Aside from their intended actions, fungicides can drive pest insect outbreaks due to virtually continuous use and pest evolution. Small brown planthopper (SBPH), Laodelphax striatellus, outbreaks occurred recently in many provinces in China, with devastating rice losses. Because exposure to the fungicide jinggangmycin (JGM) increased reproduction of the brown plant hopper, Nilaparvata lugens, via its influence on fatty acid synthase, we posed the hypothesis that JGM and carbendazim (CBM) influence SBPH reproduction via their influence on enzymes involved in other aspects of lipid metabolism. Exposure to the fungicide CBM stimulated SBPH reproduction (egg-laying up by 78%) and to another fungicide, JGM, led to decreased egg-laying (down by 47.3%). These inverse effects are mediated by down-regulated expression of l-3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD) in JGM-treated females and up-regulated expression of hydroxysteroid dehydrogenase-like protein 2-like (HSD) in CBM-treated females. RNAi knockdown of, separately, LCHAD and HSD led to reduced egg-laying (down by 52% for dsLCHAD and by 73% for dsHSD). dsLCHAD, dsHSD, and JGM treatments also led to severely reduced ovarian development in experimental SBPH, with shorted and thinned valvula and lack of egg cells in ovaries. Valvula of CBM-treated females enlarged, with banana-shaped eggs in ovaries. These data strongly support our hypothesis.