Resistance genes (R genes) are an important part of the plant’s immune system. Among insects, the Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), larva is the target of the greatest number of characterized R genes (H1–H32). The biochemical/molecular mechanism of R gene resistance to Hessian fly is not well understood. In the absence of an effective R gene, larvae caused extensive growth deficits (>30 cm) in wheat seedlings. In the presence of one of three effective R genes, H6, H9, or H13, larvae caused small growth deficits (≈3–4 cm) in two leaves (third and fourth) that were actively growing during the first days of larval attack. After larvae died on R gene plants, the fifth leaf and tiller leaves exhibited small increases in growth (2–4 cm). Growth responses of susceptible and resistant plants diverged at a time when Hessian fly larvae were establishing a nutritive gall tissue at feeding sites. The results of this study support the hypothesis that R gene resistance cannot prevent initial larval attack, but, by stopping the formation of the larval gall, it prevents the most serious consequences of larval attack.