Pecan Arthropod Management
Text from M. A. Harris and J. A. Jackman. 1991. Pecan Arthropod Management. pp. 6-15. In B. W. Wood and J. A. Payne (eds.). Pecan Husbandry: Challenges and opportunities. First National Pecan Workshop Proceedings. U.S. Dept. of Agriculture, Agriculture Research Service, ARS-96.
Pecan arthropod management practices today are a legacy inherited from our predecessors stretching back in time more than 4,000 years to "when the Sumerians used sulfur compounds to control insects and mites" (Bottrell 1979). Pest control by manipulation of natural enemies, application of chemicals, and the use of physical and cultural methods all originated before the Christian era. Human ingenuity and time have combined to provide us the tools we presently use to minimize the adverse effects pest arthropods can have on food and fiber production and storage. Despite these massive, extensive and creative efforts, we are still routinely plagued by pest arthropods. Events within the past decade involving Medfly, Ceratitus capitata; Africanized honey bee (so-called killer bees), Apis mellifera; and ticks that vector Lyme disease illustrate our continuing vulnerability to pest arthropods.
A prime example in the pecan industry occurred in 1985 when the Federated Pecan Growers Association declared pecan aphids to be the number one pest and noted that the problem was so severe that the entire industry could fail if the aphids could not be dealt with effectively. The primary point here is that pest arthropods consistently pose difficult challenges to human progress throughout our culture and, pecan production, despite technological advances, continues to suffer its share of problems from pest arthropods as well.
The reasons for these continuing difficulties are many, but two factors address the core of the problem. The majority of arthropod pest problems are both complex and dynamic. Complexity means that solutions are hard to come by and a dynamic quality implies today's solution may not solve tomorrows problem. In fact, today's solution may actually cause tomorrow's problem. This is especially true where living organisms are involved. Those capable of surviving changes in their environment confer these capabilities to their offspring thereby allowing just a few survivors of, say, pesticide treatments, to give rise to an entire, epidemic level, population of survivors.
Another kind of complexity is introduced by increased involvement and concern by the public, expressed both through laws and supermarket purchases, in how pest and other problems are solved. The alar controversy is one example and the removal of phosalone from pecan is another. Chemical agriculture will be less dependable in the future for this reason as well.
The resolution of pecan arthropod problems in production agriculture can only be accomplished by developing an understanding of this complex system and by anticipating how that system will be affected by specific solutions. The approach taken by the Pecan Insect Laboratory at Texas A&M University is to compare and contrast the survival mechanisms inherent in the unmanaged pecan with the production requirements placed on the commercial pecan (Harris 1980). This allows us to examine how pecan interacts and survives with arthropods in natural situations and then to focus on how and why particular arthropods achieve pest status in commercial situations. This broad perspective provided useful insights that resulted in effective management approaches in this complex and dynamic system. Although much progress has been made, primarily through the Southern Regional Project on Pecan Arthropods, in which my lab participates, we have only begun to understand the complex and dynamic features of this agroecosystem, and to convert that knowledge to management approaches that result in greater quantities of high quality reasonably priced pecans available to consumers everywhere throughout the year and provide a fair return to the producers. The purpose of this paper is to take a broad view of pecan arthropod management in the context of pecan production and to attempt to anticipate problems and consider options from a wider perspective than is usually taken.
Pecan Defenses Against the Pecan Pest Complex
There are five defense mechanisms whereby a plant species can survive phytophagy and these can be expressed from the subcellular to the individual to the population level of organization. These mechanisms are escape in space, escape in time, confrontation, accommodation, and biological associations (Harris 1980). These defenses allow transmission of superior phenotypes from one generation to the next by natural selection (Grant 1977).
Pecan utilizes all these mechanisms in the natural environment and what little is known has been reviewed elsewhere (Harris 1980, 1988). A principal feature for pecan is masting (synchronous nut production over wide areas at irregular intervals) to escape primary nut feeders by periodic satiation and starvation. Confrontation of pests like pecan phylloxera and pecan scab is a population phenomenon where individual trees may succumb but the diverse population is protected from an epidemic (Browning 1980; Harris 1980). Accommodation of foliage feeders by universal susceptibility to periodic defoliators like walnut caterpillar and to some extent, aphids, preserves the masting cycle for the pecan and natural enemies keep the pest in check most of the time (Harris 1980).
The selection and vegetative propagation of annually productive pecan in large genetically uniform monocultures drastically reduces mechanisms for escape in space and time and eliminates confrontational mechanisms based on population diversity. Protection with broad spectrum pesticides eliminates many favorable biological associations and supplemental fertilization and irrigation improves nutritive quality and extends windows of susceptibility to pests that could otherwise be escaped, confronted or accommodated. Pecan arthropod management programs have largely developed in a reactive capacity without providing input on long range development plans for the industry. Given that a failure to solve pest problems can eliminate a crop from a wide production area, i.e., sugar beets, sugar cane, cotton and sunflower have all been driven from production from some areas due to pests, a production system that does not capitalize on all natural defense mechanisms compatible with commercial production may increasingly risk failure. Greater attention needs to be paid to the limited information available in this area and further work should be supported to identify and capitalize upon risks and rewards using this approach.
Carya illinoensis (Wang) K. Koch is native to North America from the Mississippi River Valley to West Texas and into Mexico (Little 1971). Brison (1974) notes the pecan is the most important horticultural crop native to the United States. The American Indian relied on pecan nuts nature provided for food; but, serious domestication of the tree really began when early settlers thinned wild mixed species stands of trees to leave some pecan and create a pasture understory for cattle (Table 1). The earliest orchards were established by seed followed by development of vegetative reproduction technology in the middle of the last century allowing the "best" varieties to be propagated wherever the trees would grow. This technology and an entrepreneurial spirit resulted in an extensive expansion of the pecan industry into the southeastern U.S. in the early 1900s and to the west of the Pecos in the 1930s and continuing to the present. Conversion from thinned stands of wild pecan to vegetatively propagated orchards in the native range was very slow because nuts produced by wild trees were and are competitive with those from improved varieties.
Native pecan producers are reluctant to remove producing trees, even those a century old or more, and plant an orchard in their place because of 1) the initial expense, 2) the time required to recoup that investment from new production, 3) the change in lifestyle from a cattle/pecan mixed agriculture system to pecan monoculture, and 4) other uncertainties in such a long-lived crop like fluctuating prices and revisions of tax law. Consequently, about 50% of the 300 million lbs. of pecan nuts produced annually in the U.S. come from trees nature planted or grown from seedlings, with the remainder produced primarily from a dozen or so of the 1,000 plus vegetatively propagated "cultivars" noted by Thompson and Young (1985).
These origins provide a diversity of production situations on a large scale that has no parallel in any other crop grown in the U.S. and research opportunities that allows investigation of pecan arthropods from their natural state to intensively managed production agriculture conditions with all manner of intermediate situations available for study as well. Investigation of the complex and dynamic nature of the arthropod/pecan interaction and its impact on production agriculture benefits from this diversity because field situations can be found to actually test hypotheses in pecan whereas only speculation and computer simulation would be possible in most other crops.
The diversity of pecans available today is, however, much less than that at the turn of the century. Native pecans and seedling orchards are not to any appreciable extent being replaced as they die. Slowly but surely the base for commercial production is being shouldered by the "best" of the vegetatively propagated cultivars and this uniformity provides greater opportunities for pests previously kept off-balance in environments where every tree was genetically distinct. The effects of this increasing uniformity were recognized 60 years or more ago as pecan scab became more severe and 20 years or so ago as pecan twig phylloxera was observed to preferentially attack specific cultivars. Resistance "breaks down" is the common observation, but in reality the virulence of the pest "catches up" with the now genetically frozen pecan planted in monoculture. Wheat and other annual crop farmers can deal with this kind of problem by switching varieties (Harris 1980). That solution is of limited value with current pecan technology, and increased narrowing of the germ plasm means similar problems will occur in the future.
The trend towards having more and more of our production dependant upon a narrower and narrower genetic base is a potential time bomb in a production system with a useful life of 100-200 years. A narrowed genetic base unquestionably reduces the ability of the pecan to defend itself against arthropod and pathogen attack and artificial defenses using chemicals, sanitation, etc., are needed to maintain productivity (Harris 1980, 1983, 1988). If diversity can save one treatment at $60/hectare each year and those profits were invested at 6% annually, the compounded return would be $338 thousand after 100 years and $114 million after 200 years. The analogy is admittedly unrealistic because of the unknowns of inflation, taxes, acts of God, war, etc., but is this less visionary than planting a new orchard to the "best" cultivar and one or two pollinators and expecting current budget figures to apply over the same time frame? History shows the narrower genetic base becomes increasingly plagued with pest problems requiring more and more overt management to maintain the same level of production so that today's advantages are often rapidly lost. The inability to rapidly and economically switch cultivars should cause us to ask how much diversity is needed to prevent or delay the need for overt management?
Varietal selection and breeding programs serve as the predominant source of new cultivars. These efforts emphasized precocity, productivity, and marketability of specific selections based on limited evaluations over short time spans compared to the century plus life of orchards planted to them. This is analogous to a football team composed exclusively of wide receivers that can surprise the opposition for a while but ultimately suffers from a lack of balance. Rectifying this weakness will require modifying our selection requirements from the individual archetype to the population archetype that allows improvement while maintaining diversity.
Current management practices vary from treating the pecan as a gift of nature and harvesting the trees when they produce a crop to state-of-the-art systems of high density, carefully selected and pruned cultivars that are irrigated, fertilized and protected with the latest pesticides, and mechanically harvested, shelled and refrigerated in storage until being sold. The gift of nature grower harvests a crop every 2-5 years while the intensive manager expects a crop every year. No single system of arthropod management is compatible with this diversity of production situations. Careful examination of each production situation is needed and an arthropod management plan should be developed that is consistent with the overall production program (Harris 1983, 1985).
Earlier I noted the diversity of geographic areas from the arid climate and basic soils of the west to the humid climate and acid soils of the southeast where pecans are grown. These factors too can affect the options available in pecan arthropod management, particularly in regard to pesticide application using a spray machine, typically an air blast sprayer. Their primary uses are for application of zinc for prevention of rosette, fungicides for disease management, and acaricides and insecticides for arthropod management; and, tank mixes of all three amendments can be used simultaneously to "solve" rosette, disease and arthropod problems in a single application reducing wear and tear costs on equipment and a labor and water savings of up to 66% compared to separate applications. This makes excellent economic sense when all applications are really needed but can result in unnecessary costs or even disasters when one or more of the treatments are not needed. Tank mixing unneeded insecticides or miticides with needed zinc or disease treatments is the greatest single threat to a sound arthropod management program faced by the pecan industry.
Pecan Arthropod Management
The central consideration to a sound pecan arthropod management program is to only undertake a management action if one or more target pests are present in sufficient numbers to seriously threaten to cause economic damage and the action taken will significantly reduce or remove that threat. Following this guideline requires an ability: 1) to identify the pests, 2) to assess population levels, 3) to relate pest density to economic damage, and 4) to be able to take effective management action. Most arthropod research and extension efforts are directly related to these four aspects of pest management.
Fig. 1 shows how the major pests in the pecan arthropod complex relates to rosette, pecan scab and other diseases. This overview illustrates that pest problems responsive to management occur from before budbreak until leaf drop, a period of about 250 days. Fortunately, the problems at a particular location can usually be reduced to a subset of the general profile in Fig. 1 for many reasons. Pests differ in their distribution and intensity from location to location due to geographical barriers and climate. For example, pecan weevils do not occur west of the native range, or in Mexico, or in localized regions from Texas to Georgia, and thus pose no immediate threat to pecan production at these locations. Pecan scab is most intense where significant rainfall and high humidity coincide with rapidly growing pecan leaves and nuts and pathogen sporulation. Thus, pecan scab is of virtually no consequence in the arid west but increases in intensity as one moves east. Rosette is especially severe where pecans growing in basic soils cannot access Zn because it is tied up in such soils. Thus, rosette tends to be most severe in the limestone soils of the west and diminish in severity as soils to the east become more acidic; however, the practice of liming acidic soils to increase N uptake can also tie up Zn so that foliar Zn amendments are needed there for optimum production as well. In short, effective pecan arthropod management demands a thorough understanding of the overall context of pecan production at the specific location, as well as an understanding of the pecan arthropod complex. Extensive literature is available outlining these approaches and resulting programs (see Boethel and Eikenbary 1979; McVay and Ellis 1979; Peeples and Brook 1979; Harris 1983, 1985; Cooper et al. 1982, etc.).
The details of current IPM programs are a result of innumerable large and small changes that continuously occur to maintain profitability of pecan production. This evolution is so much a part of the fabric of production that one must deliberately reflect upon how and why we conduct them in their present form and to anticipate how future problems may be resolved.
The wild pecan produces large crops at irregular intervals and primarily defends itself against the key nut feeders, pecan nut casebearer and pecan weevil, by cycles of starvation and satiation (Harris et al. 1986; Harris 1988). Foliage feeding arthropods rarely remove more than 10% of the leaves during a season (Ring et al. 1985) and severe infestations appear to either be limited to outbreaks on single trees or widespread epidemics that affect all trees more or less equally (Harris 1980). Phylloxera devastatrix is an example of the former and Datana integerrima the latter. Note that neither type of severe foliar infestation interferes with cycles of irregular bearing in wild trees. D. integerrima delays the bearing year and P. devastatrix has virtually no effect since few trees are affected. Apparently, severe pathogen infestations are also primarily limited to single wild trees with similar effect.
The rapid adoption of vegetative reproduction a century ago has resulted in planted orchards where most trees are genetically identical contrasted to wild populations where every tree is genetically distinct. Pruning, fertilization, and irrigation in combination with selection of cultivars with a propensity to bear regular crops ensures nut production every year.
The pecan nut casebearer has been a key pest of pecan throughout the pecan-producing states east of the Pecos causing damage almost every year from at least the early 1900s until the advent of new chemicals and application equipment following WWII. Although it remained a key pest up to the present in most areas west of central Louisiana, reference to damaging populations in the southeastern U.S. virtually disappears after the advent of mechanized chemical agriculture. I believe these differences came about because of the different driving forces at work in these diverse geographical and climatological areas.
Pecan nut casebearer (PNC) was the most important pecan pest in Texas in the late spring at the advent of the post WWII period and even though rosette and pecan scab were also problems, addressing them made little sense to southwestern growers routinely devastated by PNC. B. Hancock (Pecan Horticulturist, Extension, Texas A&M Univ., 1952-present, pers. comm.) recalls how programs to manage PNC were the impetus to also tank mix with zinc sulfate for rosette and later fungicides for pecan scab control in the mid 1950s beginning in the Guadalupe River Valley. Successful PNC control in the southwest became the cornerstone for all other management programs mediated by the new chemical application equipment and chemicals that made it possible.
Southeastern growers, in contrast, were plagued with pecan scab. This pernicious disease thrives best in hot, humid environments on rapidly growing tender pecan leaves from April to June and nutlets from May to August. The massive expanses of 30-50 year old vegetatively propagated pecans with often overlapping canopies and an excess of 30 inches of rainfall from April to September provided ideal epidemic conditions virtually every year that especially frustrated the more progressive growers because most practices that increased yield, like fertilization or zinc amendments, also stimulated and prolonged growth of tender tissue that would be infected with pecan scab. Nuts could only be destroyed once and pecan scab masked other mortality forces by a wide margin. The same breakthroughs in chemistry that produced post WWII insecticides for the southwest for PNC brought fungicides to the southeast for pecan scab (Table 1).
The time window of vulnerability to first summer generation PNC for a given orchard is about 3 weeks during the late April to early June period depending on the exact location of the orchard. Growers in the southwest could provide prophylactic coverage with two insecticide treatments during this period and achieve excellent PNC control. The primary extension scientists working with pecan immediately after WWII were horticulturists and recommending this schedule in the late spring in the southwest also ensured that two zinc treatments for rosette could be applied at little additional cost with a marked benefit in tree vigor.
The time window of vulnerability of pecan scab is dependent upon having a virulent pathogen, a susceptible host and a favorable environment. Southwestern growers in the native range were buffered from pecan scab by a preponderance of genetically diverse trees that prevented pathogen specialization and a more arid climate that produced fewer rains to constitute infection periods. Often, fungicides applied with PNC and zinc treatments prevented disease establishment in the early season and an unfavorable environment combined with a low inoculum provided sufficient protection for the remainder. Producers in the southeast faced a much tougher challenge from pecan scab (Cole 1941). Vegetatively propagated trees severely limited the genetic diversity and allowed the pathogen to specialize on the limited varieties, and heavier and more frequent rainfall significantly extended the window of vulnerability. Enterprising growers began to spray fungicides every 2 to 3 weeks during April to September for scab control and also piggy-backed zinc and other pesticide treatments into this schedule when other pest problems became more evident in the absence of pecan scab (Miller et al. 1982).
Thus, the initial patterns for pecan arthropod management in the two major pecan production regions were established for the post WWII era. Results were dramatic in each region with the heaviest pesticide use occurring in the southeast where the window of vulnerability to pecan scab was so long. The ability to effectively manage PNC in the southwest and pecan scab in the southeast allowed producers to detect other problems that were not as obvious before like rosette, fertilization, and pecan weevil. Pecan weevil became a problem across the entire pecan belt because improved pecan management resulted in healthier more productive trees that produced large crops of sound pecans on a regular basis (Harris et al. 1980). This provided unlimited food for pecan weevil and insect populations soared. The initial answer to this problem was another chemical and today, 2-3 applications of carbaryl at 10-14 day intervals beginning in late August are used for control from West Texas to the Atlantic Coast wherever pecan weevil occurs.
The advent of effective chemical agriculture in pecan occurred later than in many other areas like cotton, corn, dairying and apple. This was primarily due to the lack of effective equipment to move chemicals into the tops of trees 20-30 meters above the ground. The pecan industry inherited chemicals from other agricultural sectors virtually as soon as they were developed, but the first practical and effective machines were airblast sprayers modified from the fruit industry in the 1950s and early 1960s. This only explains in part how the pecan industry escaped the drawbacks of chemical agriculture much longer than other agricultural sectors; namely, arthropods resistant to pesticides, outbreaks of secondary pests, and pollution. The pecan industry is unique in that the first pest reported to be resistant to a pesticide was a fungal pathogen rather than an arthropod. Pecan scab resistant to benomyl was reported in 1975 in Georgia followed by hickory scorch mite to some carbamates and organophosphates in 1979 in Louisiana (see Harris 1983). Dutcher and Htay (1985) reported pecan aphid resistance to pyrethroids in 1985, many decades after reports on similar arthropods on comparable crops had manifested themselves. Interestingly, however, pyrethroid resistance appeared only a few years after this new class of chemical became available for use on pecan and this is a quite respectable interval if we were competing in a race for obtaining resistance.
Arthropod resistance to pesticides in pecan is primarily due to a drastic reduction of genetic diversity of pecan through vegetative propagation that predisposes orchards to increasing problems from pecan scab. This necessitates increased fungicide treatments at shorter intervals using machinery that provides thorough coverage of the foliage. A general intolerance of pest arthropods and a plethora of initially effective insecticides makes them appear "cheap" to add to the tank mix from a short-term perspective.
The cycle of subsistence, exploitation, crisis, disaster and integrated control Dutcher (1981) foresaw as also applying to the pecan industry culminated in 1985 when the Federated Pecan Growers declared the pecan aphids (secondary pests) the most important pest problem facing producers and said in essence that if the disaster was not resolved, the industry itself was in jeopardy. The southwest participated in this disaster as well, piggybacking insecticide on needed zinc and occasional fungicide treatments to the extent that one grower in the Mesilla Valley spent more than $500,000 in one year for aphid control without success. Whereas the disasters of a generation earlier in cotton, corn, apples, etc. (Bottrell 1979), and the formal IPM programs in pecan begun in 1977 (Harris 1985), were insufficient "teachable moments" to move from exploitation to integrated control, thereby skipping the crisis and disaster phases of the cycle, the pecan aphid complex proved to be the vehicle of change that brought many producers to IPM. Mark Twain observed that some matters defy description and must be personally experienced to be appreciated, and gave as his example "carrying a cat home by the tail". Perhaps, despite my earlier optimism (Harris 1983), a catastrophe must also precede adoption of IPM in each commodity.
The disaster phase of the cycle is not over even though much progress has been made in re-establishing reliance on natural enemies for aphid control in most situations. A major root of the problem is still the genetically frozen pecan that allows genetically flexible pests to fine tune their genetic capacities to exploit these massive expanses of fixed hosts so that management becomes increasingly difficult. Novel chemistry and improved equipment have, along with other technologies, kept pace with this evolution. The trend is toward less chemical development and continued reductions in the existing arsenal due both to resistance and for economic reasons, as the loss of phosalone exemplifies. Public concerns with pollution, worker safety and a wholesome food supply also indicate that reliance on chemical agriculture will diminish in the future.
Table 1. An abbreviated history of pecan and factors affecting arthropod management
Prior to 1800 - Native Americans gathered and subsisted on pecans in their season and early explorers and settlers readily adopted them to their diets.
1800 to 1900 - Settlers thinned tree stands in native range leaving pecans and grass for grazing. Seedling orchards established in the southeastern U.S., particularly near the turn of the century. Grafting technology for pecan developed but not heavily implemented until the last decade (Stuckey 1941). Rail transportation results in shipment of nuts to urban markets.
1900 to 1930 - Vegetative reproduction inundates southeastern U.S. with many selected varieties. Land development schemes sold small acreage of subdivided orchards (Littlepage 1913). Bordeaux spray 3-10 times recommended for nursery trees to prevent scab but large orchard trees considered unreachable; also plant resistant trees like Stuart, Schley and Frotscher (Waite 1914). Early harvest, sanitation, burning and Persian insect powder used for insect control (Morris 1912; Quaintance 1914).
1930 to 1940 - Shelling machinery, transportation and consolidations of orchards into economic units increase marketability of pecans. Expanded production of the most popular varieties like Stuart is met with increasing levels of pecan scab on previously resistant varieties (Stuckey 1941). Rosette linked to foliar zinc deficiency and lead arsenate and nicotine sulfate recommended for insect control (Rainey 1960). Spray machinery expensive, labor intensive and rarely employed (Milward 1940).
1940 to 1950 - Tank mixing of nicotine sulfate with needed fungicides recommended as "cheap insurance" (Moznette 1941 a and b). Contract spray services expand with truck-mounted hydraulic sprayers (Milward 1940, Anon. 1941). DDT used for pecan nut casebearer but aphids and mites appeared in epidemic numbers; toxaphene alone or mixed with nicotine sulfate controlled pecan nut casebearer without resurgence of aphids and mites (Rainey 1960).
1950 to 1960 - Airblast speed sprayers become generally available; compared to hydraulic sprayers (Brison 1960), the cheaper speed sprayers allow a single operator to spray the same number of trees with one-fourth the water and still obtain better coverage (Shelton 1960). Effective and economical rosette, arthropod and pathogen control with conventional and newer chemicals resulted. Malathion was adopted for pecan nut casebearer control (Rainey 1960). Chemical management of pecans became widespread.
1960 to 1970 - Cyprex and then Du-Ter replaced Bordeaux for pecan scab control and new carbamates, organophosphates and systemics became available for arthropod control (Denman 1965; Denman and Hancock 1965; Littrell 1983). Mechanization for pesticide application, pecan maintenance, harvesting and processing burgeoned along with the explosion of chemicals and solutions appeared faster than problems. Chemical schedules became routine and screening for efficacy dominated research efforts.
1970 to 1980 - Carbaryl became chemical standard for pecan weevil management, phosalone for other arthropods, Benlate and Du-Ter for pathogens, and NZN or zinc sulphate + uran for rosette. The first case of pesticide resistance in pecan was a pathogen, the causal agent for pecan scab, to Benlate in 1975, followed by an arthropod, hickory scorch mite, resistance to carbamates and organophosphates in 1979. Synthetic pyrethroids, a new class of chemicals for arthropod control, introduced late in the decade. Integrated Pest Management (IPM) philosophy develops and spreads across agriculture due to widespread pesticide resistance by arthropods, secondary pest outbreaks like aphids, mites and leafminers due to broad spectrum pesticides killing natural enemies and societal concerns about environmental pollution. Pecan industry as a whole was buffered from many of these problems because of the surfeit of chemicals for all pests (Dutcher and Payne 1982) and lagging problems of resistance due to remaining management diversity. However, aphids were viewed as a major problem in the El Paso Valley of Texas and increased reliance on natural control by predators and parasites resolved this problem there. Increased attention in pecan was paid to developing economic thresholds of important pests, refining understandings of basic biologies to predict and manage pests, to identify and rely on natural enemies of pests and other IPM strategies. Pecan IPM programs initiated in Alabama, Georgia, Texas and elsewhere late in the decade. Widespread expansion of new pecan plantings of a few varieties occurred in the Southwest and Mexico inside and outside the native range epitomizing the drastic narrowing of genetic diversity in the natural pecan population compared to the cultivated varieties.
1980 to 1990 - Arthropod resistance to pesticides becomes widespread and Federated Pecan Growers (Beshears 1988) declare aphids the most destructive pest in 1985, refuting the contention by Harris (1983) that the pecan industry had adopted IPM without the normal cycle of subsistence, exploitation, crisis, disaster and finally integrated control (Smith 1969, as quoted by Dutcher 1981). Modeling efforts and basic biological studies on pecan nut casebearer, pecan weevil, hickory shuckworm, pecan aphids, pecan scab and other pests began to be implemented into management programs (Harris 1983; Hudson 1983). Phosalone was withdrawn from the market in 1989 due to the producing companies unwillingness to risk costs of federal re-registration requirements against potential revenue or perhaps refusal of registering the chemical. Implications of the continued transition from the diverse native and seedling trees to increased genetic uniformity of vegetatively propagated varieties on the ability to manage diseases and arthropods became ever more apparent, continuing a trend observed at least half a century earlier.
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