Herbivory response of Murgantia histrionica to a Ni-hyperaccumulator, Alyssum murale

Background/Question/Methods

Alyssum murale (Brassicaceae) is a nickel-hyperaccumulator native to the Mediterranean region that is generally found on serpentine soils derived from ultramafic rock. Many studies have shown that hyperaccumulation of heavy metals is advantageous in reducing feeding by insects and other herbivores. Alyssum murale also produces glucosinolates as a chemical defense, as well as physical defenses (trichomes) on leaves, stems, flowers, and fruits. Murgantia histrionica (harlequin bug) is an aposematic stinkbug native to Mexico and Central America that is currently invading North America. Murgantia primarily feeds on plants in the Brassicaceae (mustard) family. This study explored two questions: Does variation in nickel levels prevent or deter herbivory by Murgantia on Alyssum murale ? and Does Murgantia show preference for stem vs leaf feeding on Alyssum murale? Alyssum seeds were germinated and grown in four soil nickel treatments (0 ppm, 100 ppm, 500 ppm, and 2000 ppm). Soil treatments were prepared using a 1:1 ratio of nickel acetate: nickel sulfate in commercial soil mix. Thirty field-captured Murgantiaindividuals were starved 24 hours before each feeding trial. Each trial consisted of releasing one individual into a cardboard feeding arena containing one plant from each soil nickel level. 

Results/Conclusions

Murgantia visited Alyssum plants in the control soil treatment significantly more often than plants in treatments with nickel. This suggests that Murgantia can discriminate against plants that sequester nickel in their aboveground tissue. However once the insect arrived on a plant, the duration of its visit did not differ significantly in different treatments. We found no significant difference in time spent “feeding” or “not feeding” when Murgantia visited plants grown in different levels of soil nickel. Thus, although Murgantia shows a preference for visiting Alyssum plants grown on substrates without nickel, it appears that high nickel levels do not prevent herbivory. An “elemental defense” with a toxic metal may not be as effective with herbivores that use a piercing-sucking mode rather than chewing leaves. Murgantia also showed a significant preference for stem feeding rather than leaf feeding across all treatments. The preference for stem feeding may relate to physical defense, if trichome density on leaves is greater than stems. Further studies should investigate responses of a wider diversity of herbivores to determine the importance of elemental, organic, and physical defenses of metal-hyperaccumulators in natural communities. Food-chain transfer of metals from herbivores to higher trophic levels also deserves further exploration.