The volatile compounds of crofton weed infested by cotton aphids and sprayed with MeJA were collected and analyzed by the TCT-GC/MS technique. The healthy weeds were controls. Seventeen volatiles identified from crofton weed included green leaf odors, monoterpenes and sequiterpenes, and oxo-compounds. Camphene, 2-carene, α-phellandrene, ρ-cymene, and caryophyllene were the major volatiles and constituted about 77% of the total volatile emissions from the control. In the aphid-infested weeds, no new induced component was found. Among the terpenes, ρ-cymene increased markedly in the infested weeds compared with the control, whereas all sesquiterpenes decreased markedly. Levels of endogenous JA in leaves and young stems of the aphid-infested weeds were markedly higher than in the control, whereas both endogenous SA level and ABA level were not significantly different. MeJA sprayed on crofton weed with the aphid infestation had a similar effect on volatile emissions. It is suggested that JA was one of the most important signals in crofton weed and could regulate the emission of volatile compounds.

Thousands of plant species throughout tropical and temperate zones secrete extrafloral nectar (EFN) (see www.biosci.unl.edu/emeriti/keeler/extrafloral/worldlistfamilies.htm) to attract ants, whose presence leads to an indirect defence against herbivores (Chamberlain & Holland 2009, Heil 2008, Heil & McKey 2003, Rico-Gray & Oliveira 2007). Although termed ‘extrafloral’ because the nectar is not involved in pollination, EFN can also be secreted within the inflorescences (Bentley 1977, Holland et al. 2010, Martins 2009). Because ants tend to defend reliable food sources against all types of putative competitors, it has been hypothesized that the presence of extrafloral nectaries close to flowers may lead to competition among ants and pollinators, or even to direct ant–pollinator conflicts. Such antagonistic interactions would reduce the access of pollinators to flowers and, thereby, may cause significant ‘ecological costs’ of indirect, ant-mediated defences (Heil 2002).

Vaccinations are the mainstay of western preventive medicine, and they have been used to protect some crops against disease and insect pests. We consider rice as a model for protection using induced resistance since it is one of the most important staple crops and there have been significant new developments in: cross-resistance among rice insects, chemical pathways involved in induced resistance, sequencing the rice genome and expression of genes conferring resistance against rice insect pests. Insect attack has been found to cause lesions that kill planthopper eggs and early stages of gall midges. Damaged plants released volatiles that made them less likely to be chosen by planthoppers and more attractive to parasitoids. Chemical elicitors have been developed for dicotyledonous plants and these can induce resistance in rice, although rice does not fit models developed to explain signalling in dicots. For example, salicylic acid did not increase in rice after infection by pathogens and did not appear to be the mobile signal for induced resistance against pathogens although it was involved in induced responses to phloem-feeding insects. Jasmonic acid acted as a signal in some induced responses to pathogens as well as chewing insects. Many of the genes associated with induced resistance in rice have recently been mapped, and techniques are being developed to incorporate them into the genome of cultivated varieties. Attempts to control insect pests of rice will affect interactions with pathogens, predators and parasites, and other organisms in this agroecosystem.