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Tritrophic interactions, as they relate to plant defense against herbivory, describe the ecological impacts of three trophic levels on each other: the plant, the herbivore, and its natural enemies. They may also be called multitrophic interactions when further trophic levels, such as soil microbes, or hyperparasitoids (higher-order predators), are considered. Tritrophic interactions associate pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals.

Natural enemies such as predators, pathogens, and parasitoids that attack plant-feeding insects can benefit plants by hindering the feeding behavior of harmful insect. It is thought that many plant traits have evolved in response to this mutualism. They do so by making themselves more attractive to natural enemies to protect against excessive herbivory and is considered an indirect plant defense mechanism. Traits attractive to natural enemies can be physical or chemical such as in the case of domatia and nectaries, or in the case of induced plant volatile chemicals used by natural enemies to pinpoint a food source. There are numerous other plant traits that influence the success of natural enemies in controlling herbivores which still require further investigation.

This article will discuss the chemistry by which plants attract natural enemies to kill plant herbivores; it will also illustrate both the alternative morphological means by which plants attract natural enemies and will explain what value understanding these interactions can have for humans.

Chemical mechanisms of enemy attraction[ edit]
Plants produce two classes of chemicals known as primary and secondary metabolites which are also called allelochemicals. Primary metabolites such as monosaccharides, polysaccharides, and nucleic acids are produced by plants to help aid with development. Contrastingly, secondary metabolites do not aid in growth rather they help produce secondary cell wall components such as those produced via amino acid modification. A specific example of secondary metabolites are known as volatiles mediate the interactions between a plant and its environment such as with attracting, repelling, or poisoning insects. In a tritrophic system these volatiles readily escape from plants into the air, and are more efficient to surface chemicals in attracting foraging natural enemies. Furthermore, root volatiles drive tritrophic interactions among below-ground herbivores and their natural enemies. A very small fraction of plant volatiles are detectable by humans resulting in plants like basil, eucalyptus, and pine trees to have their distinctive odors. The mixture and ratios of individual volatiles emitted by a plant is referred to as a volatile profile, (also referred to as synomones in the context of natural enemy attraction). These are plant species specific and are detectable within several meters of the production source. Predators and parasitoids exploit the specificity of volatile profiles to navigate complex chemoattractant signals in their efforts to locate a particular prey species. The production of volatiles is likely to be beneficial given two circumstances: that they are effective in attracting natural enemies and that the natural enemies are effective in removing or impeding herbivores. However, volatile chemicals may not have evolved initially for this purpose, but for within-plant signaling, to attract pollinators, or to repel herbivores that dislike such odors.

Hormone Interactions

Besides chemoattractant defensins, plants have other mechanism they can utilize to defend against herbivory; this mechanism is through hormonal interactions. One hormone that plants can use to increase their resistance against insects is through the interaction of the hormone called Jasminic Acid (JA). JA is found to increase in concentration within a plant when damaged and is also responsible for inducing the transcription of enzymes that are necessary in secondary metabolite production pathways. In addition to its benefits in resistance and transcription, this hormone has been found to aid in the production of defensive proteins such as α - amylase inhibitors as well lectins. Since α - amylase has been known for having hydrolytic properties in its ability to break down starch, these inhibitory proteins prevent insects from properly breaking down starch. Contrastingly, lectins provide their own defense benefits for plants as they interfere with insect nutrient absorption as they bind to carbohydrates.

Interactions with Bacteria

An additional experiment showed that the bacterium E. aerogenes produces the volatile 2,3-butanediol which had impacts on interactions between plants, pathogens, and insects. When maize plants were grown in a soil culture containing the volatile bacterium or inoculated with the bacterium in its plant mass, researchers found that the maize were more resistant to the pathogen Setosphaeria turcica as a significant decrease in neurotic development and hyphae length when compared to bacteria absent maize. Furthermore, researchers found that the bacterium did not deter insect herbivory, rather it increased weight gain and leaf consumption in a caterpillar species known as Spodoptera littoralis . Lastly, when observing the attraction of a natural predator known as Cotesia marginiventris researchers found that these wasps were attracted more readily to maize plants that were grown in a soil culture containing either volatile producing bacterium or pure 2,3-butanediol. These results confirm the idea that soil microorganism does play a role in influencing tritrophic interactions between plants and insects.