We show that six of eight ant species limit CBB colonization of berries and that the effect of ants is independent of ant activity on branches. This study is the first field experiment to provide evidence that a diverse group of ant species limits the CBB from colonizing coffee berries.To test the effects of each ant on CBB colonization of berries, we performed an ant exclusion experiment. We surveyed bushes occupied by one of the eight target ant species. We excluded coffee bushes with few branches to control for the size of the foraging area of each ant species. On each bush, we searched for two branches of equal age and position and roughly the same number of coffee berries . On each branch, we removed all berries that had CBB entrance holes. We then removed all ants from one branch and applied tangle foot to the base of the branch near the coffee trunk. On the second branch, we left ants to forage freely . To estimate ant activity, we counted the total number of ants foraging on the stem, leaves, and berries of each branch for 1-min including those that travelled onto the branch during the 1-min survey. We also counted ants on exclusion branches after the experiment and if a branch had more than one ant individual present, we excluded the bush from analysis . To release CBB onto control and treatment branches, we created a leaf platform to aid their chances of encountering berries. The leaf was wedged between the branch stem and a cluster of berries to create a platform surrounding the cluster . A coffee leaf was used as a platform because artificial structures attract attention from many ant species. After waiting several minutes to ensure normal ant activity, blueberries in pots we released 20 CBBs on the leaf platforms of the control and exclusion branches.
After 24 h, we counted the number of berries per branch that had CBBs inside entrance holes. We did not count partially bored holes in berries, nor CBBs that had bored into twigs and leaves. Multiple bored entrance holes per berry were only counted as one bored berry. We modified the experiment slightly for P. simplex and P. ejectus because of the difficulty in locating these species within a bush using visual cues . For these two species, we used the living branch to which the nest was attached to as the control branch . This was done because we wanted to make sure that ants were actively foraging on control branches after the disturbance of removing nests. To statistically analyze experimental data, we opted to use linear mixed models instead of paired t tests because mixed models allow inclusions of experimental non-independencies through the incorporation of covariates. We included bush as a random effect in the model to pair control and exclusion branches within each bush. Ant species and treatment and the species 9 treatment interaction were included as fixed effects in the model. To control for differences between each branch and bush, we included the number of berries per branch, the number of berries in contact with the leaf platform, and the number of worker ants per branch as covariates in the model. We performed type III F tests of significance for main effects with maximum likelihood to estimate the fixed effect parameters and variance of random effects . We removed non-significant factors from models and compared nested and null models with likelihood ratio tests to determine the best-fit model. We also compared ant activity across different species to determine if this factor might correlate with berries bored and vary across ant species.
To determine if ant activity correlated with the number of coffee berries bored, we limited the dataset to only control branches and used a generalized linear model with a Poisson log-link function because data did not meet the assumptions of normality. To determine if ant activity varied by species, we again limited the dataset to only control branches and used ANOVA with Tukey’s HSD analysis. We tested the normality of the data with qq-plots and Kolmogorov–Smirnov tests of model residuals. We conducted all statistical analyses with SPSS .Our study represents one of the first field experiments showing that a broad survey of ants reduce colonization of coffee berries by the CBB. This is in contrast to previous studies that suggest ants may not have any effects on CBB, especially in field experiments . Our results are in accordance with other observational studies that show that specific ant species may limit CBB in coffee plantations, yet these studies have either focused on the most dominant or abundant species observed or investigated the broad community-wide impacts of ants on the CBB . Our experimental approach is limited to our understanding of how ants control CBB colonization of berries and not other life stages of the CBB. Our study suggests that ant occupation of coffee bushes is very important during a seasonal period when new coffee berries develop and the CBB begins to disperse from old infested berries to developing un-infested berries . It is surprising that Crematogaster spp. and S. picea did not limit the colonization of berries, considering that other studies have shown species within these two genera have important effects on herbivores .
Low ant activity on coffee bushes with Crematogaster spp. or S. picea cannot explain these results because thesespecies had greater activity per branch than P. ejectus and P. simplex and equivalent activity to A. instabilis and P. synanthropica, species that did limit CBB damage. One explanation could be that because we grouped five Crematogaster spp. together into a single treatment, effects of individual species may be masked. Solenopsis picea may have an effect on CBB colonization, but only with higher ant activity or when CBB are in closer proximity to nest entrances. This species also has a small body size and moves relatively slowly in comparison to the species that did have an effect, which might have limited it from removing or easily capturing CBBs. Wasmannia auropunctata is of similar size to S. picea and still had strong effects on CBB. However, W. auropunctata had significantly higher ant activity on branches as compared to S. picea. Perhaps the combination of low activity, small body size, and slower movement limited S. picea from affecting the CBB. While we found no effect of S. picea on CBB colonization of berries, it may be that S. picea, and other smaller ants, have important impacts on the CBB at other stages of the CBB life cycle because they can pass into entrance holes of the CBB . Experiments with both P. simplex and P. ejectus employed slightly different methodologies than the other ant species, which may have intensified the effect of these ants. For these two species, hollow twigs that contained ants were attached to a branch with berries and this branch was used as the control branch in the experiment. This likely elevated the number of ants per branch per minute. However, in the lab, P. simplex had similar effects on the CBB . Additionally these two species had the lowest densities on control branches of all other species, averaging 3.6 and 3.7 ants per branch for P. ejectus and P. simplex, respectively. Thus, these species have effects at very low numbers, and the results of this study should only pertain to branches for which the density of these species reaches this mark. Certain aggressive ants that limit CBB colonization of berries might also benefit CBB after colonization. Larger ants cannot enter berries, but if they are aggressive competitors for space, square plant pots they will prevent other ants from occupying the branches they patrol . These ants, likely A. instabilis and P. synanthropica, may provide CBB with enemy free space after the CBBs colonize berries in their territories. In conclusion, we find that six of eight ant species limited CBB colonization of coffee berries suggesting that ants, generally, provide important pest control services within coffee agroecosystems. This is the first field experiment to demonstrate general ant limitation of CBB colonization. This finding is important considering that chemical pesticides are thought to be ineffective at controlling the CBB . Nonetheless, ants do not completely control the CBB, other control agents like birds, parasitoids, and fungal pathogens also aid in the control of the CBB . Further work should look at larger scale impacts of ants on the CBB, such as farm scale impacts. Also, more theoretical work is needed to understand how ants impact the CBB at different stages of its life cycle and to reveal which stage of the life cycle is most important for population regulation. Nonetheless, this study provides strong evidence that ants defend coffee from CBB colonization.
Seminal work by Thouless and coworkers pointed out that band insulators are not identical, but can differ in fundamental respects, that are characterized by a topological property of the bands. The central example discussed was the integer quantum Hall state, whose topological properties are characterized by an integer which is essentially the Hall conductance. Realizing such a state naturally requires breaking of time reversal symmetry, typically by the application of a strong magnetic field on a two-dimensional system. The topological nature of the integer quantum Hall state is also revealed by studying the edge of a two-dimensional sample, where chiral edge states occur at energies within the bulk energy gap. Recently, it has been realized that band insulators with spin orbit interactions can also be characterized by their band topology. In two dimensions, the quantum spin hall phase is closely analogous to the quantum Hall state. However, since it preserves time reversal symmetry, it has a pair of counter-propagating one-dimensional modes at the edge. Such a state can occur with SOIs that preserve spin rotation symmetry about an axis. It was shown in Ref., that even in the absence of such spin rotation invariance, the counter-propagating modes remain protected by time reversal symmetry. The topological property of these insulators are characterized, not by an integer, but by aZ2 number, so that all topologically non-trivial insulators of this kind fall within the same topological class. An experimental realization of this phase has been reported in HgTe heterostructures. Turing to three dimensions, an insulator with nontrivial band topology can be realized just by stacking such two-dimensional QSH states. These are called the weak topological insulators . However, a more surprising possibility, the strong topological insulator , has been predicted theoretically. Once again, the surface physics is exotic, which provides a physical characterization of this phase. STIs have an odd number of Dirac nodes on their surface, which are stable against moderate perturbations that preserve time reversal symmetry. Such a band structure cannot be realized in any two-dimensional system with time reversal invariance. There have been experimental realizations of these predictions in bismuth antimony and in bismuth selenium, which have been verified by angle resolved photoemission spectroscopy. Note, in contrast to the QSH state, in order to realize the STI the SRS must be completely broken. The topological insulator and QSH phases normally exist in systems with strong SOI that explicitly breaks SRS. However, as pointed out in Ref.an extended Hubbard model on a two-dimensional honeycomb lattice can have spontaneous SRS breaking and result in a QSH phase, with the right kind of repulsive interactions. SRS is only preserved about an axis ˆn, which is spontaneously chosen, leading to gapless Goldstone modes. This was termed a topological Mott insulator – the separation of energy scales between the low lying magnetic excitations and the gapped charge excitations being typical of Mott insulators. We will also adopt this nomenclature although it must be noted that local moment physics, often associated with Mott insulators, does not occur here. Subsequently, it was argued in Ref. that skyrmions of ˆn carry charge 2e. Here, we consider the analogous problem of a three-dimensional system without bare spin orbit couplings, and full SRS, being driven into a TI state by strong interactions. The key difference from the two-dimensional case, is that in order to realize the STI, SRS must be completely broken. Hence the order parameter in this case is a rotation matrix ←→R ∈ O, similar to super fluid Helium-3 A and B-phases. Physically, this order parameter describes the orientation of the spin coordinate system, relative to the spatial coordinates. Spatial variations of the order parameter lead to a rich set of topological textures.