Searching for Pheromone Strains in the Pecan Nut Casebearer Emilie A

DOI: 10.1111/j.1570-7458.2010.01033.x Searching for pheromone strains in the pecan nut casebearer Emilie A. Hartfield, Marvin K. Harris & Raul F.Medina* Department of Entomology, Texas A&M University, 2475 TAMU, College Station, TX 77843, USA Accepted: 3 June 2010 Key words: AFLP, Acrobasis nuxvorella, pecan, sex pheromone, Lepidoptera, Pyralidae, Juglanda- ceae, Carya illinoinensis, monitoring program, genetic isolation, assortative mating, pherotypes Abstract The pecan nut casebearer, Acrobasis nuxvorella Nuenzig (Lepidoptera: Pyralidae), is the most damaging insect pest of pecan, Carya illinoinensis (Wang) K. Koch (Juglandaceae), in the USA and Mex- ico. A pheromone monitoring program for A. nuxvorella has been developed to assist pecan growers in the timing of insecticide applications. The discovery that there are two pheromone types produced by A. nuxvorella has led to complications in the implementation of pheromone monitoring programs. One pheromone (referred to as standard) is attractive to moths in the southern USA, but not in Mexico. The other pheromone (referred to as Mexican) is attractive to moths in the southern USA and in Mexico. Because most male lepidopterans respond only to a specific pheromone, it was suspected that there were two pheromone strains of A. nuxvorella, one exclusively present in the northern distribution of A. nuxvorella (USA strain) and the other widely distributed from Sonora, Chihuahua, and Durango in northern Mexico to Texas, Georgia, and Oklahoma in the USA (Mexican strain). The goal of this research was to determine whether differences in pheromone response are sufficient to genetically isolate A. nuxvorella into pheromone strains. To confirm the existence of the two alleged pheromone strains, amplified fragment length polymorphism (AFLP) markers were obtained and analyzed. Four primer combinations were used to obtain a total of 483 polymorphic AFLP markers. Our results indicated that the standard and Mexican pheromones did not group A. nuxvorella into pheromone strains. However, three genetically distinct populations of A. nuxvorella were identified. Two of those three populations are sympatric throughout the southern USA and one is allopatric relative to the other two and occurs exclusively in pecan growing regions of Mexico. larvae are capable of consuming 3–4 nutlets in a cluster Introduction (Ring & Harris, 1984). The larvae of subsequent genera- The pecan nut casebearer, Acrobasis nuxvorella Nuenzig tions require fewer nutlets to complete development and (Lepidoptera: Pyralidae), is the most damaging pest of thus are less damaging to the crop. Acrobasis nuxvorella is pecan, Carya illinoinensis (Wang) K. Koch (Juglandaceae), monophagous on pecan. in the southern USA and Mexico (Harris et al., 1998). Pecan is indigenous to the alluvial plains of the south- Damage to pecans is a result of A. nuxvorella larvae bur- central USA and mountain valleys of Mexico (Harris, rowing into and feeding on pecan nutlets. Acrobasis nuxvo- 1983). Native pecan in Mexico is geographically isolated rella is mutivoltine, having typically 3–4 generations per from plantings in the USA (Harris et al., 2008). Acrobasis year, depending on climate and geographic location nuxvorella is distributed throughout the pecan growing (Neunzig, 1972). The first-generation larvae are more regions east of the Rocky Mountains in North America destructive to pecan crops due to the small size of the nut- (Figure 1). The range of this pest reaches from Florida west lets on which they are feeding. These first-generation to New Mexico in the USA and as far west as Chihuahua in Mexico (Harris, 1983). Acrobasis nuxvorella has recently expanded to pecan growing regions in Sonora, Mexico (Fu *Correspondence: E-mail: [email protected] Castillo et al., 2005; Harris et al., 2008). Ó 2010 The Authors Entomologia Experimentalis et Applicata 137: 11–18, 2010 Journal compilation Ó 2010 The Netherlands Entomological Society 11 12 Hartfield et al. Legend Pecan native range A. nuxvorella range Collection sites Figure 1 Native pecan distribution, range of Acrobasis nuxvorella, and collection sites. Pecan is native to the alluvial plains of the south- central USA and mountain valleys of Mexico, but has expanded due to cultivation. Acrobasis nuxvorella is present in pecan growing regions in the USA east of the Rocky Mountains and in pecan growing regions in northern Mexico. The current IPM strategy for controlling A. nuxvorella pheromone does not attract A. nuxvorella male moths in consists of monitoring for this pest with sex pheromone Mexico, even though A. nuxvorella is known to be present baited sticky traps and using a degree-day prediction in Mexico (Fu Castillo et al., 2005; Harris et al., 2008). model (Stevenson et al., 2003) combined with a sequential Due to the failure of the standard pheromone to attract sampling plan (Ring et al., 1989) to ensure only needed A. nuxvorella in Mexico, the second A. nuxvorella phero- insecticide treatments are applied at the correct time. mone (henceforth Mexican pheromone) was developed These strategies have been helpful in timing pesticide (Harris et al., 2008). The Mexican pheromone was identi- applications, allowing for more efficient pesticide use. fied as 1:1 (9E,11Z)-hexadecadien-1-yl acetate (9E,11Z- Chemical control against A. nuxvorella, when needed, tar- 16:Ac):(9E,11Z-16:Ald). This pheromone attracts moths gets hatching first instars to prevent nut entry and to kill in the USA and in Mexico. The discovery that there are them before they gain access to a refuge inside the pecan two sex pheromones able to attract A. nuxvorella (Millar nut. The current IPM strategic plan for pecan has signifi- et al., 1996; Harris et al., 2008) has improved the phero- cantly reduced the amount of pesticides used to control A. mone monitoring program currently in place for this pest. nuxvorella compared to when calendar sprays were used Sex pheromones are used by many insects to facilitate to control this pest (Stevenson et al., 2003). This reduced mate location and recognition (Lo¨fstedt, 1993). In the pesticide use was achieved without a reduction of crop majority of moth species, it is the female that produces the quality or yield and has conserved natural enemies to chemical signal and the male that responds. Sex phero- more effectively manage foliar pecan pests, such as aphids, mones are the primary source of communication among mites, and leaf miners, that occur later in the pecan grow- mating moths (Tamaki, 1985). The responder is capable of ing season (Harris et al., 1998). distinguishing among pheromones and is attracted to a Two synthetic sex pheromones have been developed very specific chemical signal produced by a conspecific and are currently used to detect the flight of A. nuxvorella female (Svensson, 1996). There are several examples of adults (Millar et al., 1996; Harris et al., 2008). The first lepidopterans that have polymorphic sex pheromones pheromone developed (henceforth standard pheromone) within the species. A thoroughly studied example is Ostri- was identified to be a single aldehyde (9E,11Z)-hexadeca- nia nubilalis Hu¨bner: females of one pheromone race pro- dienal (9E,11Z-16:Ald) (Millar et al., 1996). This phero- duce and males respond to a 3:97 E ⁄ Z-11-tetradecenyl mone has been used to attract male A. nuxvorella in the acetate pheromone blend, whereas in a second pheromone USA for over 10 years, but interestingly, the standard race, females produce and males respond to a 99:1 E ⁄ Z Pheromone strains of pecan nut casebearer 13 pheromone blend (Klun et al., 1973; Kochansky et al., differential response to pheromone blends) (Zada et al., 1975). Studies have found that O. nubilalis pheromone 2008) at each of our study sites. production and response can be attributed to insect genet- ics (Klun & Maini, 1979; Dopman et al., 2004) and to host plant use (Pelozuelo et al., 2004). Likewise, pheromone Materials and methods races in Spodoptera frugiperda JE Smith are linked to host Amplified fragment length polymorphism markers (Vos plant preference (Groot et al., 2008). In contrast, an aber- et al., 1995) were used to detect differences in the geno- rant pheromone blend resulting from a single autosomal types of male A. nuxvorella that responded to different sex gene mutation was discovered in a laboratory population pheromones. Amplified fragment length polymorphism of Trichoplusia ni Hu¨bner in which mutant females were markers were chosen because they are capable of surveying unattractive to normal males (Hunt & Haynes, 1990). Two the whole genome of an organism when no prior genetic distinct pheromone blends and one intermediate phero- information is available. Additionally, they can be mone blend were discovered for Hemileuca eglanterina obtained fairly inexpensively and they produce many Boisduval (McElfresh & Millar, 2001). It is believed that markers in a relatively short period of time (Pejic et al., these two distinct blends are a result of reproductive char- 1998; Althoff et al., 2007). acter displacement and that the intermediate blend is a result of mating between the two pheromone races where Specimen collection they occur in sympatry (McElfresh & Millar, 2001). Other Acrobasis nuxvorella used for this study were collected by examples of moth species that exhibit polymorphic phero- peer and producer cooperators from pheromone-baited mone production include Agrotis segetum Denis & Schif- sticky traps (Trece, Adair, OK, USA) from 21 locations fermu¨ller (Lo¨fstedt et al., 1986; Hannson et al., 1990), in the southern USA and Mexico (Figure 1). Each trap Argyrotaenia velutinana Walker (Miller & Roelofs, 1980), was baited with a rubber septum impregnated with and Pectinophora gossypiella Saunders (Collins & Carde´, 100 lg of synthetic pheromone. At each of the locations 1985). Each of these cases offers a unique angle to learn in the USA and Mexico, 3–6 Trece Pherocon IIIÔ sticky more about the evolution of chemical signaling in Lepido- traps baited with synthetic standard pheromone lures ptera. and 3–6 sticky traps baited with synthetic Mexican Lo¨fstedt (1990) hypothesized that aberrant pheromone pheromone lures were used to capture male moths.

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