Density dependence and induced resistance or behavioural response of the shot-hole borer of tea, Xyleborus fornicatus (Coleoptera:Scolytidae) to conspecifics and plant odours

Abstract

The two tea plant cultivars TRI2023 and TRI2025 are known to show different susceptibilities for attack by the shot-hole borer beetle, Xyleborus fornicatus Eichh. (Coleoptera:Scolytidae). Olfactometric studies showed that female beetles were attracted to both tea cultivars TRI2023 and TRI2025 but preferred the susceptible cultivar TRI2025 to the resistant cultivar TRI2023. In a no-choice experiment where the settlement of beetles on twigs of both cultivars was studied, it was found that in the first hour of attack a greater number of beetles attacked twigs of the resistant cultivar TRI2023 than twigs of the susceptible cultivar TRI 2025. However, after the first hour the rate of attack was inverted, i.e. the established view on the resistance of the two clones was registered in terms of attacks. When twigs had been exposed to beetles for 24 h before attack by a fresh set of beetles, a lesser number of beetles attacked the already infested stems of both cultivars consistently throughout the period of observation compared with attack on healthy stems. The possibility of an induced resistance stimulated by the initial beetle attack is suggested and is hypothetically discussed in terms of a plant-resistance strategy based on using a strong initial attack of beetles as a releaser for induced effects which counteract further attacks.