Lepidoptera cannot synthesise α-18:3 and must therefore obtain it from dietary sources. An α-18:3 deficiency has little effect on larval growth, but leads to abnormalities in pupation and adult eclosion. In order to compare the α-18:3 requirements of Heliothis virescens (a generalist) and Heliothis subflexa (a specialist), two artificial diets with varying concentrations of α-18:3 were prepared. Developmental characteristics and the fatty acid content of the adults were monitored. Although H. virescens and H. subflexa responded differently to alterations in α-18:3 content, it remains essential for both species. Thus, in contrast to a previous report, H. subflexa does indeed require dietary α-18:3. Larvae of H. virescens were reared on a range of α-18:3 supplemented diets. There was a positive correlation between the α-18:3 content of the mother and the molar composition of the eggs she produced. This suggests the passive transfer of α-18:3 from females to their eggs. Larvae were started on α-18:3 containing diet and transferred to a deficient diet, or transferred to α-18:3 containing diet after being initially raised on deficient diet. Insects obtaining α-18:3 later in development performed better than those obtaining α-18:3 earlier in development. Although the maternally transferred α-18:3 may be sufficient for the growth of neonates, the α-18:3 acquired during later stages appears to be more important to successful development and pupal emergence. Helicoverpa armigera larvae were reared on the leaves of wild type tomato plants, jasmonic acid (JA) insensitive plants, and plants with reduced levels of α-18:3. On the wild type plants, larvae had a reduced growth rate, and none survived to pupation. Larvae on the JA insensitive plants performed better than those reared on the α-18:3 deficient plants. While decreased α-18:3 contents may offer the plants some protection against herbivory, the inherent JA induced defences are likely to be more effective.