between different research locations possible.
REFERENCES Abernathy, C.O. and Thurston, R., 1969. Plant age in relation to the resistance of Nicotianato the green peach aphid. J. Econ. Entomol., 62: 1356-1359. Beland, G.L., Akeson, W.R. and Manglitz, G.R., 1970. Influence of plant maturity and plant part on nitrate content of the sweetclover weevil-resistant species Melilotus infesta. J. Econ. Entomol., 63: 1037-1039. Borlaug, N.E., 1983. Contribution of conventional plant breeding to food production. Science, 219: 689-693. Campbell, B.C. and Duffey, S.S., 1979. Tomatine and parasitic wasps: Potential incompatibility of plant antibiosis with biological control. Science, 205: 700-702. Chapman, R.F., 1974. The chemical inhibition of feeding by phytophagous insects: a review. Bull. Entomol. Res., 64: 339-363. Cook, A.G., 1976. A critical review of the methodology and interpretation of experiments designed to assay the phagostimulator activity of chemicals to phytophagous insects. In: T. Jermy (Editor), The Host Plant in Relation to Insect Behavior and Reproduction. Plenum Press, New York, pp. 47-54. Dowd, P.F., Smith, C.M. and Sparks, T.C., 1983. Influence of soybean leaf extracts on ester cleavage in cabbage and soybean loopers (Lepodoptera: Noctuidae). J. Econ. Entomol., 76: 700-703. Kennedy, G.G., 1984. 2-tridecanone, tomatoes and Heliothis zea: potential incompatibility of plant antibiosis with insecticidal control. Entomol. Exp. Appl., 35: 305311. Kennedy, J.S., 1977. Behaviorally discriminating assays of attractants and repellents. In: H.H. Shorey and J.J. McKelvey (Editors), Chemical Control of Insect Behavior. Wiley, London, pp. 215-229.
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