IOBC wprs Bulletin Vol 22(9), 1999

Chemical communication in codling moth: towards environmentally safe control methods

Peter Witzgall, Marie Bengtsson, Ashraf El-Sayed, Anna-Carin Bäckman, Stefan Rauscher,1 Anna-Karin Borg-Karlson,2 C. Rikard Unelius,2 and Jan Löfqvist

Abstract: The sex pheromone of codling moth, C. pomonella, consists of at least 5 components: (E,E)-8,10-dodecadienol (E8,E10-12OH; codlemone), its EZ-isomer (E8,Z10-12OH), the monounsaturated (E)-9-dodecenyl acetate (E9-12OH), and the previously identified saturated compounds dodecanol (12OH) and tetradecanol (14OH). This blend attracts more males than codlemone alone in the wind tunnel. Female codling moths are attracted to branches with green apples by upwind flight. The volatiles released from leaves and green fruitlets have been identified, a range of esters and sesquiterpenes elicit a strong antennal response. Knowledge of the natural products medating mate- and host-finding in codling moth will allow efficient and environmentally safe control of this key pest of pome fruit.

Key words: sex pheromone synergist, plant volatile compounds, apple, Cydia pomonella, Tortricidae, Lepidoptera

Introduction

Codling moth, Cydia pomonella (L.) (Lepidoptera, Tortricidae), is the most serious pest of pome fruit orchards worldwide. The development of new control methods against this species is a must, as conventional broad-spectrum insecticides meet increasing criticism and dissatisfaction, due to health hazards, environmental problems, and insect resistance. In addition, the use of a number of well-established insecticides has recently been restricted or banned.

Few alternatives to insecticide treatments are currently available. Mating disruption by synthetic pheromones is the most promising tool for environmentally safe control of codling moth. An important disadvantage of this technique, however, is that female behaviors are not affected. The use of mating disruption against codling moth is therefore restricted to low population densities and to isolated orchards, where immigration of gravid females is precluded (Minks 1996; Barnes and Blomefield 1997; Rama 1997; Thomson 1997; Waldner 1997; Thomson et al. 1998).

Two strategies are pursued. (1) The addition of pheromone synergists or antagonists to the main compound (E,E)-8,10-dodecadien-1-ol (E8,E10-12OH; codlemone) will enhance the behavioural effect of dispenser formulations used for mating disruption. (2) Volatile compounds from apple mediating attraction of gravid females to the oviposition site will allow manipulation of female behaviours.

Mating disruption with an attractant/antagonist blend

Orchard treatments with the main pheromone component (E,E)-8,10-dodecadienol (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. 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 (Witzgall et al. 1999a).

Codlemone acetate, (E,E)-8,10-dodecadienyl acetate (E8,E10-12Ac), is a potent antagonist of male attraction to codlemone (Hathaway et al. 1974; Witzgall et al. 1996). A receptor neuron type on the male antenna is tuned to this compound, which elicits a strong response even from codlemone receptor cells (Bäckman et al. 1999a).

Table 1. Codling moth male behaviours in orchards treated with dispensers containing codlemone (E8,E10-12OH; OH), the equilibrium isomer blend (8,10-12OH; OH), a blend of codlemone and codlemone acetate (E8,E10-12Ac; OH/Ac), and both OH and OH/Ac.

Males emerging into orchard treatments with a blend of codlemone and codlemone acetate (OH/Ac) fly upwind along tree rows as observed under treatment with codlemone alone (OH); and some are even attracted to dispensers. Whether or not males migrate out of OH/Ac-treated orchards remains to be determined. However, in contrast to treatments with codlemone alone,

Figure 1. Ratio of C. pomonella males landing (n = 60) at each of 2 sources (choice test), comprising calling females, pheromone gland extracts and synthetic codlemone (E8,E10-12OH). Codlemone was formulated on red rubber septa (A), gland extracts and codlemone were sprayed from capillary tubes (B, C, D). Asterisks show significant differences between adjacent columns (Chi-square P = 0.05).

the OH/Ac-blend treatment does not attract males from surrounding untreated orchards, as this is the case with OH-treatment. A promising strategy may be the combination of spatially separate OH and OH/Ac dispensers, which greatly enhances male attraction to and close-range searching at OH dispensers. The equilibrium isomer blend of codlemone (8,10-12OH; McDonough et al. 1994, 1996) has a weaker antagonistic effect than the OH/Ac-blend (Table 1, Witzgall et al. 1999a).

Codling moth pheromone is multicomponent

Observations in orchards treated with large amounts of codlemone, at application rates of up to 250 g codlemone/ha, resulting in aerial concentrations of ca. 1 ng/m3, showed that males are capable of distinguishing the female signal over at least a couple of m (Bäckman 1997; Witzgall et al. 1999a). A comparison of male attraction to calling females, female sex gland extracts containing known amounts of codlemone, and pure synthetic codlemone in a laboratory wind tunnel corroborates that codling moth sex pheromone is multicomponent (Fig. 1; El-Sayed et al. 1999). The pheromone sprayer (Goedde et al. 1999) facilitates comparisons between different stimuli, as it maintains a known and constant release rate throughout the experimental period. The compounds are dispensed from solutions, containing test compounds at known quantities and chemical purity.

Subsequent tests with female gland components (Arn et al. 1985) showed that the saturated alcohols dodecanol (12OH) and tetradecanol (14OH) do not account for the difference between female extracts and codlemone. Only further admixture of the E,Z isomer of codlemone and the monounsaturated E9-12OH resulted in a significant increase of male attraction and landings at the source (Figure 2; El-Sayed et al. 1999).

Figure 2. Ratio of C. pomonella males landing (n = 60) at each of 2 sources (choice test), comprising synthetic codlemone alone, and blends of codlemone female gland components. Compounds were released from the pheromone sprayer at 10 pg codlemone/min. Landings at the 5-component-blend are significantly increased (Chi-square test; P = 0.05).

However, in the field, the 5-component-blend did not significantly enhance trap capture compared to codlemone alone (El-Sayed et al. 1999a). The ratio of codlemone to the other compounds is expected to change, mainly due to differences in vapor pressure and isomerization of the dienic compounds on the rubber septa used in the traps. Therefore, this blend would need to be adapted to the respective dispenser material for field use. In comparison, the pheromone sprayer used in the wind tunnel study evaporates the blends without bias, at the proportions formulated.

Even in the wind tunnel, the 5-component-blend blend is not as active as female gland extracts, especially with respect to close-range and copulatory behaviours at the source. Codling moth sex pheromone contains probably further compounds.

Development of mating disruption technique for control of codling moth

The newly identified blend is clearly more active than codlemone alone or the 3-component-blend of codlemone, 12OH and 14OH, and its effect should be evaluated in mating disruption trials, in comparison with incomplete pheromone blends and pheromone/antagonist blends.

Figure 3. Diel periodicity of codlemone release and male codling moth flight in a Swedish apple orchard.

Adding pheromone synergists or antagonists to codlemone will enhance the behavioural effect of formulations, and may even allow to reduce application rates.

A most important drawback of commercially available dispenser materials is their temperature-dependent release of the active compound. Codling moth is nocturnal and gets activated only at temperatures below 19°C in Swedish orchards, while pheromone release from the dispensers increases with temperature (Brown et al. 1992; Witzgall et al. 1999a; Fig. 3). As a result, almost 90% of the active ingredient went to waste during field trials in Sweden. Field-EAG recordings do not indicate a long-term build-up of pheromone within tree canopy (Milli et al. 1997; Witzgall et al. 1999a). The recent development of pheromone sprayers, which allow to regulate the diel periodicity of pheromone emission (Shorey and Gerber 1996) is one step towards a more economic use of the active ingredient.

Apple volatiles mediate attraction of gravid females

It is known since long that volatiles from apple are attractive to ovipositing codling moth females (e.g. Wildbolz 1958). Behavioural observations in orchards have shown that egg-laying females orient upwind towards branches with green fruit, showing a flight pattern similar to males flying upwind towards pheromone sources. The female attraction towards twigs with green fruitlets was corroborated in an olfactometer assay (Yan et al. 1999) and in the wind tunnel (Fig. 4). We have therefore initiated a study of the volatile compounds released by apple and the female behavioural response.

The volatiles emitted from apple have been studied in detail with respect to flavors important for human consumption, but few studies have been done from an entomologist's viewpoint (Takabayashi et al. 1991; Boevé et al. 1996). We have reexamined headspace collections of flowering twigs, twigs with and without apples, and apples in various developmental stages, from different cultivars.

Figure 4. Attraction of female codling moths to a branch with green apples or apples alone.

Collections were made in an all-glass apparatus in the labora- tory and polyester bags in the field, using Tenax and Porapak Q filters, as well as solid-phase microextraction (Bäckman et al. 1999b; Witzgall et al. 1999b). Analysis by gas chromatography - mass spectrometry (GC-MS) showed a range of compounds, most of which have been identified previously from apple (Flath et al. 1969; Maarse et al. 1989). Collections were screened by gas chromatography - electroantennography (GC-EAD) for compounds with antennal activity in codling moth females.

The volatile emission of flowering twigs contained a number of benzenoids which were not emitted by leaves or fruit. The most abundant compound, benzyl alcohol, was EAD-active, as well as methyl salicylate and b-linalool, which were released in smaller amounts also from fruitlets. Flowers emitted few esters, such as (Z)-3-hexenyl butanoate and (Z)-3-hexenyl 2-methylbutanoate. A range of esters were characteristic for fruit; their proportions increased with ripeness, and several were detected only in collections from ripe apples. Among the compounds with strong antennal activity were butyl butanoate, hexyl butanoate, butyl hexanoate, (Z)-3-hexenyl butanoate, 2-methylhexyl butanoate, and hexyl hexanoate. Terpenoids were released predominantly from leaves and small fruit. The compounds eliciting the strongest antennal responses were 4,8-dimethyl-1,3,(E)-7-nonatriene, b-linalool, b-caryophyllene, (E)-b-farnesene, germacrene D, and the (E,E)- and (Z,E)-isomers of a-farnesene (Witzgall et al. 1999b).

Collections were made from cut branches in the laboratory and from both cut and living branches in the field (Bäckman et al. 1999b). The proportions of all terpenoids with antennal activity increased strongly after cutting the branches. This relative increase was most pronounced for 4,8-dimethyl-1,3,(E)-7-nonatriene, b-linalool, and a-farnesene.

The behavioural activity of the compounds identified from apple is being investigated for the development of a lure attractive to female codling moths.

Acknowledgements

This work was supported by the Foundation for Strategic Environmental Research (MISTRA), the Swedish Council for Planning and Coordination of Research (FRN), the Wenner-Gren Center Foundation, the Swedish Council for Forestry and Agricultural Research (SJFR), and the Swedish Board of Agriculture (Jordbruksverket).

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