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PLANTS FEED ANTS FOR PROTECTION

Researchers have discovered the means by which plants provide nectar to ants, in return for bodyguard-like protection. 

Nectar is normally produced by plants to attract insects, birds and mammals to help pollinate the flowers. However, some plants have extrafloral nectaries that are not connected to pollination; they are used to attract predatory insects in order to help protect the plant from herbivorous arthropods. But do the predatory insects only receive payment in carbohydrate or are there additional benefits of the relationship?

If a plant needs protection from arthropod herbivores, it makes sense that they have evolved ways to attract predatory insects to act as bodyguards. Furthermore, the more aggressive the predatory insect, the better the level of protection. Extrafloral nectaries, which offer a free carbohydrate feed, would appear to be an attractive food source for such predatory insects. However, attraction is all relative. In an environment where there are plenty of other food sources, the level of attraction provided by extrafloral nectaries may not be sufficient to attract enough predatory insects to keep herbivores at bay.

A group of Brazilian researchers have tested this hypothesis by investigating whether the frequency of attendance by more aggressive ants and the efficacy of plant defence by ants is higher in environments when the general availability of carbohydrates and/or proteins is low.1

By mapping ant activity across various plots varying in available carbohydrate (active extrafloral nectaries) and protein (biomass of soil arthropods), the researchers concluded that neither carbohydrate nor protein availability influenced the probability of interacting with the more aggressive ant species.

However, in a series of experiments where the researchers added the larvae of the peanut beetle, Ulomoides dermestoides, to plants with extrafloral nectaries, they observed that more of these larvae were removed from the plants by the predatory ants in plots with lower soil arthropod biomass. The researchers concluded that overall ant aggressiveness towards other arthropods increases in protein-poor sites. In such situations the benefits of plants having extrafloral nectaries to aid with protection against herbivorous arthropods becomes more obvious.

Extrafloral nectaries on a wild cherry tree

These observations from the natural world provide some good reminders about some of the considerations required during an ant baiting program. Firstly, before starting a baiting program take time to observe what the ants are currently feeding on, how big the food resource is and what other food resources are in the area. Secondly, in choosing a suitable ant bait, it should obviously be attractive to the ant species in question. But if it is different to the food currently available (protein or carbohydrate), it may provide an attractive alternative as a food type that is lacking in their environment.

Most of the leading ant baits are very attractive, but if there are larger and equally attractive food resources elsewhere, it can make it harder to get them to feed. So when pest managers observe an ant species not feeding on a bait that they had previously accepted, it probably isn’t due to a manufacturing problem with the bait. It is most likely down to either a change in feeding preferences due to changing food requirements of the colony and/or the impact of competing food sources in the surrounding area.

1 Felipe C S Passos, Laura C Leal. Protein matters: ants remove herbivores more frequently from extrafloral nectary-bearing plants when habitats are protein poor. Biological Journal of the Linnean Society, 2019; 127 (2): 407 DOI: 10.1093/ biolinnean/blz033.