Combining 1,4-dimethoxybenzene, the major flower volatile of wild strawberry Fragaria vesca, with the aggregation pheromone of the strawberry blossom weevil Anthonomus rubi improves attraction
Introduction
Plants produce numerous volatile compounds utilized by insects in host plant recognition and location. To develop novel crop protection strategies, it is therefore of utmost importance to understand insect-plant interactions (Agelopoulos et al., 1999). These volatiles may attract insects alone (Städler, 1992). However, the response of insects to their sex or aggregation pheromones may be enhanced or inhibited by host volatiles (Landolt and Phillips, 1997). The effect of enhancement of aggregation pheromones by plant volatiles has been documented for the boll weevil [Anthonomus grandis Boheman (Coleoptera: Curculionidae)] (Dickens, 1989), a species of the same genus as the strawberry blossom weevil [Anthonomus rubi Herbst, (Coleoptera: Curculionidae)] studied in the present work. In the northern part of Europe, A. rubi is one of the major pests on cultivated strawberry [Fragaria ananassa L. (Rosaceae)]. Early in the spring, adult weevils migrate from overwintering shelters to strawberry fields. Here they start feeding on the foliage and mate at the onset of bud formation. The eggs are deposited singly into flower buds which the weevils partially sever from their stalks. The bud formation is thus terminated and the bud will not open, and this leads to a direct loss of crop (Popov, 1996, Cross and Easterbrook, 1998).
The pheromone system of A. rubi was investigated by Innocenzi et al. (2001) and led to the identification of three male-specific compounds. Two of these compounds are also components of the aggregation pheromone of the boll weevil, A. grandis, Grandlure I and II (Tumlinson et al., 1969). Traces of Grandlure III and IV were also present but not necessary for attraction. The third male-specific compound of A. rubi was lavandulol. In the presence of pheromone-producing weevils on F. ananassa plants, germacrene D was collected in increased amounts (Innocenzi et al., 2001). This is a known volatile also from strawberry plants thus this increase may be due to production by the weevils or a plant response to feeding-induced damage.
Cross et al., 2006a, Cross et al., 2006b showed that sticky stake traps baited with the aggregation pheromone can successfully be used as a monitoring tool, but were not effective in controlling A. rubi. These sticky stake traps with glue for catching weevils were not practical for strawberry producers, thus never became a success for farmers in monitoring weevils. However, funnel traps with A. rubi aggregation pheromones for monitoring are sold in United Kingdom and used by several farms.
Bichão et al. (2005a) showed that the neurons on the antenna of A. rubi are narrowly tuned by responses to a few structurally related sesquiterpenes, aromatics or monoterpenes. Five olfactory receptor types were identified according to which compound elicited the strongest neuron response. These potent compounds were (-)-germacrene D, (-)-β-caryophyllene, methyl salicylate, E-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). All these compounds are present in the intact strawberry plant and are induced in higher amounts by weevil feeding.
The present study aimed to investigate the significance of two plant volatiles in combination with the aggregation pheromone for enhanced attraction of A. rubi. The plant volatiles chosen for this study were (-)-germacrene D and 1,4-dimethoxybenzene. The first compound was tested because previous studies by Innocenzi et al. (2001) and Bichão et al. (2005a) showed that (-)-germacrene D was involved in the insect-plant relationship for A. rubi and its host. The second compound 1,4-dimethoxybenzene was chosen as test compound when the results of the present chemical analysis of wild strawberry plants [Fragaria vesca L. (Rosaceae)] were available. This compound was shown to be the dominant compound in the volatile plume of F. vesca flowers, in contrast to many F. ananassa cultivars where only traces of 1,4-dimethoxybenzene were found (Innocenzi et al., 2001).
Section snippets
Plant volatiles, collection and identification
Wild strawberry (F. vesca) plants were brought into the laboratory during May–June, and volatiles of flowers were collected in the laboratory during flowering period (flowering seasons May–June) by SPME (Solid Phase Micro Extraction).
Each flower (still attached on a potted plant) was inserted and enclosed in a glass bulb with two openings used for bulb distillation (5 ml volume). In total, five flowers of separate plants were analysed. The openings of the glass cylinder and the bulb were
Chemical analysis
The volatiles from the five single flowers of F. vesca, trapped by SPME during 24 h and analysed by GC–MS were dominated by 1,4-dimethoxybenzene (mean 96.6%, SE ± 1.29). Methyl salicylate (mean 2.2%, SE ± 0.86) and others minor compounds (mean 1.2%, SE ± 0.80) were also present but these compounds constituted only minor part of the total volatiles.
Field experiments
Altogether, 3515 weevils were caught in the funnel traps across both years and all sites. The three sites with the highest insect caught catch
Discussion
The results of the chemical analysis of flowers of the wild strawberry F. vesca show that one major compound measured by SPME, 1,4-dimethoxybenzene, at a considerable concentration is released. Earlier, only traces of this compound had been found in the headspace of flowers of the strawberry cultivar Korona (Innocenzi et al., 2001, and own unpublished results). Since this compound is dominant in the volatile profile of F. vesca flower and in accordance with the results of the present study,
Acknowledgement
We would like to acknowledge the owner of the following farms for letting us use their strawberry fields as study sites: Lundasletta farm, Ørland, Norway; Sogge farm, Åndalsnes, Norway; Skjønsby farm, Ringsaker, Norway; Troldebakkens Frugtplantage, Denmark; and Middle Pett farm, UK. We will also acknowledge the Danish Agricultural Advisory Services with advisers Charlotte Hansen and Bodil Damgaard Petersen and the Norwegian Agricultural Advisory Services with adviser Jørn Haslestad for their
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