The role of foliage in mating disruption in apple orchards

D. Max Suckling and Gerhard Karg

• Introduction
• Materials and methods
• Results and discussion
• References

Key words - sex pheromone, mating disruption, release rates, foliage absorption, apple orchard, Epiphyas postvittana , Tortricidae, Lepidoptera

Introduction

Mating disruption relies on maintaining a sufficiently high pheromone concentration over the treated area, to disrupt intraspecific communication. Plant foliage has been recognized for some time to be capable of pheromone uptake and release (Wall et al. 1981). However, the potential importance of absorption and release of pheromone by plants on the temporal and spatial distribution of pheromone in the context of mating disruption has not been widely investigated.

Karg et al. (1990) and Karg & Sauer (1992) showed the importance of grape vine foliage to the mean pheromone concentration in a pheromone-treated vineyard. Hrdy et al. (1990) recovered pheromone from stonefruit foliage. Karg et al. (1994) demonstrated foliage uptake and release of pheromone by apple leaves. These results led us to hypothesise that leaves could affect the atmospheric pheromone concentration and its temporal distribution, and thereby enhance the success of mating disruption in apple orchards. Here we summarise two behavioural experiments on the influence of foliage on disruption, following the removal of polyethylene dispensers.

Materials and methods
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Dispensers contained 54.9 mg (E)-11-tetradecen-1-yl acetate, 2.5 mg (E,E)-9,11-tetradecadien-1-yl acetate, 19.7 mg (Z)-11-tetradecen-1-yl acetate, as well as 16.8 mg of other substances such as stabilisers (Shin-Etsu Chemical Co., Tokyo). These dispensers have been used in large scale trials of disruption (Suckling & Shaw 1995). Delta traps were baited either with rubber septa containing 1 000 µg of 100/5 E11-14Ac/ E9,E11-14Ac (Bellas et al. 1983), or virgin female moths in gauze cages. Traps were checked daily. Disruption was calculated from catch in the treatment/(treatment plus control).

In experiment 1, the first behavioural experiment of the effect of foliage on disruption (Suckling et al. 1996), we released 52 to 125 naive laboratory-reared moths ca. 1 hr after the removal of dispensers (700/ha), which had been in a 0.05 ha apple block for 48 hr. Feral moths, which had been present in the area during the treatment, were also trapped. The experiment was repeated on four nights, each time in a different apple block. Nine delta traps baited with virgin females were used in each block, which had similar numbers of moths released into a similar untreated control block, about four hours before moth flight at dusk. Traps were hung after the dispensers were removed.

In experiment 2, the treated area was similar to the above, but dispensers were placed into the four replicated blocks for two days, and removed, as above. Feral moths, which had been present in the area during the treatment were trapped in nine delta traps baited with 1 000 µg were used in each block. This lure strength (equivalent to tenfold the standard monitoring lure) was used to ensure high moth catches, after previous trappping had shown increased catch in treated and untreated apple blocks with high-dose lures (Suckling & Karg 1996). Control blocks were operated as above.

Results and discussion
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The recapture of naive male moths in traps baited with virgin females was lower in the pre-treated blocks than in the equivalent control blocks (Figure 1), and was equivalent to 65% disruption. These results were obtained when dispensers were no longer present, and when the sole source of pheromone for disruption was from the apple trees. Since it has been demonstrated elsewhere that apple foliage can absorb and release pheromone (Karg et al. 1994, Suckling et al. 1996), it can be argued that the most probable source of pheromone in this experiment was the apple foliage, which is covered by leaf waxes (Baker 1982). The use of naive moths prevents the possibility of pre-exposure.

The same level of disruptive effect was observed for feral moths (Figure 2), which had been present in the blocks before dispensers were removed.

Figure 1 Rate of recapture of unexposed male E. postvittana  in four paired apple orchard blocks, with moth release after removal of disruptant dispensers (after Suckling et al. 1996).

Figure 2 Catch of wild male E. postvittana  in four paired apple orchard blocks, after removal of disruptant dispensers (after Suckling et al. 1996).

The proportion of moths caught in the previously-treated blocks was the same (Table), and again significantly lower (P<0.05) than the expected proportion of the total catch (0.5). This finding suggests that the potential pre-exposure of the feral moths was not sufficiently lasting to cause greater disruption than occurred for the naive moths.

The next experiment, which used synthetic lures (at 10 fold the standard lure strength) produced a similar result, with a proportion of 0.22 of all moths caught in the blocks which had been previously treated (Table). This result indicates that the magnitude of the estimated effect of disruption from foliage sources was independent of lure. A similar effect of disruption for one night, despite removal of dispensers, was reported using synthetic lures at the standard rate (Suckling et al. 1990).

Apple foliage is capable of absorbing and releasing pheromone in biologically important amounts, and is therefore likely to play a role in aiding disruption. While the level of disruption caused by foliage following the removal of dispensers is less than would be desirable from the perspective of insect control (ca. 60-70% disruption), dispensers are normally left in place. The buffering effect we have demonstrated is likely to supplement pheromone from dispensers, by raising the background concentration in the air. Pheromone trapped on foliage in this way would remain in the block, to be released later, rather than being immediately lost from the system. Pheromone concentrations in foliage are a function of atmospheric concentration, and decline logarithmically with distance from dispensers (Suckling et al. 1996). The buffering effect would therefore be expected to be most important in the vicinity of dispensers. It would be least effective in orchards with missing foliage, or during times of limited leaf area density, such as in the spring.

References
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Baker AE (1982) Chemistry and morphology of plant epicuticular waxes, pp. 139-165 in Cutler DF, Alvin KL, and Price E (eds.) The Plant Cuticle. Academic Press, London

Bartell RJ, Lawrence LA (1973) Reduction of responsiveness of Epiphyas postvittana (Lepidoptera) to sex pheromone following previous brief pheromonal exposure. J Insect Physiol 19, 845-855

Bellas TE, Bartell RJ, Hill A (1983) Identification of the two components of the sex pheromone of the moth, Epiphyas postvittana (Lepidoptera, Tortricidae). J chem Ecol 9, 503-512

Hrdy I, Borek V, Kalinov B, Kudlov J, Vrkoç J (1990) Male confusion of the Oriental fruit moth Cydia molesta, and the plume fruit moth, Cydia funebrana: Small trials of pheromone dispensers, pp.401-406 in Hrdy I (ed.) Proc Conf Insect Chem Ecol. Tabor. Academia Praha, The Hague

Karg G, Sauer AE, Koch UT (1990) The influence of plants on the development of pheromone atmospheres measured by EAG method, pp. 301, in Elsner R, Singer W (eds.) Brain-Perception-Cognition, Proc 18th Göttingen Neurobiology Conference. George Thieme, Stuttgart

Karg G, Suckling DM, Bradley SJ (1994) Absorption and release of pheromone of Epiphyas postvittana (Lepidoptera: Tortricidae) by apple leaves. J chem Ecol 20, 1825-1841

Suckling DM, Shaw PW (1995) Large scale trials of mating disruption of light brown apple moth. N Z J Crop Hort Sci 23, 127-137

Suckling DM, Karg G (1996) Parameters affecting catch of lightbrown apple moth in pheromone-treated orchards. Proc. 49th New Zealand Plant Prot Conf in press

Suckling DM, Khoo JGI, Rogers DJ (1990) Disruption of lightbrown apple moth Epiphyas postvittana (Lepidoptera: Tortricidae) trapping in Nelson, New Zealand. Aust J Zool 38, 363-373

Suckling DM, Karg G, Bradley SJ (1996) Apple foliage enhances mating disruption of lightbrown apple moth. J chem Ecol 22, 325-341

Wall C, Sturgeon DM, Greenway AR, Perry JN (1981) Contamination of vegetation with synthetic sex attractant released from traps for the pea moths, Cydia nigricana. Entomol exp appl 30, 111-115
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