Peter Witzgall, C. Rikard Unelius,1 Franco Rama2, Jean-Pierre Chambon3 and Marie BengtssonDepartment of Plant Protection Sciences, Swedish University of Agricultural Sciences,
Abstract - The sex pheromone of pea moth, Cydia nigricana, is (E,E)-8,10-dodecadien-1-yl acetate (E8,E10-12Ac). Small amounts of the geometric isomers E,Z-, Z,E-, and Z8,Z10-12Ac strongly inhibit male attraction to E8,E10-12Ac. In pea fields treated for mating disruption, males are attracted to fresh dispensers releasing >98% pure E8,E10-12Ac. After formation of >6% of the antagonistic geometric isomers in the dispensers, males are no longer attracted, but fly out of the treated fields. The main pheromone compound of the codling moth, C. pomonella, is (E,E)-8,10-dodecadien-1-ol (E8,E10-12OH; codlemone). The nonpheromonal isomers do not prevent attraction to codlemone; males are attracted to mating disruption dispensers releasing an equilibrium isomer blend of 66% E,E, 17% E,Z-, 13% Z,E-, and 3% Z8,Z10-12OH. Codlemone acetate, E8,E10-12Ac, significantly inhibits male attraction to codlemone, but treatment with a 1:1-blend of codlemone and codlemone acetate does not have a repellent effect on males emerging within the orchard. However, males are not attracted from surrounding orchards, in contrast to mating disruption treatments with codlemone alone.
Key words - sex pheromone, attraction antagonist, mating disruption, Cydia nigricana, pea, Cydia pomonella, pome fruit orchard, Tortricidae, Lepidoptera
Public concern about the use of toxic chemicals in fruit production and increasing resistance against insecticides demand the development of new techniques for control of codling moth, Cydia pomonella L. Mating disruption is the most promising method for environmentally safe control of this key pest of pome fruit, and is being applied on more than 20 000 ha worldwide. For widespread use, however, it needs to be made more reliable and cost-effective. At high population densities, additional insecticide sprays are still necessary and even multiple insecticide applications against resistant strains are usually cheaper than the pheromone treatment (Charmillot 1990; Vickers & Rothschild 1991; Barnes et al. 1992; Howell et al. 1992; Pfeiffer et al. 1993; Barnes & Blomefield 1997; Becid 1997; Rama 1997; Thomson 1997; Waldner 1997).
Orchard treatments with the main pheromone component (E,E)-8,10-dodecadien-1-ol (E8,E10-12OH; codlemone) alone or in combination with the saturated alcohols dodecanol (12OH) and tetradecanol (14OH) enhance search flights of male moths in tree crowns and even attract males from untreated orchards (Witzgall et al. 1996b; Bäckman et al. in prep.). This may lead to matings particularly along borders and in tree tops, where lower aerial concentrations and less homogenous dispersal of synthetic pheromone are to be expected.
In the pea moth, C. nigricana F., a blend of sex pheromone, (E,E)-8,10-dodecadien-1-yl acetate (E8,E10-12Ac) and the antagonistic isomers E,Z-, Z,E-, and Z8,Z10-12Ac repells male moths from pea fields and is thus efficient to prevent matings even at high population densities (Witzgall et al. 1993, 1996a; Bengtsson et al. 1994).
We have therefore investigated the effectiveness of pheromone/antagonist blends for disruption of mating in codling moth. Codlemone acetate, E8,E10-12Ac, is the most potent antagonist of male attraction to codlemone, E8,E10-12OH (Hathaway et al. 1974) and has been shown to be an effective disruptant on its own (Hathaway et al. 1979, 1985). The geometric isomers E,Z-, Z,E-, and Z8,Z10-12OH have a moderate inhibitory effect on male attraction (McDonough et al. 1993), blends with codlemone gave better communication disruption than codlemone alone (McDonough et al. 1994, 1996).
Materials and methods
Pea moth, C. nigricana
Pea moth pheromone, E8,E10-12Ac (>99.1% isomeric and 99.7% chemical purity, by gas chromatography), was formulated in polyethylene tubes (Shin-Etsu Chemical Co., Tokyo) and puzzlepiece trilaminate dispensers (Hercon Environmental Co., Emigsville, Pennsylvania) at 50 mg/dispenser. Dispenser were placed in two unsprayed pea fields (0.6 ha and 3 ha) at 600 dispensers/ha (30 g E8,E10-12Ac/ha) and 333 dispensers/ha (17 g E8,E10-12Ac/ha), respectively.
Dispenser release rates were measured by glass adsorption in a static atmosphere. Aerial pheromone concentrations were determined by field-EAG and air sampling. Communication disruption was monitored by field trapping with 100 µg E8,E10-12Ac on rubber septum or 3 mg "propheromone" (Streinz et al. 1993) on filter paper; behavioural observations during pea moth diel flight in the late afternoon; and larval counts after the seasonal flight period (Bengtsson et al. 1994; Witzgall et al. 1996a).
Codling moth, C. pomonella
Resin-treated paper dispensers (Isagro Ricerca, Novara; Rama 1997) were placed at rates of 400/ha and 1000/ha in five unsprayed apple orchards (2 to 15 ha) in 1995 and 1996. These dispensers contained (1) 250 mg codlemone, E8,E10-12OH (99.3% isomeric and 99.7% chemical purity, by GC); (2) 250 mg of an equilibrium blend of codlemone and its geometric isomers (26% E,Z; 20% Z,E; 5% Z,Z, relative to E8,E10-12OH); (3) 250 mg codlemone plus 250 mg codlemone acetate, E8,E10-12Ac (96.4% isomeric and 97.9% chemical purity).
Polyethylene tubes (Shin-Etsu Chemical Co., Tokyo) containing ca. 100 mg E8,E10-12OH (unknown isomeric and chemical purity of formulated compound), 56% dodecanol (12OH) and 20% tetradecanol (14OH), were applied at 1000 dispensers/ha in two of these orchards (5 ha and 15 ha) in 1994.
Male flight behaviours were observed during the diel flight period between 20:00 and 23:00, over three seasons. Trap captures were recorded in treated and untreated orchards (tetra traps; 100 µg and 1 mg E8,E10-12OH/septum). The effect of the geometric isomers of codlemone on male attraction was determined by trap tests in untreated orchards (Bäckman et al. in prep).
Pea moth, C. nigricana
Pea moth, C. nigricana
In the pea moth, male attraction to sex pheromone, E8,E10-12Ac, is strongly inhibited by all three non-pheromonal geometric isomers. A 5%-addition reduces trap catch significantly (Figure 1; Witzgall et al. 1993), even captures in the control trap seemed to be affected by neighbouring traps with isomer blends. C. succedana D.& S. and C. jungiella Cl. have a similar diel flight period and they feed on Leguminosae; they are attracted to blends of E,E/E,Z and E,Z/Z8,E10-12Ac, respectively (Witzgall et al. 1996c).
Pea moth males were attracted to fresh rope and trilaminate dispensers, formulated with rather pure E8,E10-12Ac. Already after two days of field exposure, these dispensers released 6% of the other isomers: this blend of pheromone and attraction antagonists had a repellent effect on male moths, they were observed to fly out of the treated fields.
At a density of 330 and 600 dispensers/ha, three and no males at all, respectively, were attracted to monitoring traps throughout the season, compared to &200 males/trap in control fields. The absence of males in treated fields was corroborated by visual observations, including cages with calling females. Male pea moths respond to synthetic pheromone up to one hr before the onset of female calling. For mating disruption, it may be effective to repel males by attractant/antagonist blends during this premating flight period, rather than to attract them (Bengtsson et al. 1994; Witzgall et al. 1996a).
Figure 1 Attraction of C. nigricana (above; data from Witzgall et al. 1993) and C. pomonella males (below) to traps baited with sex pheromone and 5 or 20% of the non-pheromonal geometric isomers E,Z; Z,E; and Z,Z (N = 10)
Codling moth, C. pomonella
Codling moth, C. pomonella
Of the geometric isomers of codlemone, only a 20%-addition of E8,Z10-12OH reduced trap catch with codlemone (Figure 1); mating disruption dispensers formulated with an isomer blend (100% E,E-, 26% E,Z-, 20% Z,E-, 5% Z8,Z10-12OH) were attractive to male codling moths (Bäckman et al. in prep.). None of the species closely related to codling moth is known to use the isomers of codlemone as sex pheromone (Arn et al. 1997b).
It is not possible to release isomerically pure codlemone from currently available dispenser material. Two commercial dispensers, formulated with rather pure codlemone, release several percent of the other isomers immediately after field application. However, even after field exposure of up to one year, the isomer blend proportion remains well below the equilibrium blend (Brown et al. 1992; McDonough et al. 1994; Knight 1995; Knight et al. 1995; Bäckman 1997).
Codlemone acetate, E8,E10-12Ac, and its geometric isomers significantly diminished male attraction to codlemone, at a 20%-addition (Figure 2; Witzgall et al. 1996b), trap catch with a 1:1-blend was reduced to 3%, compared to codlemone (unpubl. res.). Codlemone acetate is the sex pheromone of pea moth (see above), and pear moth, C. pyrivora Danil., a sibling species of codling moth. Females of the beech tortrix, C. fagiglandana Z., produce a blend of codlemone and codlemone acetate (Witzgall et al. 1996c).
Figure 2 Attraction of C. pomonella males to traps baited with codlemone E8,E10-12OH and 5 or 20% of codlemone acetate E8,E10-12Ac and its geometric isomers EZ, ZE, and ZZ (N = 10; data from Witzgall et al. 1996b)
For mating disruption, three orchard treatments were compared: E8,E10-12OH; an isomer blend (delta-8,delta-10-12OH); and a 1:1-blend of E8,E10-12OH and E8,E10-12Ac (Bäckman et al. in prep.).
Communication disruption by the isomer blend was not significantly different from the codlemone/codlemone acetate blend, according to trap catch and visual observations (Bäckman et al. in prep.). Further trials on larger surfaces will be needed to determine whether such blends could be better suited for codling moth mating disruption than codlemone alone (see also McDonough et al. 1994, 1996).
In orchards with high overwintering populations, none of the two pheromone/ antagonist-treatments had a pronounced repellent effect. E8,E10-12Ac and E8,Z10-12OH are only moderate antagonists of codling moth attraction - the nonpheromonal acetates had a much stronger effect on pea moth (Figures 1, 2).
Male flight activity was much enhanced by all three treatments, compared to untreated plots. The males "searched" mainly the upper half of the trees by flying about branches and top shoots, where they frequently landed on leaves - and where matings occurred. Males terminated searching by flying off downwind or across the wind line, usually well above the trees. It was impossible to determine how far these males travelled, but there was no noticeable decrease in the density of male moths in any of the treatments.
It is important to note that our observations did not allow to distinguish males emerging within the orchard from those arriving from outside. Field wind tunnels (Cardé et al. 1993, 1997) allow to investigate the behavioural effects of long-term exposure to different disruptant chemicals. In comparison, pea moths emerge outside the pheromone treatment, as peas are sown in different fields each year.
Codling moth males were attracted upwind to codlemone treatments from nearby untreated orchards, over at least 50 m, while long-range attraction to E8,E10-12OH/ E8,E10-12Ac-treatments was not observed.
The behavioural modifications leading to disruption of mating result from the chemicals used - permeating the atmosphere with pheromone blends, single components, incomplete or off-blends will induce different male behaviours (e.g., Flint & Merkle 1983; Chisholm et al. 1984; Palaniswamy et al. 1984; Palaniswamy & Underhill 1988; Bengtsson et al. 1994; Suckling & Burnip 1996).
The chemicals used for mating disruption and their purity must therefore be known. This is the sine qua non for reproducible experiments, comparative analysis of applications, and for the interpretation of behavioural studies (see also Arn et al. 1997a; Millar et al. 1997). Tests with the pea moth underline that it is necessary to monitor the dispenser release by chemical analysis: isomerization of formulated pea moth pheromone reverses its behavioural effect within a few days.
Which chemicals should be used for disruption of mating? A complete pheromone blend should be best suited to preclude male perception of females (see Charlton & Cardé 1981; Sanders 1981; Minks & Cardé 1988; Cardé 1990; Suckling & Clearwater 1990; Cardé & Minks 1995; Felland et al. 1995). The behavioural activity of the main pheromone component is much enhanced by the addition of pheromone synergists: the same amount of active ingredient will produce a stronger behavioural effect (Linn et al. 1986) and this may even compensate for the cost of additional compounds.
It will rarely be possible to apply the complete pheromone blend at the female-released proportions. Incomplete blends, however, may still lead to attraction of males from surrounding, untreated crops. These males may be capable of distinguishing female signals from the synthetic background, as long as they haven't become adapted or habituated, and this may account for matings especially along treatment borders. Attraction of males can be overcome by area-wide mating disruption, or by pheromone antagonists, as in pea moth.
Male codling moths are attracted to codlemone-treated orchards, but not to treatments with blends of codlemone and codlemone acetate. It is therefore surprising that this pheromone/antagonist blend enhanced search flights within the orchard. A tentative explanation is selective sensory adaptation or central habituation to codlemone acetate, perhaps in combination with a synergistic interaction of codlemone and plant volatiles. The addition of the codlemone isomers or codlemone acetate to codlemone dispensers seems to have little effect on males emerging into the treated orchards.
This study was supported by the Swedish Council for Forestry and Agricultural Research (SJFR), the Swedish Board of Agriculture (Jordbruksverket) and the Erik and Ebba Larssons & Thure Rignells Foundation.
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