Gall-Colonizing Ants and Their Role as Plant Defenders: From ‘Bad Job’ to ‘Useful Service’

Abstract

Galls are neoformed structures on host plant tissues caused by the attack of insects or other organisms. They support different communities of specialized parasitic insects (the gall inducers), and can also provide refuge to other insects, such as moths, beetles and ants, referred to as secondary occupants. This study focuses on galls induced by the oak gall wasp Andricus quercustozae and secondarily colonized by ants in a mixed oak forest. A field survey and two experiments were carried out to a) study ant (species-specific) preferences for different features of the galls, b) describe differences in gall architecture due to ant activity, c) analyse the effects of the presence of gall-dwelling ants on plant health. The results show that there are differences between ant species in gall colonization and in the alteration of gall opening and inner structure. We verified that gall-dwelling ants protect their host plants efficiently, offering them an indirect defence mechanism against enemies (predators and pathogens). The data suggest a new paradigm in ant–plant relationships mediated by the presence of galls on the plants whose ecological and evolutionary implications are discussed.

New Tools for Conservation Biological Control: Testing Ant-Attracting Artificial Nectaries to Employ Ants as Plant Defenders

“…In both cases, trees with a high ant activity were significantly less affected, which may indicate the role of ants in limiting the spread of this epiphytic fungus. Indeed, effects on plants by ant antimicrobial secretions have already been documented in several cases, but the ecological weight of these interactions-not to mention their possible applications in agriculture-is far from being understood and in need of investigation [17,48,79]. The ability of ants to reduce the damage by apple scab (Venturia inequalis (Cooke) G. Winter (1875)) was recently documented [48].…”

“…Only a few studies have dealt with ants in Italian agroecosystems (e.g., [71][72][73][74][75][76][77]). However, native ant species common in Italy and in the Mediterranean basin could be effective control agents against highly problematic pests such as the exotic brown marmorated stink bug Halyomorpha halys (Stål, 1855) [78], fungi and other herbivores [79]. In some cases, common harvester ants may have a positive role in the control of weeds [80].…”

First Report on the Acrobat Ant Crematogaster scutellaris Storing Live Aphids in Its Oak-Gall Nests

“…In the laboratory, all the galls were checked and those displaying physical damage, mold or signs of the presence of other arthropods (e.g., spiders or beetles, detected by the presence of silk or feces near the entrance hole) were excluded from further analyses. Each of the remaining galls was then cut into two halves tracing the height line keeping the gall’s hole approximately in the middle of one of the halves ( Figure 2 ) as in [ 4 ]. The presence of ant queens (Q), workers (W), brood (b) or aphids (a) was checked.…”

“…Later, the trees were isolated with adhesive strips (glue bands, Stocker art. 45118) at the base of the trunk to prevent any successive ant colonization [ 4 , 37 ]. The galls were left on the trees for one year before being retrieved (from September 2016 to October 2017, coinciding with the period of Crematogaster scutellaris (Olivier, 1792) nuptial flights and foundation of new colonies [ 38 ]).…”

“…From the individual gall-inductor perspective, the changes in growth and development that are elicited in the plant are unique, for the most part being species-specific [ 1 , 2 , 3 ]. However, after the hatch and departure of the inductor’s offspring, galls may be occupied by secondary fauna, in particular by ants that can start a close relationship with the host plant, which in turn can gain important advantages from the ants’ presence by gaining protection from phytophagous insects and pathogens [ 4 ].…”