Theories of parasite-mediated sexual selection predict a positive association between immune function and the expression of sexually selected ornaments. Few studies, however, have investigated how an immune challenge affects the performance of sexually selected weaponry. Male Wellington tree weta (Hemideina crassidens) (Orthoptera: Anostostomatidae) possess enlarged mandibles that are used as weapons in fights for access to females residing in tree galleries. Intense sexual competition appears to have favoured the evolution of alternative male mating strategies in this species as males have a trimorphic phenotype in which weapon size varies across morphotype: 8^th instar males have the smallest jaws, 10^th instar males have the largest and 9^th instar males being intermediate to the other two. After injecting males with either lipopolysaccharide (LPS; immune challenge) or saline (control), I measured their bite force to assess the functional performance of their jaws. Contrary to prediction no male morph exhibited a significant decline in bite force.