Ants in obligate defense mutualisms with plants protect them against potentially damaging organisms. In the swollen-thorn acacias, organisms linked to the plant inform about the interaction between the tree and the resident ant colony. Some organisms coexist with the aggressive mutualistic ants: specialized herbivores and organisms using the enemy-free space. Conversely, trees inhabited by non-defending ants usually hold a greater load of generalist herbivores and are avoided by organisms looking for the ant protection. We aimed to elucidate the association type between swollenthorn acacias (Vachellia collinsii) and the almost unstudied Pseudomyrmex simulans ants from Panama. We compared the presence of non-ant organisms on trees inhabited by P. simulans, a well-known mutualist (P. spinicola) and a facultative parasite (non-defending ants; Crematogaster crinosa). We recorded non-ant organisms (e.g., stem galls, acacia true bugs, spiders) that nest, lay eggs, or live on the trees. Except for stem galls, all other non-ant organisms were mostly or exclusively found on trees with the mutualists, which is also the most common resident ant. P. simulans is less able to deter galling midges (Cecidomyiidae) than C. crinosa and even less than P. spinicola, because trees with P. simulans were more likely to have galls and in greater densities than on C. crinosa-trees, and even more than on P. spinicola-inhabited trees. The mechanism by which the Cecidomyiids occur in greater proportion on trees with P. simulans and C. crinosa is still unknown, but the pattern indicates an herbivory specialization or a potentially obligate weaker defender of the swollen-thorn acacias.