As you would expect, I understand and appreciate regional native flora. And after all, this blog is also much about regional native fauna. Clearly, the stories put together by our NPWG team prove that we can hardly talk about one without being drawn into the vital connections to the other. Ant-plant mutualism is endlessly engaging to me and bigger than one post. So this first installment relates a bit about ants as defenders, with some references that may draw you into the nearest university science library to immerse yourself in really juicy studies.
Pat Sutton’s post about Prunus serotina (wild black cherry) jogged my memory about another fascinating aspect of this species and its relationship to ants. Young leaves of P. serotina are tender and juicy and attract various leaf-eating insects. However, this plant is smart enough to put up a round-about defense to being devoured.
Most of the new leaves have extra-floral nectaries, or glands – near the leaf blade on the petiole. For a few weeks, while the leaves are most vulnerable, the glands secrete nectar that is favored by ants (Formica obscuripes). And ants are very defensive about their food supplies. They patrol the plant and will attack and sometimes kill unlucky predators – mainly young eastern tent caterpillars (Malacosoma americanum Fabricus) – that try to make a meal of the cherry leaf (Tilman 1978). On the other hand, when surviving tent caterpillars are attacked, they regurgitate a fluid that is a serious repellant to ants, because it contains cyanide from eating the cherry leaves (Peterson et al. 1987). When the leaf tissue toughens, it coincides with the time the caterpillars are too large and yucky for ants to mount a successful attack. The ant sentries give up. Conserving the tree’s energy, the glands stop secreting nectar. Some defoliation continues, but exclusion studies have shown that stands of P. serotina have a better survival rate if they are protected by ants during early leaf-out (Tilman 1978). What astounding timing and choreography!
There is at least one interesting case involving plant-ant and insect-ant mutualism. DeVries and Baker (1989) studied the caterpillar of a South American riodinid butterfly (Thisbe irenea). Its food source is a type of Croton that attracts ants by extra-floral nectaries. The ants favor the tree leaves against predators and reap the benefit of nectar. The nectar is also sipped by the very young caterpillars, who are nourished to grow more quickly. This in turn favors the caterpillar to its third instar stage when it (a) develops ant-attracting organs that secrete fluid and (b) begins to eat the leaf tissue. The caterpillar secretions apparently taste better than the plant nectar to the ants, and they begin to tend and protect the caterpillar. You can see the implications for the Croton leaves at that point…. At any rate there must be some evolutionary benefit to all parties.
There are many examples of ants protecting plants from which they derive some benefit – a classic is that of certain acacia trees that have thorns with large bases (check Google for abundant images). Ant colonies not only live inside these hollowed-out thorns but also feed their young from extra-floral nectaries called Beltian bodies at the tips of the acacia leaflets – all in return for protecting the acacia from herbivory (Janzen, 1966). Even fire ants (Solenopsis invicta) are advantageous to some agricultural crops in the South.
Design and management applications of this knowledge about ants are cautionary…soil that supports healthy ant communities (not to mention a huge array of other soil animals) is essential to the web of life. Above-ground features, such as leaf litter and fallen logs, are hospitable to ant nests and should not be “cleaned up” too thoroughly. I’ll write more about this in a post about ants as landscape restorationists.
DeVries, P.J. and Baker, I. (1989) Butterfly Exploitation of an Ant-Plant Mutualism: Adding Insult to Herbivory. Journal of the NY Entomological Soc. 97(3): 332-340.
Janzen, D.H. (1966) Coevolution of mutualism between ants and acacias in Central America. Evolution, 20: 249-275.
Peterson, S.C., Johnson, N.D., and LeGuayader, J.L. (1987) Defensive Regurgitation of Allelochemicals Derived From Host Cyanogenesis By Eastern Tent Caterpillars. Ecology 68 (5): 1268-1272.
Tilman, D. (1978) Cherries, ants and tent caterpillars: Timing of nectar production in relation to susceptibility of caterpillars to ant predation. Ecology 59 (4): 686-692.
© 2012, Ruth Parnall. All rights reserved. This article is the property of Native Plants and Wildlife Gardens. We have received many requests to reprint our work. Our policy is that you are free to use a short excerpt which must give proper credit to the author, and must include a link back to the original post on our site. Please use the contact form above if you have any questions.