Adjusting Crop Management Recommendations for Insect-Resistant Crop Varieties'

Transcription

1 Adjusting Crop Management Recommendations for Insect-Resistant Crop Varieties' George L. Teetes Department ofentomojogy Texas A&M University College Station TX USA J. AgTic. Enlomol. 11(3): (July 1994) ABSTRACT Insectresistant crop varieties can be an important component of an integrated pest management program. To achieve use in production agriculture however is more difficult than might be expected. The process of deployment after development of the insect resistant eullivar by research entomologists and plant breeders involves Extension Service cooperation and industry commitment. Education of professionals and growers relative to the role inscct l'esistant cultivars play in integrated pest management is key to successful deployment and continued use. Similarly the need to adjust crop management recommendations for insect-resistant crop varieties requires changes in tradjtional information made available to growers_ For example information on economic threshold levels based on economic injury level research data is required when making insecticide-use decisions to control insect pests on resistant and susceptible cultivars_ Insecticide use requirements for control of insect pests on resistant cultivars likewise often change. The information required and adjustments to crop management recommendations must be such that there is confidence in the use of insect-resistant cultivars. Examples of adjustments to management recommendations are presented using insect-resistant sorghums as a case history. KEY WORDS Plant resistance sorghum integrated pest management greenbug Schizaphis gramin.unl Homoptera Aphididac sorghum midge ContariTlia sorghicola Diptera Cecidomyiidae. [nsect-resistant crop varieties are of value in production agriculture only when they are used. To achieve use of an insect-resistant cultivar in production agriculture is difficult as growers are often reluctant to change and use innovations especially if they lack familiarity with the innovation and perceive unacceptable risk. The process of deployment after development of an insectresistant cultivar by research entomologists and plant breeders involves Extension Service cooperation and pl"ivate industry commitment. Education on the role insecl-resistant cultivars play in integrated pest management is key to deployment by professional crop protection specialists and to successful and continued use by growers. Similarly adjusting crop management recommendations for the use of insect-resistant cultivars requires that I Heceived for pub1iction 23 JUlle 1993; llccepted 12 November

2 192 J. Agric. Entomol. Vol. 11 No.3 (1994) additional information be made available to growers and to those who provide technical assistance to growers. The reluctance of growers to use insectresistant varieties is most often due to lack of trust or confidence as growers arc risk-avoiders. Those involved in the science of plant resistance to insects believe that using insect-resistant cultivars helps avoid the risk but growers arc reluctant to accept this. Instead growers use insecticides to avoid risk. Despite economic social and political pressure to reduce insecticide use and implement more non-chemical bio-intensive management strategies many growers rely on insecticides to lessen the risk. No claim is made to knowledge of why brr'owers make the decisions they do in protecting their crops from insect pests. The purpose of this article is not to try to explain why growers make the decisions they make but to try to relate experiences on how crop management recommendations need to change when an insect-resistant cultivar becomes available. Examples are from research on insect-resistant sorghums and from working with Extension Service and industry personnel to encourage growers to use resistant cultivars. Painter (951) knew that insect-resistant cultivars would seldom function alone to provide adequate insect pest control. He described insect-resistant cultivars as an adjunct or component of a strategy that encompassed the use of multiple management tactics. It is the role insect-resistant cultivars play in integrated pest management that makes this topic of adjusting crop management recommendations so critical. Insectresistant cultivars as a component or control tactic of integrated pest management function in several ways to suppress insect pest damage to crops. Insect-resistant cultivars can act to reduce insect pest numbers because of nonpreference or antibiosis or to elevate plants' damage tolerance level because of a tolerance mechanism. In addition using insect-resistant crops allows natural and biological control to function more effectively and dependably and improves the efficacy of cultural management tactics and insecticide applications. An important advantage to the use of insect-resistant cultivars as a component in integrated pest management is that resistant cultivars arc compatible with other direct control tactics. Insect-resistant cultivars have a distinct advantage over biological control meaning the use of natural enemies to suppress the abundance of an insect pest because plant resistance to insects is compatible with insecticide use whereas biological control usually is not. However insect resistant cultivars must be used like other non-chemical control tactics especially cultural controls. Insect-resistant cultivars can prevent development of damaging insect infestations but resistant cultivars cannot cure or prevent insect damage when insect abundance is close to the damaging level. The major advantage in using insecticides is that they have curative capability. Because resistant cultivars are used to prevent insect pest damage to plants insect-resistant cultivars must be part of the agriculture production planning process. To illustrate this point growers in Texas need to decide in December what crop varieties to use the next season. It is interesting to wonder how many growers consider in advance of the growing season all the non-chemical control tactics including the use of insect-resistant varieties that they need to use to prevent insect pest damage.

3 TEETES: Adjustments for Insect-Resistant Cultivars 193 A matter related to this topic of grower acceptance of the use of insectresistant cultivars is the attitude of the seed industry that develops and markets resistant varieties. Insect-resistant varieties or inbred lines developed by public institutions are released to private industries for seed increase and sale. Seed companies usually use the novel characteristics of a variety to promote sales. This approach elevates the expectations of growers who might be led to believe that if they grow a resistant variety they will encounter no threat from insect pests. However insect resistance in crop varieties is usually not high enough to prevent crop damage when insect infestation levels are high. It is hoped that in the future marketability of a crop variety will be based not on a single appealing attribute such as resistance but on a trait such as the variety's capability of sustaining productivity undel' insect pest stress situations. Because the term resistance conveys different expectations to different people and because growers desire no additional risk the use of economic injury levels to define the level of resistance of a newly released crop cultivar is critical. Growers understand the concept of economic injury and threshold levels because these terms also are used to justify the application of insecticides. One of the first and most important adjustments to crop management recommendations that must be made is to define the economic threshold or action threshold. This approach is in concert with the integrated pest management approach and adds to the security growers demand. The topic of methodology used in determining economic injury levels is important to this subject of adjusting crop management recommendations. The economic injury level is defined as the insect pest abundance or amount of damage that results in economic yield loss. The economic injury level is an objective determinable value. In contrast the economic threshold level an insect abundance level at which remedial control is required to prevent an insect pest that is increasing in abundance from reaching the economic injury level (Stern et a ) is subjective. In most cases the economic threshold action threshold or treatment threshold is lower than the economic injury level to allow time for curative action to be taken. If insect pest abundance is not increasing the economic injury and threshold levels are the same. Currently insecticides are almost always the curative action. Determining the economic injury level requires determination of the relationship between yield loss and insect pest abundance or level of damage. Traditional ways to do this include: 1) observations of natural insect pest infestations which are then related to yield loss 2) modifications in insect pest abundance which is then related to yield loss 3) using artificial infestation levels and relating insect pest abundance to yield loss and 4) simulating damage by mechanical means. Factors that need to be considered in determining density/damage relationships include: 1) time of injury with respect to plant growth 2) part of the plant injured 3) type of injury (direct or indirect) 4) intensity of the injury and 5) environmental eflects on the plant's ability to withstand injury. Adding an insect-resistant cultivar to this agenda doubles the amount of work. In some cases there are several different resistant cultivars of a crop that have different resistance levels. However general or common resistance levels

4 194 J. Agric. Entomo!. Va!. II No.2 (1994) usually occur for which relative differences between resistant and susceptible cultivars can be determined. Experience and empirical data are very valuable in determining the level of resistance ofa cultivar. Most people would expect that insect resistant cultivars have an economic threshold level higher than that of a susceptible cultivar. Often this is true if the resistance is due to tolerance. However if resistance is due to 000 preference or antibiosis that reduce the ability of the insect to reach the economic threshold level but do not necessarily result in a plant that can withstand higher insect pest infestation. the economic threshold level of the resistant cultivar may not be higher than that of the susceptible cultivar. Also changes in the economic injury level depend on whether the economic threshold level is based on insect numbers or on plant damage. The economic threshold often changes when it is based on insect numbers but most often does not change when based on plant damage. It is usually best to base economic threshold levels on plant damage rather than on insect numbers. Plant damage is often more directly related to yield loss than are insect numbers. These relationships are not always linear. Also some insect pests occur in numbers or locations that make them very difficul t to count. In other cases insect damage does not repair itselfwhen the insect pest is removed. Once the relationship between yield loss and insect pest abundance or plant damage is determined crop value and control costs need to be considered. Calculation of the value of the crop multiplied by injury units multiplied by damage per unit injury divided into the value of the crop provides a dynamic economic threshold level. A similar approach is to calculate gain threshold and relate it to the value ofyield lost per insect or per unit damage. The greenbug Schizaphis graminum (Rondani) is a pest that is so abundant that accurately counting these insects is almost impossible. The insect has the capability of causing serious damage and yield loss at almost any plant growth stage from seedling to grain maturity. Consequently it is more advantageous to base infestation severity on plant damage than on insect numbers. A primary issue once greenbug-resistant sorghums were developed was to determine if the plant damage to yield loss relationship was the same for greenbug resistant and susceptible sorghum hybrids. Comparisons between susceptible and resistant cultivars in numbers of greenbugs and leaves killed per plant showed that more greenbugs were required to kill leaves of a resistant hybrid (Fig. 1) (Teetes 1982). Similarly a comparison was made of greenbug number and effects on yield. Results showed that at the same infestation level per plant more green bugs were requjrcd to cause yield loss on a resistant than on a susceptible hybrid (Fig. 2) (Teetes 1982). However the most important comparison between the resistant and susceptible hybrids was of the relationship between the amount of plant damage and yield loss. In this comparison. the leaf damage to yield loss relationship was the same for a greenbug-resistant or susceptible hybrid (Fig. 3) (Teetes 1982). Consequently because the economic threshold for greenbug is based on plant damage the economic threshold level for either a susceptible or resistant cultivar is the same. In most instances. an economic threshold table describing symptoms at different plant growth stages (Table 1) (Fuchs et al. 1988) is used to determine when insecticide treatment is needed.

5 TEETES: Adjustments for Insect Resistant Cultivars :.. B 7.-. SUSCEPTIBLE.---. RESISTANT w 6 " j ;< 5 VI 4 " 0 3 0: w OJ /. ::J z _. '" MEAN NUMBER OF GREEN BUGS PER PlANT Fig. 1. Sorghum leaves killed per plant in relation to greenbug abundance on resistant and susceptible hybrids (Teetes 1982). c " '" 4000.=. w " 3000 >= '\"..-. SUSCEPTIBLE.. _.. RESISTANT ' MEAN NUMBER OF GREENBUGS PER PLANT Fig. 2. Sorghum grain yield pe!' hecta!'e in relation to g!'eenbug abundance on resistant and susceptible hybrids (Teetes 1982).

6 196 J. Agric. Enlomol. Vol. 11 No.3 (1994) :: SUSCEPTIBLE RESISTANT i 4000 s:: ' o j o MEAN NUMBER OF leaves KIUEO PER PLANT Fig. 3. Sorghum grain yield per hectare in relation to number of leaves killed by greenbugs (Teetes 1982). Other important considerations that affect but not necessarily change crop management recommendations are effectiveness and frequency of insecticide application differences and yield differences that occur either in the presence or absence oran insect pest infestation. Determining yield capabilities of insectresistant cultivars is critical. t\.'iany growers falsely believe that in the absence of an insect pest yields of a resistant cu1tivar are always lower than those of a susceptible cultivar. Testing in recognized yield performance trials is important to the education associated with deployment and to grower confidence in the use of insect pest-resistant cultivars. Adjusting management recommendations for sorghum hybrids resistant to sorghum midge Contarinia sorghicola. (Coquillett). requires different considerations than those for greenbug. Greenbug feeding on leaves causes indirect damage. Sorghum midge causes direct damage by larvae feeding on developing kernels that prevents normal grain formation. The economic injury level is based on the abundance of ovipositing sorghum midges when sorghum panicles are in anthesis. The economic threshold level for sorghum midge is based on a non-injurious insect life stage the adult. Basing economic thresholds on non-injurious insect life stages is disadvantageous for many reasons. Also whereas a plant damaged some by greenbugs can recover to produce normal yields after the greenbugs are killed by insecticides sorghum midge damage is permanent. Because the economic threshold level for sorghum midge is based on the abundance of adult sorghum midges and the resistance mechanism is 000

7 TEETES: Adjustments for Insect-Resistant Cultivars 197 Table 1. Insecticide treatment thresholds for greenbug on grain sorghum (Fuchs et al. 1988). Plant size Emergence to about 15 cm Larger plant to boot Boot to heading Heading to hard dough When to treat Visible damage (plants beginning to yellow) with greenbugs on plants. Greenbug colonies causing red spotting or yellowing of leaves and before any entire leaves are killed. At the death of one functional leaf. When greenbug numbers are sufficient to cause death of two normal-sized leaves. preference for adult oviposition the density/ damage relationships for sorghum midge-susceptible and resistant hybrids are not the same. Sorghum rnidgeresistant hybrids sufter fivefold less damage than do susceptible hybrids at the same abundance of adult sorghum midges (Fig. 4) (Hallman et al. 1984). To put it differently the economic threshold level for sorghum midge-resistant hybrids is five times higher than it is for susceptible sorghums (Tables 2. 3) (Fuchs et al. 1988). Although insecticides often need to be applied when sorghum midge abundance exceeds the economic threshold level using resistant sorghum hybrids allows fewer insecticide applications than are required for a susceptible hybrid (Table 4) (Teetes et al. 1986). This saves time money and effort. Normally insecticidal control on sorghum midge-susceptible sorghum is only moderately effective because of the continuous re-infestation by adult sorghum midges the fact that all life stages of the insect except the adult are protected within the spikelet the changing morphology of the sorghum panicle and the short residual properties of cun-ently registered insecticides. Using resistant hybrids and applying insecticides when the economic threshold level is exceeded provides a synergistic effect. These changes in insecticide scheduling require adjusting crop management recommendations when a sorghum midgeresistant cujtivar is used. The economic and environmental benefits of using insect-resistant cultivars in production agriculture must be made known to growers and crop protection specialists. One economic benefit is reduced insecticide use. For example using a sorghum midge-resistant instead of susceptible hybrid saves $ per hectare in sorghum grain protected and insecticide costs ellminated when insecticides are applied to resistant and susceptible sorghums in amounts required to provide the most practical control of sorghum midge. When insecticides are applied to neither the resistant nor susceptible sorghum the economic advantage of the resistant hybrid is much greater than for the

8 198 J. Agric. Enlomol. Vol. ll. No.3 (1994) Vl Vl 9 w-' z w '" "" 'Z w u '" ẉ a.-. SUSCEPTIBLE.. _.. RESISTANT Ii :/. / / FEMALE SORGHU IOGES PER FLOWERING PANICLE PER OAY Fig. 4. Percent sorghum kernel loss in relation to abundance of sorghum midge adults infesting resistant and susceptible hybrids (modified [l'om Hallman et al ) Table 2. Economic injury levels for sorghum midge-susceptible hybrids (Fuchs et al. 1988). Mean number of adult sorghum midges per panicle Control Market value ($) per hectare cost ($lha)

9 TEETES: Adjustments for Insect Resistant Cultivars 199 Table 3. Economic injury levels for sorghum midge.resistant hybrids (Fuchs et al. 1988). Mean number of adult sorghum midges per panicle Control Market value ($) per hectare cost ($/ha) Table 4 Percent greater yield of a sorghum midge resistant hybrid over that of a susceptible hybrid following identical insecticide treatments (Teetes et a ). % Increase Net over return Treatment susceptible ($/ha) Every day Three times starting at 25% anthesis Twice starting at 50% anthesis Twice starting at 25% anthesis Once starting at 75% anthesis Once starting at 25% anthesis Once starting at 50% anthesis Untreated

10 200 J. Agric. EnlomoL Vol. 11 No.3 (1994) susceptible sorghum hybrid but slightly less than when the resistant hybrid is appropriately treated with insecticide. Similar results occur and economic benefits are derived when a greenbug-resistant sorghum hybrid is used instead of a susceptible hybrid. The economic consequences of insect pest resurgence and the occurrence of secondary insect pest outbreaks are not considered in these calculations. Crop protection specialists and growers should be educated as to the benefits both economic and environmental that can be gained from the use of insect-resistant cultivars and the need to make adjustments to crop management recommendations. Also it is likely that government regulations will pressure the development and use of insect-resistant crop cultivars. Accordingly the information needed to adjust crop management recommendations must be readily available and disseminated. Aeknowledgment I thank Dr. Bonnie B. Pendleton for her helpful comments during preparation of this manuscript. This research was supported in part by grant A. I. D. IDAN-1254-G from the Agency for International Development through the Sorghum/Millet Collaborative Research Support Program (INTSORMIL); and a grant from the Texas Grain Sorghum Producers Board. This paper is approved as TA by the Director Texas Agricultural Experiment Station College Station. References Cited Fuchs T. W. H. A Turney J. G. Thomas & G. L. Teetes Manabring insect and mite pests of Texas sorghum. Texas Agricultural Extension Service Bulletin 1220 College Station 16 pp. Hallman G. J. G. L. Teetes & J. W. Johnson Relationship of sorghum midge (Diptera: Cecidomyiidae) density to damage to resistant and susceptible sorghum hybrids. J. Econ. Entomol. 77: Painter R. H Insect resistance in crop plants. The University Press of Kansas Lawrence 520 pp. Stern V. M. R. F. Smith R. vnn den Bosch & K. S. Hagen The integrated control concept. Hilgardia 29: 8]-1Ol. Teetes G. L Sorghum insect pest management - T pp In L. R. House. L. K. Mughogho & J. M. Peacock (eds.! Sorghum in the eighties Vol. 1 Proceedings lntcrnotional Symposium on SOI ghum Paiancheru A. P. India: ICRISAT 473 pp. Teetes G. L. t M. J. Becerra & G. C. Peterson Sorghum midge (Diptcra: Cecidomyiidae) management with resistant sorghum and insecticide. J. Econ. Entomol. 79: