JungleWahine, T. ovacus is a leaf gall, so the effected areas will show galls on the leaves (looking like a smaller version of the NOT RELATED galls on most non-native Hibiscus leaves - again these are not at all related - I am only mentioning them for a comparison)
The gall will effect the vigor of the plant, but does not directly effect the fruit, over than the potential of reducing the yield per tree, and most likely will not kill any of the trees here, only reduce the total number of fruit on an effected tree...and potentially slowing the increasing population
From 'Strawberry Guava (Psidium cattleianum) – Prospects for Biological Control' by C. WIKLER1, J. H. PEDROSA-MACEDO, M. D. VITORINO, M. G. CAXAMBÚ, and C. W. SMITH
"LEAF GALL - Tectococcus ovatus (Homoptera, Eriococcidae)
The gall is convex oval on one side of the leaf, and acuminate oval on the other. The
acuminate portion is generally on the upper side of the leaf whether or not that is the abaxial
surface. Occasionally galls may have acuminate or convex forms on both sides of the
leaf. The size of the galls is very variable, depending on the developmental stage and the
sex of the insect, those containing adult males are narrower and more acuminate than
females. The maximum diameter of the gall varies from 0,95 - 7.9 mm on the acuminate
side and 1,55-7,0 mm on the convex side. The depth of the galls from the acuminate tip
to the top of the convex portion varied from 1,80-8,5 mm. The galls are the same colour
as the leaf though the tips are frequently red (Vitorino, 1998)."
ADDED FOR DAYNA, this link is the abstract for a paper that has studied the potential of T. ovatus crossing over to common guava, of you need access to the full paper, I can send you the pdf
http://link.springer.com/article/10.1007...3-3#page-1
This quote from the abstract may help you out:
"In total, 57 plant species representing 21 families were included in the host range tests. First instar nymphs of Tectococcus ovatus Hempel fed on two closely related guava species, Brazilian guava (Psidium friedrichsthalianum O. Berg), and Costa Rican guava (Psidium guineense Sw.). However, none of the nymphs completed their development on these two non-target species"