The pecan weevil is a key pest of pecans. In this project, the potential to control pecan weevil using entomopathogenic (insect-killing) fungi was investigated. In laboratory and field studies the following factors were found to affect efficacy: fungus strain or species, application method, and formulation. The most promising treatments consisted of trunk sprays including a novel UV-protecting formulation, and fungus-impregnated cloth bands wrapped around the tree trunk; these treatments produced approximately 80% mortality in emerging pecan weevils. The results indicate that using fungus as a natural environmentally sound measure for pecan weevil control is promising.
Our overall goal is to reduce reliance on chemical insecticides by incorporating biological control into a sustainable pest management program for C. caryae. Specific objectives are to:
1. Enhance persistence of C. caryae control with B. bassiana by one or more of the following:
A. Selecting superior B. bassiana strains.
B. Addition of soil amendments to improve B. bassiana persistence.
C. Developing improved formulations.
2. Determine the optimum area of B. bassiana application
3. Conduct feasibility and economic analyses to determine the potential of incorporating B.
bassiana into an integrated pest management system.
4. Extend the project findings to grower clientele.
Pecan, Carya illinoensis, is the most valuable North American native nut crop (Browne 1985). There are over 492,000 acres of managed pecans in the US (Wood 2001). Major pecan production occurs in the southeastern US (AL, AR, GA, FL, LA, MS, NC, and SC) as well as the southwest and parts of the mid-west (AZ, CA, KS, NM, OK, and TX). Total pecan production value in the US over the past three years (1999-2001) averaged approximately 260 million dollars (www.usda.gov/nass/pubs/).
Insects and mites can cause severe crop losses in pecan (Dutcher 1991). Of major concern is the pecan weevil, Curculio caryae, which may cause up to 90% damage resulting in over 75% crop losses (Dutcher 1991). Due to the seriousness of this pest, and its potential to establish in non-endemic areas, (e.g., it was established in New Mexico orchards in 2000 & 2001), C. caryae may be considered a major threat to pecan growers throughout the US. The insects have a two or three-year life-cycle (Harris 1985). Most adults emerge from soil over a 4-6 week period beginning in late July to early August, feed on developing nuts, and oviposit into the nuts after the dough stage (Harris 1985). Larvae develop within the nut and fourth instars drop to the soil, burrow to a depth of 8-25 cm and form an over-wintering cell. The following year 90% of the larvae pupate and continue to spend the next 9 months in the soil as adults (Harris 1985). The remaining 10% of the population spend 2 years in the soil as larvae emerging as adults in the third year (Harris 1985).
Control recommendations for C. caryae currently consist solely of aboveground applications of chemical insecticides (e.g., carbaryl) to suppress adults (Harris 1999; Hudson et al. 2002). Late season applications of carbaryl, however, can result in resurgence of damaging aphid populations because carbaryl suppresses certain aphid predators (e.g., coccinellids) but does not suppress the pecan aphid complex (Dutcher and Payne 1985). Due to problems associated with aphid resurgence, as well as other environmental and regulatory concerns (Cohen 2000), research on developing alternative control strategies is warranted.
A highly promising alternative for management of C. caryae is the entomopathogenic hyphomycete fungus group, particularly B. bassiana (Gottwald and Tedders 1983; Tedders 1985; Harrison and Gardner 1991; Harrison et al. 1993; Fuxa et al. 1998; Shapiro-Ilan et al. 2003; 2004). Fungi in the class Hyphomycetes (e.g., B. bassiana) invade the insect host through the cuticle, replicate within the host’s hemocoel, and form external conidiophores to disperse their spores (Tanada and Kaya 1993). Beauveria bassiana can be applied using diverse standard agricultural spray equipment (Goettel et al. 2000).
The promising nature of B. bassiana as a C. caryae control agent, has led to several investigations concerning its ecology and factors affecting efficacy. The fungus is naturally widely distributed in pecan orchards (Harrison and Gardner 1991; Shapiro-Ilan et al. 2003). Transmission of B. bassiana from infected C. caryae adult to healthy adult or larvae to larvae has been demonstrated (Gottwald and Tedders 1983, 1984). Pesticides, particularly fungicides, may suppress the ability of B. bassiana and other entomopathogenic fungi to control C. caryae (Tedders 1981; Harrison and Gardner 1992; Jaros-Su et al. 1999). However, Shapiro-Ilan et al. (2002) demonstrated that harmful pesticide effects on B. bassiana can be overcome through artificial selection or isolation of naturally resistant strains. Based on environmental constraints of the fungus (Fuxa 1987), higher field efficacy against the adult weevil than against larvae (Gottwald and Tedders 1983; Harrison et al. 1993), and the biology of the weevil (its relatively short adult emergence period compared to larval drop, Boethel and Eikenbary 1979; Harris and Ring 1979), the most promising approach for fungal control of C. caryae is to target adult weevils as they emerge from soil.
Statement of Problem
Pecan, Carya illinoensis Wang., K. Koch, is the most valuable North American native nut crop. The two largest producing states, Georgia and Texas, account for 50% or more of the total pecan production in the US (www.usda.gov/nass/pubs/). The total value of the US crop over the past 3 years (1999-2001) averaged approximately $260 million per year (www.usda.gov/nass/pubs/).
The pecan weevil, Curculio caryae (Horn), is a key pest of pecans throughout the Southeast, OK, KS, and TX (Harris 1999). Control recommendations consist solely of applications of chemical insecticides (e.g., carbaryl) to suppress adults (Harris 1999; Hudson et al. 2002). Chemical insecticides may promote secondary pest outbreaks, accelerate the development of resistance, destroy natural enemies, and create hazards for humans and the environment (Luckman and Metcalf 1982; Dutcher and Payne 1985). As a result, US regulatory agencies have imposed restrictions on chemical pesticide usage, e.g., the Food Quality and Protection Act has led to accelerated restrictions and removal of chemical insecticides (Cohen 2000); thus, the need for alternative pest control measures has increased.
Biological control using predators, parasitoids, or pathogens, can be an effective alternative for arthropod pest management (Stehr 1982; Tanada and Kaya 1993). In contrast to chemical insecticides, biological control agents are generally not harmful to humans or the environment, and have minimal or negligible potential to cause resistance or harm nontarget organisms (Coppel and Mertins 1977). Although predators and parasitoids provide substantial natural control of C. caryae, research toward developing these organisms in biocontrol programs has been negligible (Tedders 1985). In contrast, research on developing microbial control agents as alternatives to chemical insecticides for C. caryae has been significant (Fuxa et al. 1998). The most studied and most promising, microbial agent for C. caryae control to date is Beauveria bassiana (Balsamo) Vuillemin (Gottwald and Tedders 1983; Tedders 1985; Sikorowski 1985; Harrison and Gardner 1991; Harrison et al. 1993; Fuxa et al. 1998; Shapiro-Ilan 2003).
Prior research (Gottwald and Tedders 1983), and our own (Shapiro-Ilan et al. 2003; 2004), has indicated that high levels of adult C. caryae mortality (e.g., >90%) can result following soil applications of B. bassiana around the tree. Two critical hurdles that must be overcome before this approach can become a viable management tactic for C. caryae are (1) extending the persistence of control, and (2) optimization of fungus application.
1. Laboratory experiments-Screening fungal strains. Multiple strains of two pathogenic fungus species (B. bassiana and Metarhizium anisopliae) were screened for virulence and persistence based on procedures described by Shapiro-Ilan et al. (2003). Briefly, experimental units consisted of 30 ml plastic cups filled with oven dried soil from the USDA-ARS pecan orchard in Byron GA (sand: silt:clay = 84:10:6, pH = 6.1, organic matter = 2.8%) and a final moisture level of field capacity (14%). Approximately 10,000 viable conidia in aqueous suspension were applied to soil in each cup (viability was assessed and standardized prior to application). One pecan weevil was added per cup. There were a minimum of 3 replicates of 7-10 weevils per treatment and each experiment was repeated at least once. Weevil mortality was assessed 10-14 days post-application. Two promising strains of fungi were the subject of additional studies under field conditions: Beauveria bassiana GHA strain, and Metarhizium anisopliae F52 strain. Also, to some extent B. bassiana BBMS1 was also tested in the field. Additionally, laboratory experiments were conducted to determine the affect of soil amendments (e.g., compost, manure et) on fungal persistence and virulence. The amendment deemed most promising (composted peanut hulls) was tested further in field experiments.
2. Field experiments. A variety of application approaches were tested under field conditions in pecan orchards 2004-2006. For B. bassiana GHA strain treatments included a ground applications (bare ground), ground applications with a cover crop (Sudan grass), ground application with cultivation, application directly to the trunk, and trunk applications with a UV-protecting adjuvant. 2004 trials also included an additional B. bassiana strain, i.e., BBMS1. For M. anisopliae trials treatments included a cloth band containing the fungus stapled onto the tree trunk, a ground application, and a combined trunk band and ground application. All experiments also contained a non-treated control. The compost amendment was tested in 2006. All experiments were conducted on USDA-ARS research station in Byron, GA except in 2006 three Commercial pecan grower fields were also included (two in Georgia and one in Texas). Field experiment were based on procedures described by Shapiro-Ilan et al. 2004. Naturally occurring weevils were captured in Circle traps and subsequently assessed for fungal infection. All experiments were conducted in randomized block designs; the fungal applications rate was 5 x 1012 conidia per tree (applied in a 4 m radius for ground applications).
Laboratory data indicated statistical differences among fungal strains and species (Fig. 1). Based on virulence, culture ability, and commercial availability B. bassiana GHA strain and M. anisopliae F52 strain were chosen for research in field experiments.
In field experiments focusing on B. bassiana GHA strain, in 2005 and 2006, experiments on USDA-ARS Byron station indicated that all fungal treatments showed significant weevil suppression relative to the control; weevil mortality reached 80% over 10-14 day periods (Fig. 2). In 2006, when analyzed by sample date, some evidence indicated trunk applications were superior to ground applications (Fig. 3).
In field experiments focusing on M. anisopliae application 2006 results indicated the trunk band method caused significantly greater weevil mortality than the control whereas direct ground application with or without compost amendment failed to cause a significant effect (Fig. 4). 2005 results also indicated a significant effect of the band application approach but only at 15 d post-application (data not shown).
In grower trials (2006), the trunk application approach with B. bassiana caused 90-100% suppression (in 2 GA locations), yet the M. anisopliae treatment did not provide significant control (tested in 1 location) (Fig. 5). The grower trial in Texas failed showed variable effects of the fungus and possibly due to low weevil counts and poor replication.
Overall, the results indicate that using fungus as a natural environmentally sound measure for pecan weevil control is promising. Particularly promising is the trunk spray approach, but also the trunk band (cloth impregnated with fungus) appears to have potential.
Educational & Outreach Activities
Four presentations were made and 2 manuscripts (non-refereed proceedings) were published.
1) Shapiro-Ilan, D. I., Cottrell, T. Gardner, W. and Wood, B. W. Biologically-based insecticides for pecan pest management. Western Pecan Growers Association Meeting, 2005, Las Cruces New Mexico.
2) Shapiro-Ilan, Cottrell, Gardner, Behle, & Wood. Using fungus for biological control of pecan pests. Georgia Pecan Growers Association Annual Meeting. May 4, 2006, Perry, GA.
3) Shapiro-Ilan, Cottrell, Gardner, Behle, Nyczepir, & Wood. Alternative Control Tactics in Pecan. Southeastern Pecan Growers Association Annual Meeting. 2006, Panama City, FL.
4) Shapiro-Ilan, D. I. Fungus for control of control of pecan pests. Georgia Pecan Growers Fall Field Day, September 2006, Byron, GA.
Shapiro-Ilan, D. I., Cottrell, T. Gardner, W. and Wood, B. W. Biologically-based insecticides for pecan pest management. Western Pecan Growers Association Proceedings 39: 4-12. 2005. (Proceedings)
Shapiro-Ilan, D.I., Cottrell, T.E., Gardner, W.A., Behle, R.W., Nycepir, A.P. and Wood, B.W. 2006. Alternative pest control tactics in pecan. Southeastern Pecan Growers Meeting Proceedings 99: 86-91.
Additionally: We anticipate substantial impact yet to come. We anticipate 2 (possibly 3) refereed scientific manuscripts to be published based upon the data collected in this project. Furthermore, several additional grower events will continue to extend the information we have collected (e.g., a scheduled presentation at Texas Pecan Growers Association Meeting this coming Summer and the Georgia Pecan Growers meeting).
We have found that both B. bassiana and M. anisopliae, when applied in novel fashions, can cause high levels of pecan weevil control. The results have been presented at Grower meetings and in Grower publications with more to come.
Based on a 40.00 per A treatment application, the approach seems viable for organic and other pecan growers. More complete economic analysis is pending and will be delved into in future studies.
Several farmers have inquired about using the fungus. There is a real interest in organic pecan production and it is these farmers that have shown the most interest. We recommend conducting some more in-depth large plot studies to measure level of crop protection before growers adopt the technology. We anticipate the fungus applications will be incorporated as part of sustainable IPM programs in grower orchards.
Areas needing additional study
Our research shows clearly that fungal applications can cause high levels of pecan weevil mortality under field conditions. The next step is to conduct large plot studies and determine the level of crop protection (yield). Assuming the level of protection is positive, at that point farmers can begin to adopt the technology.