As is frequently noted, the green coffee scale insect is a persistent but relatively benign coffee herbivore, only rarely reaching important pest status, although with clear potential to do so . A casual walk on a coffee farm reveals what appears to be a regulating factor. Searching coffee bushes, one finds scale insects here and there and, importantly, a small species of beetle that is evidently feasting on them . The predatory beetle is Azya orbigera, in the family Coccinelidae. Without a doubt, this observation can easily lead to the conclusion that the relatively rare scale insect is kept under control by the relatively common coccinellid beetle. But a closer look reveals a dramatic variability: Some bushes are very heavily laden with the scale insects, and some have none at all. There is another classical ecological notion that emerges in this system. Surrounding the tree in which an Azteca nest is located is a region containing coffee plants that are routinely patrolled by the Azteca ants that were described above. The ants harvest the sweet secretions the scale insects produce and, in turn, scare away or kill the natural enemies seeking to attack the scales , a well-known mutualism . Because the coffee bushes located near the shade trees that contain Azteca nests are where the scale insect is at least partially protected from the predatory beetle and various parasitoids, this area represents a refuge for the scale insect. It is therefore tempting to conclude that the ant itself is an indirect herbivore on the coffee . Although such is the case at a very local level , because of the complexities induced by the beetle predator, such is not the case at a larger scale. The ants effectively provide an area of high food availability for the beetle. Furthermore, the ants protecting the scale insects also, inadvertently, dutch bucket for tomatoes protect the beetle larvae from its own parasitoids, providing an effective refuge for the beetle as well .
Predator–prey systems that contain a refuge are well studied in theoretical ecology , usually with an emphasis on their stabilizing properties. Expanding our view to a larger spatial scale, we deduce an evident contradiction from easily observable patterns. The scale insects are inevitably eaten by the predatory beetle unless they are protected by the ants. However, the ants cannot provide protection if they have not yet created a foraging pattern at the site where the scales are located. Therefore, the scale insect is unable to form a successful population unless under protection from the ants but is unable to attract the ant protection unless it builds up at least a small population. This pattern is well known in ecology as an Allee effect: An organism cannot form a successful population unless a critical number of individuals first become established, a mechanism generally understood to frequently be involved with the idea of critical transitions. In figure 4, we illustrate the system with a cartoon diagram approximately summarizing a simple population model . On one hand, as the dispersion of scales moves from a position far removed from the refuge toward it, the adult beetle predators that have already located the scales will tend to move with it, until they encounter the protective ants , as is presented in figure 4a. A snapshot at some particular time therefore might look like the pattern in figure 4b. On the other hand, as the dispersion of scales moves from a position within the refuge away from it, the encounter with the beetle predators will not occur until the scales are far removed from the refuge, as is presented in figure 4c. A snapshot at some particular time therefore might look like the pattern in figure 4d. Finally, combining the pattern of figure 4b with that of figure 4d, we obtain the combined graph presented in figure 4e. Note that there is a broad region in which the scales could be very high while at the same time could be very low, effectively depending on where the scales are dispersing from, a structure typically referred to as hysteresis. Selecting 20 different shade trees containing Azteca nests, we examined all coffee bushes within 2 meters of the nest and a number of bushes further removed .
We estimated the activity of Azteca ants on each of the bushes before counting the scale insects, to get an estimate of where the actual refuge was located . Note that the ant activity within 1 meter of the nest was high for almost all bushes surveyed , although positions greater than 1 meter awaty were highly variable, with some bushes having high activity levels and others having none. Further than 4 meters from the nest, ant activity was effectively nonexistant, and bushes further than about 4 meters from the nest were completely out of the refuge. Plotting the number of bushes with a saturated density of scale insects and those with less than 10 scales, we obtain a pattern corresponding quite closely to what is expected from the hysteretic pattern predicted by the theoretical considerations . A further complication enters with a more complete natural history understanding of the beetles and their larvae. Although the adult beetle can fly and therefore forage over long distances for its food source, the larvae are largely restricted to terrestrial movement; that is, they are restricted in space . Female beetles therefore must choose their oviposition sites in such a way that the larvae will mature in an environment that contains a locally abundant food source. One major food source for predatory beetles is the general kinds of insects that are relatively sessile and suck the juices from plants, precisely the characteristics of the green coffee scale. They are easy targets for predators because they are normally slow moving and have few defenses. The problem for a potential predator is that they are very frequently defended by ants, precisely in areas where they are good sources of food for a beetle larva. Consequently, a whole group of beetles has evolved the habit of seeking out ants and ovipositing in areas where ants are abundant and defending the hemipterans. These myrmecophilous beetles must obviously have a strategy of protecting their larvae from the aggressive action of the ants and of enabling oviposition in sites of high ant activity . In the case of the beetle A. orbigera, the larva is covered with waxy filaments that tend to stick in the ants’ mandibles whenever they try to attack it .
But more importantly, female beetles take advantage of an unusual behavioral pattern of the ants in order to oviposit where the scales are abundant . When a phorid fly attacks an ant, that ant exudes a pheromone that effectively says to the other ants in the general vicinity “Look out! Phorids attacking,” and the surrounding sisters all adopt a sort of catatonic posture, heads up, mandibles open, and stationary . Although the phorid is able to detect the alarm pheromones of the ant and is therefore attracted to it, it is unable to actually oviposit on the ant unless it sees some movement . Therefore, not only the ant under potential phorid attack, but also the sisters surrounding her assume this semistationary posture, a result of the very specific pheromone that alerts all ants in the vicinity that a phorid is lurking about. Remarkably, the adult female beetle is able to detect and react to this specific chemical, apparently using it as a cue that the time is propitious to enter into the ant-protected zone to sneak in some ovipositions . Therefore the phorid, in addition to being an important player in the Turing process that forms the basic spatial structure of the system, imposes a trait-mediated indirect interaction , in which the effect of the ant on the beetle is reduced. There is more to this story: first, from simple theoretical considerations and, second, from some evident natural history observations of the system. The theoretical considerations emerge from the knowledge that the refuge is dynamic. That is, past ecological theory has shown that when a prey species is able to retreat from its predator in a fixed refuge space, the basic instabilities of the predator–prey arrangement can be cancelled. But, in the present example, the refuge is effectively a pattern formed by another element in the system , blueberry grow pot the Azteca ant. And the Azteca ant is dynamic in the system, increasing its numbers in proportion to the resources it gains . If the scale insect population increases, there is more food for the ant, and it will therefore make more nests and expand its territory, creating even more refuge area for the scale insect. However, as the ant expands its area of influence , an increasing fraction of the area becomes refuge and, therefore, not available to the adult beetles . At the extreme, there must be some point at which the beetle is unable to find enough prey to continue its population expansion, because almost all of the area would now be a refuge for the scale insect. Therefore, theoretically, the inevitable expansion of the refuge would lead to the eventual local extinction of the beetle predator. It could, of course, be the case that this expected instability of the system does not express itself for diverse reasons or perhaps for an excessively long time. However, purely theoretically, it represents a potential problem for persistence of this control agent.
The theoretical problem is resolved by some very simple natural history observations. A fungal disease, known as the white halo fungus , almost inevitably becomes epizootic , especially when local population densities of the scale insect become large . The fungus can occasionally be found on isolated scale insects, but almost always is most evident when scale insects have built up a significant local population density, and such a buildup can only happen when they are under the protective custody of the Azteca ant.In the end, we see that the Azteca ant plays a key role in the control of this pest. On one hand it protects the scale insect from its adult beetle predator but only in the area of the refuge of the scale, which is defined by the ant itself . On the other hand, it permits the scale insect to build up such large local populations that the white halo fungus frequently becomes epizootic and drives the scale insect to local extinction. It is a curious inverse application of Gause’s traditional competitive exclusion principle, which might be expected to apply between the fungus and the beetle because they share this same food source. It seems unlikely, however, that the scale could be controlled completely by either the beetle or the fungal disease, except in the context of a spatial pattern generated by the Azteca ant . The massive expansion of the ants that might be expected theoretically never happens, partly because of the local effect of the fungal disease and the beetle larvae together reducing the scale insect population locally. Therefore, the dynamic nature of the ant cluster mosaic , always provides a small set of refuges that allows the beetle predator to be maintained throughout the coffee farm. From the point of view of the beetle, it is perhaps ironic that the beetle itself may be involved in the organization of the spatial pattern that is required for its own persistence . There is yet an additional complication. The fungal disease, once it arrives, multiplies extremely rapidly. But, as was noted above, it does not arrive in the first place unless the scale population is large and locally concentrated. Therefore, once the disease gets there, it increases to epidemic levels and wipes out the entire population of scale insects , creating a classical situation of boom and bust and hysteresis in space . Although it is a somewhat complicated argument that has been made in a couple of different ways elsewhere , the disease can clearly generate a locally chaotic dynamic trajectory. Furthermore, as the relevant population gets closer to the ant nest , the oscillations with its disease are expected to be more and more extreme. Eventually, they become so extreme that they transcend the boundaries of a critical value and both scales and disease completely disappear. Note that chaotic trajectories have boundaries , and the equilibrium point at zero is constrained within a basin of attraction.