The possibility of exploiting differential ant attendance to generate new intercropping or selected plants combinations represents a practical opportunity to increase the resilience of cultivated systems. In this study we confirmed the initial prediction, i.e. that the supplement of a specific plant within a perennial cropping system would divert ant attendance from an aphid attacking the perennial crop to an aphid feeding on the newly introduced plant. The effect of intercropping on ant was initially measured in the greenhouse and subsequently confirmed in a full field setting. In a previous study, intercropping was associated with season-long reductions in ant activity in a pecan tree orchard21, although influence on a specific pest was not shown.
Previous studies showed a relationship between reduction of ant attendance, increase of antagonists in colonies, and drop in aphid populations in apple and other cropping systems. Nagy et al.38 used different artificial feeders and sugars attaining a reduction of D. plantaginea by reducing L. niger attendance in all cases. They reported the highest ant diversion and aphid reduction for sucrose feeders attached to the tree trunk and argued that proximity to the ant nest, as opposed to feeders on the branches, may increase diversion. A similar effect was observed for the mutualism between Aphis spiraecola Patch and Lasius grandis Forel39. A solution comprising different sugars decreased attendance and aphid populations. However, an increase in antagonists was only noticeable when feeders were placed on branches. Our results, using a sentinel colony methodology, did not evidence a significantly higher biological control in terms of colony reduction over time. Nonetheless, colonies remained active in the control treatment the last two weeks of assessment whilst they were almost eradicated in the presence of bean plants (Fig. 5). The same active colonies grew larger over time under ant-attendance. A pronounced drop in colonies during the first two weeks in both treatments can probably explain the lack of differences observed. A high predator population, in particular of Anthocoris nemorum (L.), that peaks at this time of the year, could have outcompeted L. niger in the location of the small aphid colonies after clipcage removal. This could have partially prevented the establishment of mutualism in the control treatment in our field setting. Porcel et al.18 showed the efficiency of this natural enemy to quickly locate D. plantaginea colonies that are not under ant-attendance protection. Additionally, it is possible that ants had already established trails to track naturally occurring colonies at the time of year the experiment was carried out. Such trails could have deterred the presence of antagonists in the apple + bean treatment diluting the treatment effect. Given the complexity at the field level, further research should evaluate the population of natural enemies, the effect of temperature and the possible influence of already established non-inoculated aphids on aphid-ant-natural enemies interaction.
Fischer et al.22 showed that there is a competition for the mutualistic services of L. niger between the aphid A. fabae, Metopeurum fuscoviride Stroyan and Brachycaudus cardui (L.) utilizing the same plant Tanacetum vulgare. While M. fuscoviride was the most attended species, A. fabae was the least intensively attended. Authors argued that qualitative and quantitative differences in honeydew determine the aphid’s selective attractiveness to ants, with the least competitive species being predated by L. niger. Although in our experiments with sentinel colonies we did not observe predation by ants neither upon A. fabae nor upon D. plantaginea (data not shown), the spatial isolation of the species may have played a role in moderating such competition. Probably, L. niger simply visited more intensively A. fabae to optimize the rate of honeydew harvest, made possible by a shorter distance from the nest compared to D. plantaginea23. As an alternative explanation, L. niger may prefer bean over apple due to the presence of extra floral nectaries.
Offenberg24 observed an increased predation on A. fabae when artificial nectaries were offered to L. niger, whereas alternative prey had no significant effect. Consequently, a higher density of V. faba in the orchard or the addition of artificial nectaries may restore the balance. The shift of ants from extrafloral nectaries to aphid colonies seems to occur when the total reward from aphids will exceed that from extrafloral nectaries25.
The chemical analysis of the honeydew highlighted a rather specific profile for the two aphid species, with a higher content of the trisaccharides melezitose and raffinose but a lower presence of 1 out of 17 amino acids (asparagine) in D. plantaginea’s honeydew. This result is in contrast with Woodring et al.26, who found that aphid species with the highest total amino acid concentration in the honeydew always had the highest concentration of sugars. It seems however, that the content of specific saccharides plays a major role in ant attraction. Woodring et al.27 discussed that the sugar composition of the honeydew of the ant-attended M. fuscoviride indicated a rapid digestion of sucrose into glucose and fructose, and the simultaneous synthesis of considerable amounts of melezitose and some trehalose. Melezitose, a trisaccharide synthetized by aphids, has been reported to be used by ant scouts as a cue indicative of a long-lasting productive resource for collective exploitation and defence against competitors or aphid predators28. Trail mark laying was exclusively triggered by raffinose, sucrose and melezitose. A plant-specific honeydew production seems to be common in polyphagous species such as A. fabae, where the quality of the plant directly affects honeydew composition and indirectly ant attendance30. In our experiment, an increased preference for A. fabae could perhaps have been achieved by selecting plants with a higher release of melezitose, because A. fabae from E. europaeus produces a honeydew low in this compound29,30. However, A. fabae was more tended than D. plantaginea in both greenhouse and field experiments in spite of releasing less melezitose than D. plantaginea. The role of melezitose in the establishment of the mutualism is not entirely clear for A. fabae, because low-producing clones manage to attain high frequency of attendance31. Perhaps also the quality and the density of extrafloral nectaries on V. faba as well as the size of the colony and the related amount of produced honeydew need to be taken into account when evaluating the efficacy of ant diversion in apple-bean intercropping32.
It is nonetheless intriguing that in both our greenhouse and field experiments, A. fabae attracted a large majority of L. niger, disregarding the lower content of raffinose and sucrose and the complete absence of melezitose. The higher level of the sugar alcohol sorbitol in D. plantaginea’s honeydew is somehow expected, as it is often found in the apple phloem. The carbocyclic sugar myo-inositol has not previously been retrieved from aphid honeydew and it seems to be related to glucose metabolism and cold tolerance in hemipterans33,34.
The amino acid composition is reported as a relevant factor in the establishment of ant-aphid mutualism35. Whilst amino acid analyses of A. fabae honeydew are available in the literature, no information could be found on D. plantaginea’s honeydew profile. In our study we retrieved 6 major, 5 minor and 6 “in traces” amino acids. Two of the major ones, asparagine and isoleucine, differed substantially between the aphid species. Schillewaert et al.29 reported arginine, asparagine and glycine as major amino acids in A. fabae, whereas isoleucine and 13 other amino acids were detected at an intermediate or lower content. Although the lack of information on the possible effect of specific amino acids such as asparagine and isoleucine on ant preference does not allow us to interpret further our result, it appears that ant preference falls under the category of context-dependent behaviours. Because L. niger did not discriminate between A. fabae and D. plantaginea honeydew mimics in our laboratory and field assays, we argue that the result observed in the intercropping experiment is mainly driven by other factors than honeydew contents. It is possible that a combination of factors such as honeydew profile, aphid colony growth and distance of the colony from the ant nest may affect the final outcome of the mutualism. In addition, plant architecture may shape the distance between colonies, which could in turn affect the foraging activity of ants36. Because of plant quality in deciduous trees may change with phenological development, the ant-aphid interaction observed in our study could be differentially shaped at a later stage in the season.
Recent studies showed that L. niger scouts were attracted not only to the complete plant system and honeydew, but also to the microorganisms in the absence of plant or honeydew; more specifically to a bacterium from A. fabae’s honeydew. Staphylococcus xylosus emits a blend of volatiles that attract ant scouts37. In the present study we did not examine the possible effect of honeydew volatiles on ant attendance. However, it might be relevant to measure such attraction in order to shed additional light on the factors regulating the increased preference of L. niger towards A. fabae.
An aspect to consider is that the intensity of attendance reduction observed in Nagy et al.38 and Wackers et al.39, by using artificial sucrose feeders, is comparable to what we report in this study (Fig. 4). This leads us to infer that, under certain circumstances, equivalent biological control increases may be obtained with the natural system that we present here.
In this study we demonstrated the potential of intercropping an aphid–bearing plant in orchards as a method to reduce ant attendance in aphid colonies in order to increase biological control. Future studies should clarify whether the same effect can be achieved by only sowing beans without inoculating the aphid. Additionally, alternative companion plants and aphids would be needed to increase diversity and provide a more robust system as aphid population can vary between years. We conclude that the concept of using a plant capable of diverting ant attendance from the main cultivated crops holds the potential to be included in conservation biological control of horticultural crops as a bottom-up approach to enhance the action of natural enemies. Together with other ecosystem-based measures, intercropping should be thus evaluated when planning ecosystem redesign to decrease the use of synthetic inputs such as pesticides.
Source: Ecology - nature.com
