Green leaf volatiles

Undamaged neighboring plants have been shown, in some cases, to respond to GLV signals.[3] Both the plant emitting the GLVs and its neighboring plants can enter a primed state in which plants activate their defenses systems more quickly and in a stronger concentration.[7][4]

The first study to clearly demonstrate anti-herbivore defense priming by GLVs focused on corn (Zea mays). Neighboring plants responded to the release of GLVs by priming against insect herbivore attack, reacting more rapidly and releasing greater levels of GLVs.[3][8] Similar results have been shown in tomato plants. Neighboring plants reacted more strongly to GLVs from the plants exposed to the herbivore, by releasing more of the proteins related to the plants' defense mechanisms.[9]

In positive plant-insect interactions, GLVs are used as a form of defense. They attract predators to plants that are being preyed upon by herbivores.[4] For example, female parasitoid wasps from two different families, Microplitis croceipes and Netelia heroica, can be attracted to plants that are emitting GLVs due to wounding from caterpillars.[10] Maize plants emit volatiles to attract the parasitic wasps Cotesia marginiventris and Microplitis rufiventris to attack African cotton leafworm.[11][12] In some species GLVs enhance the attraction of sex pheromones.[4][13] For example, green leaf volatiles have been found to increase the response of tobacco budworm to sex pheromone. Budworm larvae feed on tobacco, cotton, and various flowers and weeds, and in turn can be fed on by the larvae of cohabiting species that are attracted by GLVs.[14]

In another study, a multi-plant relationship was reported. The parasitic wasps (Vespula germanica and V. vulgaris) prey on caterpillar (Pieris brassicae)-infested cabbage leaves that emit GLVs. The same GLVs are emitted by the orchids (Epipactis purpurata and E. helleborine) through pheromone release. The orchids benefit from attracting the wasps, not to protect them from insects, but because the wasps aid in pollination.[15][16]

Benefits of GLV release have also been reported in soybeans grown in Iowa.[17] When these soybean plants became heavily infested by aphids, the amount of GLV released far surpassed normal levels and as a result, more spotted lady beetles were attracted to the pheromone releasing plants and preyed on the bugs eating the plant. The stimulus of aphid predation is chemically transmitted through the plant to coordinate an increase release of GLV’s. The particular chemical released is unique to these spotted lady beetles and when different species of beetles were tested, there wasn’t any extra inclination for them to move towards GLV releasing plants.[17] This indicates that these soybeans evolved ability to release species-specific pheromones to aid in their survival.

GLV release is correlated with fruit ripeness.[18] Although this may be of effect in attracting pollinators, it also can cause issues if these GLV’s attract predators. One such example of this is with boll weevils, as an increase of GLV release when the plants are ripe has been found to increase the predation rate of these beetles.[18]

Another issue with GLV release and increasing predation is with populations that alter GLV emissions from the affected plants. In one case, it was noted that secretions from certain species of caterpillars significantly decrease the effect amount of GLV emissions.[19] In order to determine what was being done to decrease GLV emissions, a study was run on four unique species of caterpillars to measure their effectiveness in decreasing GLV levels released from the predated plant. It’s been found that compounds in the gut and salivary glands, as well as modifications to those compounds in these various species, has been able to mute a large part of the effect of GLV released into the external environment.[19] How this is done is though stopping the flow of pheromone molecules, so they can’t interact with receptors on the leaves of other plants.[19]