Male Phyllotreta striolata (F.) Produce an Aggregation Pheromone: Identification of Male-specific compounds and Interaction with Host Plant Volatiles

Abstract

The chrysomelid beetle Phyllotreta striolata is an important pest of Brassicaceae in Southeast Asia and North America. Here, we identified the aggregation pheromone of a population of P. striolata from Taiwan, and host plant volatiles that interact with the pheromone. Volatiles emitted by feeding male P. striolata attracted males and females in the field. Headspace volatile analyses revealed that six sesquiterpenes were emitted specifically by feeding males. Only one of these, however, elicited an electrophysiological response from antennae of both sexes. A number of host plant volatiles, e.g., 1-hexanol, (Z)-3-hexen-1-ol, and the glucosinolate hydrolysis products allyl isothiocyanate (AITC), 3-butenyl isothiocyanate, and 4-pentenyl isothiocyanate also elicited clear responses from the antenna. The active male-specific compound was identified as (+)-(6R,7S)-himachala-9,11-diene by chiral stationary phase gas-chromatography with coupled mass spectrometry, and by comparison with reference samples from Abies nordmanniana, which is known to produce the corresponding enantiomer. The pheromone compound was synthesized starting from (-)-α-himachalene isolated from Cedrus atlantica. Under field conditions, the activity of the synthetic pheromone required concomitant presence of the host plant volatile allyl isothiocyanate. However, both synthetic (+)-(6R,7S)-himachala-9,11-diene alone and in combination with AITC were attractive in a two-choice laboratory assay devoid of other natural olfactory stimuli. We hypothesize that P. striolata adults respond to the pheromone only if specific host volatiles are present. In the same laboratory set up, more beetles were attracted by feeding males than by the synthetic stimuli. Thus, further research will be necessary to reveal the components of a more complex blend of host or male-produced semiochemicals that might enhance trap attractiveness in the field.

Phyllotreta striolataflea beetles use host plant defense compounds to create their own glucosinolate-myrosinase system

“…In contrast, the "mustard-oil bomb" in aphids is activated when an individual is attacked by a predator, which benefits the survival of the clonal aphid colony by repelling the predator (17,19,47). Although we cannot rule out the participation of sequestered glucosinolates in a similar defense in Phyllotreta, an additional signaling role is also suggested by previous studies showing that ITCs enhance the response to the male-produced aggregation pheromone (26)(27)(28)(29). Elucidation of the spatial organization of the glucosinolatemyrosinase system, as well as the roles of glucosinolate hydrolysis products in communication and defense, will be required to understand the ecological significance of the remarkable myrosinase activity in Phyllotreta beetles that has evolved convergently with myrosinase activity in its host plant.…”

“…However, ITCs are suggested to play a role in interspecific communication of Phyllotreta spp. because they were shown to enhance or synergize the response to the male-produced aggregation pheromone in several species (26)(27)(28)(29).…”

Improved synthesis of the chrysomelid pheromone (6R,7S)-(+)-himachala-9,11-diene via spontaneous bromination and didehydrobromination of 2,6,6,9-tetramethyl-bicyclo[5.4.0]undec-8-ene

“…5 Recently we published a more efficient route starting from the natural sesquiterpene (-)-α-himachalene (1), that is readily available from the essential oil of Cedrus atlantica. 6 Although the improved synthesis required only three steps (Scheme 1, i-iia-iii), the access to the pheromone remained unsatisfactory, since 5 was only a minor component (4%) in a complex mixture that needed extensive purification by preparative GC. 6 Since field studies and behavioral assays require larger quantities of the bioactive compound, we were disposed to improve the sequence.…”

“…6 Although the improved synthesis required only three steps (Scheme 1, i-iia-iii), the access to the pheromone remained unsatisfactory, since 5 was only a minor component (4%) in a complex mixture that needed extensive purification by preparative GC. 6 Since field studies and behavioral assays require larger quantities of the bioactive compound, we were disposed to improve the sequence. While the initial hydrogenation of the exocyclic double bond of (-)-α-himachalene proceeded quantitatively, 6 the spontaneous one-pot bromination dehydrobromination sequence appeared to be a critical transformation yielding several products, among which an aromatic analogue (6) of the pheromone dominated after prolonged reaction times (48 h).…”

“…6 Since field studies and behavioral assays require larger quantities of the bioactive compound, we were disposed to improve the sequence. While the initial hydrogenation of the exocyclic double bond of (-)-α-himachalene proceeded quantitatively, 6 the spontaneous one-pot bromination dehydrobromination sequence appeared to be a critical transformation yielding several products, among which an aromatic analogue (6) of the pheromone dominated after prolonged reaction times (48 h). Here we report a detailed analytical and quantum chemical approach to characterize the bromination/dehydrobromination sequence of 2,6,6,9-tetramethylbicyclo[5.4.0]undec-8-ene (2).…”