Alan Hern

Systemically Induced Plant Volatiles Emitted at the Time of “Danger”

Feeding by Pieris brassicae caterpillars on the lower leaves of Brussels sprouts (Brassica oleracea var. gemmifera) plants triggers the release of volatiles from upper leaves. The volatiles are attractive for a natural antagonist of the herbivore, the parasitoid Cotesia glomerata. Parasitoids are attracted only if additional damage is inflicted on the systemically induced upper leaves and only after at least three days of herbivore feeding on the lower leaves. Upon termination of caterpillar feeding, the systemic signal is emitted for a maximum of one more day. Systemic induction did not occur at low levels of herbivore infestation. Systemically induced leaves emitted green leaf volatiles, cyclic monoterpenoids, and sesquiterpenes. GC-MS profiles of systemically induced and herbivore-infested leaves did not differ for most compounds, although herbivore infested plants did emit higher amounts of green leaf volatiles. Emission of systemically induced volatiles in Brussels sprouts might function as an induced defense that is activated only when needed, i.e., at the time of caterpillar attack. This way, plants may adopt a flexible management of inducible defensive resources to minimize costs of defense and to maximize fitness in response to unpredictable herbivore attack.

Response of female Cydia molesta (Lepidoptera: Tortricidae) to plant derived volatiles

Peach shoot volatiles were attractive to mated female oriental fruit moth, Cydia molesta (Busck), in a dual choice arena. No preference was observed between leaf odours from the principle host plant, peach, and the secondary host plant, apple. Twenty-two compounds were identified in headspace volatiles of peach shoots using gas chromatography-mass spectrometry. Green leaf volatiles accounted for more than 50% of the total emitted volatiles. A bioassay-assisted fractionation using different sorbent polymers indicated an attractant effect of compounds with a chain length of 6-8 carbon atoms. The major compounds of this fraction were tested either singly or in combinations for behavioural response of females. Significant bioactivity was found for a three-component mixture of (Z)-3-hexen-1-yl acetate, (Z)-3-hexen-1-ol and benzaldehyde in a 4:1:1 ratio. This synthetic mixture elicited a similar attractant effect as the full natural blend from peach shoots as well as the bioactive fraction.

Sexual dimorphism in the olfactory orientation of adult Cydia pomonella in response to α-farnesene

The role of host plant‐derived volatile substances on the behaviour of adult codling moth, Cydia pomonella L. (Lepidoptera: Tortricidae) is poorly understood. We tested the response of females and males to a range of α‐farnesene dosages. Natural α‐farnesene from apples contains the E,E and Z,E isomers in varying proportions. No difference in the response of C. pomonella to preparations containing two different proportions of the E,E and Z,E isomers was noted (77:20.7 or 1.2:84.7% E,E and Z,E isomers respectively), indicating a similar bioactivity of E,E and Z,E α‐farnesene on codling moth. A marked sexual dimorphism was found to increasing dosages of α‐farnesene. Females were attracted to low dosages (starting from 63.4 ng) and repelled by high dosages (ending at 12 688 ng). The dose response over this concentration range was linear with a negative slope. Both mated and virgin females responded similarly in kind but differently in degree, both attraction and repellency being more pronounced in mated females. Males were neither attracted nor repelled over a large dose range (63.4 to 12 688 ng) except the highest rate which was attractive. This indicates a stronger dependency of females on plant‐derived volatiles.

A female-specific attractant for the codling moth, Cydia pomonella, from apple fruit volatiles.

Host plant-derived esters were investigated as potential female-specific attractants for the codling moth (CM), Cydia pomonella (L.), a key pest of apples worldwide. The behavioural effects of single and combined volatile compounds and of a natural odour blend were examined using olfactometry and wind-tunnel bioassays. The apple-derived volatile butyl hexanoate attracted mated females while it was behaviourally ineffective for males over a dosage range of more than three orders of magnitude in olfactometer assays. Female CM preferred this kairomone to the headspace volatiles from ripe apples. Both no-choice and choice trials in the wind-tunnel suggested that female moths might be effectively trapped by means of this compound. In contrast, headspace volatiles collected from ripe apple fruits as well as a blend containing the six dominant esters from ripe apples were behaviourally ineffective. A female-specific repellency was found for the component hexyl acetate in the olfactometer, but this ester had no significant effect in the wind-tunnel. Butyl hexanoate with its sex-specific attraction should be further evaluated for monitoring and controlling CM females in orchards.

New insights in analysing parasitoid attracting synomones: early volatile emission and use of stir bar sorptive extraction

Summary.It is well known that feeding by Pieris brassicae caterpillars on cabbage leaves triggers the release of volatiles that attract natural antagonists such as the parasitoid Cotesia glomerata. The temporal dynamics in the emissions of parasitoid attracting volatiles has never been elucidated in this system. In a time course experiment, caterpillar infested leaves attracted the parasitoid within one hour after infestation. At such an early stage of infestation, as much as fifty percent of the parasitoids flew towards the infested plant in a wind tunnel bioassay, while only five percent flew towards the non-infested control plant. Three hours after infestation and later, the response to the volatiles from the infested plant reached its maximum and then continued at a constantly high level for the remaining 14 hours of the experiment. Chemical analyses of volatiles collected from infested leaves at short time intervals during the first 24 hours identified a total of ten compounds, comprising green leaf volatiles, terpenoids, and a nitrile. Significant increase of emission within the first 5 hours following initial herbivory was detected for (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate, cineole and benzylcyanide. Subsequently, a coupled bioassay-chemical analysis procedure was developed allowing for testing and analyzing the same sample for future identification of the bioactive compounds. This was achieved by using stir bar sorptive extraction for the analysis of solvent extracts of caterpillar-damaged leaves.

Induction of volatile emissions from ripening apple fruits infested with Cydia pomonella and the attraction of adult females

We studied the effect of herbivory by Cydia pomonella L. (Lepidoptera: Tortricidae) first instar larvae on the volatile emissions from immature apples collected at the end of August. Volatiles from infested, artificially damaged, and healthy fruit were collected using solid‐phase microextraction and analysed using combined gas chromatography mass spectrometry. The results show that neonate C. pomonella larvae induce the release of E‐β‐ocimene from immature fruit compared to healthy or artificially damaged fruit. Seven further compounds, a terpene, an alcohol, four aldehydes, and a ketone, were consistently found in all treatments. There were no significant differences amongst these latter compounds, although the amount of the terpene E,E‐α‐farnesene was three to five times higher from the infested as compared to healthy fruits. Headspace volatiles were also collected on Tenax to test for behavioural responses to the volatiles. Volatile collections from infested fruit attracted mated adult females in a Y‐tube olfactometer, whereas no preference was evinced for volatiles from healthy or artificially damaged fruits when compared to a solvent control.

Preimaginal Environment Influences Adult Flight in Cydia molesta (Lepidoptera: Tortricidae)

Abstract Dispersal of adult tortricid moths between habitats may have important consequences for pest management in orchards, but little is known about how flight parameters are affected by environmental conditions during preimaginal development. The influence of changing temperature and photoperiod (both singly and in combination) as well as of larval crowding and food deprivation were investigated in Cydia molesta Busck (Lepidoptera: Tortricidae), a species that has been found to disperse after peach harvest and colonize pome fruit orchards. Comparative assessments of flight parameters were made on a computer-linked flight mill and life history traits were evaluated. A significant increase in flight performance was correlated with preimaginal exposure to decreasing photoperiod. In addition, pupal development was delayed and larger individuals emerged, but preimaginal survivorship was reduced. Decreasing and increasing temperature regimens and increasing photoperiod did not influence adult flight. Larval crowding was associated with increased flight, but the differences were not statistically significant. Food deprivation was associated with accelerated preimaginal development, lower pupal weight, less fecund adults, and reduced flight. We propose that the main factor eliciting dispersal in this tortricid is decreasing photoperiod.

Induced emissions of apple fruit volatiles by the codling moth: changing patterns with different time periods after infestation and different larval instars.

The changes in the emission of volatiles from mature apple fruits in response to larval feeding by the codling moth (Cydia pomonella) under laboratory conditions are reported. A time course experiment investigated the emission of volatiles throughout the period of larval development following infestation. The volatiles consisted mainly of esters, a few aldehydes, and the terpene alpha-farnesene. Infested apples emitted the same compounds as healthy apples. The quantities of volatiles released were much higher for infested as compared to healthy fruits for an initial three day period. Following this period there was a decrease in volatile emissions (days 6-9), eventually declining back to the levels emitted from healthy apples or below by 9-21 days after infestation. In a separate experiment, the volatile emissions from healthy and artificially damaged fruits were compared to those from herbivore damaged fruits for each of the five larval instars of C. pommonella. The results from the discriminant analysis indicate that the most effective induction of volatiles occurred when fruits were infested with first instar larvae. Induction by first instar larvae was generally higher than after infestation by later instars, and for most compounds it also exceeded the emission from artificially damaged fruits.

Statistical modelling of insect behavioural responses in relation to the chemical composition of test extracts

Abstract. The use of generalized linear models (GLM) for relating changes in insect behaviour to changes in the chemical composition of a plant extract is presented and applied to data from an experimental study of the olfactory response of Cydia pomonella L. (Lepidoptera: Tortricidae) to apple volatiles. The volatiles were collected from healthy apples, artificially damaged apples or apples infested with C. pomonella larvae (either instar I, IV or V). These treatments produced a blend of 23 major components and the chemical composition of the blends differed substantially amongst the treatments.

Bioassay approaches to observing behavioural responses of adult female Cydia molesta to host plant odour

Abstract:  Three different olfactometers were evaluated in order to develop a bioassay procedure testing for the olfactory responses of Cydia molesta. Females were tested individually using a linear and a Y‐tube olfactometer, and in groups using a dual‐choice arena. Room temperature, relative humidity, light intensity, airflow, age of the moths and odour source were kept constant during experiments. The odour source tested was a green shoot of peach excised 10 min before experiments started. Cydia molesta females showed a significant response to this plant odour in all olfactometers. A number of qualitative aspects were found to be in favour of the dual choice arena as a tool for screening potentially attractive odour sources. It allowed for a differentiation of the response of mated and virgin females. Experimental conditions allowed the circadian rhythm of insects to be mimicked. Manipulation of individuals is reduced and flight is not precluded.