Destructive consumption of extrafloral nectaries: An overlooked cost of an indirect defense mechanism

Background/Question/Methods

Many plant species bear extrafloral nectaries (EFN). By attracting predators and parasitoids to the nutritious nectar they secrete, EFN’s form an important line of defense against herbivorous insects. As in many other mutualisms, exploiters sometimes consume extrafloral nectar without giving back beneficial services. The theft of extrafloral nectar has been documented in the past, but we report on a targeted destructive predation of the EFN’s of the plant Vicia faba (fava bean). In field experiments that we conducted with V. faba plants we noticed that many of the EFN’s were eaten and destroyed by insects. We were curious whether this was a targeted consumption of the nectaries themselves or just random feeding damage, and whether the insects that were responsible for damaging the EFN’s were herbivores that may benefit from crippling the plant’s indirect defense mechanism. We collected and scanned damaged EFN-bearing stipules from field and lab experiments. The scanned images were analyzed using computer software. We also collected insects from the experimental plot and tested which of them were capable of doing this type of damage to the EFN’s.

Results/Conclusions

We found that after 8 days in the field, 23% of the stipules on the plants had some degree of feeding damage. Out of those stipules that were damaged, 81% had some degree of damage to the EFN itself and 57% had severe damage to the EFN. A heat map that was created from superimposing all the damaged areas in the stipules revealed that the feeding damage in the field was not random, but targeted to the EFN. Out of all the insects that were collected in the field and kept in cages with V. faba plants, crickets, caterpillars and grasshoppers were capable of making targeted damage to the EFN. Ground crickets (Allonemobius sp.) where the most precise ones, and seemed to have a special preference for the sweet tissue of the nectary. EFN predation is a cost that the plant pays for maintaining an indirect defense mechanism. The fact that the targeted damage is done by insects that are not necessarily herbivores that live on the plant, indicates that the predation of EFN’s is probably not a specific adaptation of the plant’s herbivores.