R.W.H.M. van Tol

Female-induced increase of host-plant volatiles enhance specific attraction of aphid male Dysaphis plantaginea (Homoptera: Aphididae) to the sex pheromone

All aphid species studied so far share the same sex pheromone components, nepetalactol and nepetalactone. Variation by different enantiomers and blends of the two components released by different aphid species are limited and can only partially explain species-specific attraction of males to females. While some host-plant odours are known to enhance specific attraction of aphid species, herbivore-induced plant volatiles that synergise attractiveness to the sex pheromone are unknown. Here, we demonstrate that for the host-alternating rosy apple aphid (Dysaphis plantaginea (Passerini)) specificity of attraction of males to females is triggered by female-induced tree odours in combination with a 1:8 ratio of (4aS,7S,7aR)-nepetalactone and (1R,4aS,7S,7aR)-nepetalactol. Female aphid infestation induces increased release of four esters (hexyl butyrate, (E)-2-hexenyl butyrate, (Z)-3-hexenyl 3-methylbutyrate and hexyl 2-methylbutyrate) from apple leaves. Two different combinations of three esters applied in a 1:1:1 ratio increase the number of male D. plantaginea and decrease the number of other aphid species caught in water traps in the presence of the pheromone components. The ester blend alone was not attractive. Combination of the pheromone blend with each single ester was not increasing attraction of male D. plantaginea. The demonstration that sexual aphid species use herbivore-induced plant volatiles as a species-specific attractant for mate finding adds a new dimension to our understanding of insect species using or manipulating chemical cues of host plants for orientation.

Behavioural responses of the vine weevil, Otiorhynchus sulcatus, to semiochemicals from conspecifics, Otiorhynchus salicicola, and host plants

The vine weevil Otiorhynchus sulcatus is a parthenogenetic reproducing species which forages for suitable host plants at night, but is found congregated in dark places during the day. Frass of this weevil species is suspected to contain attractive compounds that are host‐plant related. Using a still‐air olfactometer, we tested adult vine weevils at night for their behavioural response to odours from conspecifics, feeding on a mixture of spindle tree (Euonymus fortunei) and yew (Taxus baccata), and to a sexually reproducing related species (Otiorhynchus salicicola), feeding on a mixture of ivy (Hedera helix) and cherry laurel (Prunus laurocerasus). Their attraction to conspecifics and O. salicicola appeared to be related to frass production. Freshly collected frass from O. sulcatus and from O. salicicola males and females was attractive. Prunus laurocerasus and H. helix have not been observed to be hosts of the vine weevil in the field. However, our tests showed that the vine weevil was attracted to mechanically damaged leaves of both plant species, whereas undamaged leaves were not attractive. Only undamaged young unfolding leaves of H. helix were also attractive. The attraction to odours from mechanically damaged host and non‐host plants suggested the involvement of compounds that are commonly found in many plant species. The involvement of plant compounds and/or aggregation pheromones in attraction to frass of the vine weevil and frass of the related weevil species O. salicicola is discussed.

Methyl isonicotinate induces increased walking and take-off behaviour in western flower thrips, Frankliniella occidentalis

Timely detection of western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is important to help prevent economic damage to crops. Coloured traps are commonly used for thrips monitoring and their efficacy can be markedly improved with the addition of semiochemicals such as methyl isonicotinate (MI). However, the behavioural response of thrips to these compounds is poorly understood. In leaf/petal disc laboratory experiments the effect of MI on adult female WFT was examined with respect to walking and take‐off behaviour. Clean air or air with MI was blown over the leaf/petal discs at an angle of 30° via a glass pipette connected to a wash bottle containing a cotton dental roll with or without a defined amount of MI. At airspeeds of 0.6–1.2 m s−1 and at doses of 1, 10, 50, 100, and 1000 μl (equivalent to indicative concentrations of 1.7, 4.5, 7.3, 8.8, and 14.9 ng MI l−1 air blown over the leaf discs), WFT spent a greater proportion of time walking on chrysanthemum leaf discs in the presence of MI than the non‐attractive thrips compound β‐caryophyllene or clean air. Western flower thrips showed a dose‐dependent take‐off response to the thrips attractant MI, but not to β‐caryophyllene (virtually no take‐off). At 100 μl of MI (8.8 ng l−1 air), WFT spent a greater proportion of time walking on host plant tissue than in clean air, depending on plant disc substrates. Furthermore, WFT took off more often from plant discs exposed to an air current containing MI than from those exposed to clean air. Western flower thrips exposed to MI took off at a similar rate from chrysanthemum leaves, chrysanthemum flower petals, and sweet pepper leaves, but much less readily from bean leaves. The results show that MI stimulates walking and take‐off behaviour for adult female WFT under these experimental conditions and these responses are dose‐dependent and mediated by the host plant. These results are placed in the context of previous research on the behavioural response of thrips to semiochemicals and their implications for thrips pest management are discussed.

Methyl isonicotinate - A non-pheromone thrips semiochemical - And its potential for pest management

Methyl isonicotinate is one of several patented 4-pyridyl carbonyl compounds being investigated for a variety of uses in thrips pest management. It is probably the most extensively studied thrips non-pheromone semiochemical, with field and glasshouse trapping experiments, and wind tunnel and Y-tube olfactometer studies in several countries demonstrating a behavioural response that results in increased trap capture of at least 12 thrips species, including the cosmopolitan virus vectors such as western flower thrips and onion thrips. Methyl isonicotinate has several of the characteristics that are required for an effective semiochemical tool and is being mainly used as a lure in combination with coloured sticky traps for enhanced monitoring of thrips in greenhouses. Research indicates that this non-pheromone semiochemical has the potential to be used for other thrips management strategies such as mass trapping, lure and kill, lure and infect, and as a behavioural synergist in conjunction with insecticides, in a range of indoor and outdoor crops.