The tree secretes sap to attract ants to protect itself against herbivores, leaf-eating insects and other pathogens.
"The presence of mutualistic ants greatly reduces bacterial abundance on surfaces of acacia leaves and has a visibly positive effect on plant health. Study results indicate that symbiotic bacteria colonizing the ants inhibit pathogen growth on the leaves." (Science Daily, 2014)
Another incredible defense mechanism is that acacia trees have ‘bodyguards’ in the form of biting ants. The whistling thorn is a good example of this; at the base of the thorns is a hollow bulb. These bulbs provide shelter for the ants and the tree provides plenty to eat in the form of nectar and sap. The ants return the favour by attacking any herbivore that tries to eat the leaves ( Nassrulla 2016)
Aphids suck tree sap then "defecate" (secrete) sugar-rich liquid which ants love.
In short, aphids provide food to ants, and in return, they get protection from the ants. In the long term, trees are usually weakened as ultimately, the sap is tree blood which is being sucked to dry by these aphids
As this Swollen Thorn Acacia tree can provide everything to the ants, there is no need for aphids.
The Aquilaria trees
When the Aquilaria tree attacked by physical (insects, physical damaged), chemical or biological stress, it triggers the tree's defence mechanism, which results in forming agarwood. In short, agarwood is absent from a healthy tree.
The progression of the darkened zones observed on wounded stem surfaces at different times after wounding. (a) Two days,light yellow; (b) 5 days,light brown; (c) 2 weeks,brown; (d) 4 weeks,dark brown; (e) 6 months,blackish brown; and (f) 9 months,black. The diameter of the hole is 16 mm (red arrow) (Source: Mohamed et al.2014a)
The fungi were detected in high numbers during the first few hours and days after wounding and in low numbers many months later. On the contrary, the darkened zone where agarwood is formed continued to enlarge over time (Fig. 3.2). The accumulation of agarwood compounds at the wounding site could have caused a decline in fungal abundance, which is consistent with its role in defense response. ((Source: Mohamed et al. 2014a)
According to Mohamed, the wound is being healed and the number of fungi which attacked the Aquiaria trees has been decreasing. It makes sense as the Agarwood is ultimately its defense mechanism which drives the diseases away.
The beauty: Once the fight is over, the battle scar (agarwood) is left, and we appreciate it. See the black stripe below?
Without fighting the pathogen, these stripes (agarwood patterns or agarwood formation) would have never been created. These battle scars tell a fantastic story, the story of the winner who fights through and survives. The Glory of the Pain, the Scent of the Victory