Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 23, 2008
Publication Date: October 8, 2008
Citation: Stipanovic, R.D., Lopez, J., Dowd, M.K., Puckhaber, L.S., Duke, S.E. 2008. Effect of racemic, (+)- and (-)-gossypol on the survival and development of the specialist herbivore Heliothis virescens (Fabricius). Environmental Entomology. 37:1081-1085. Interpretive Summary: Cottonseed is a nutritious source of protein. However, it cannot be fed to animals such as chickens and pigs because the seed naturally contains a toxic compound called gossypol. Gossypol occurs in two forms that are referred to as the (+)-form and the (-)-form. The (+)-form of gossypol is not toxic to animals such as chickens and pigs. The seed from most commercial cottons grown in the United States have a mixture of from 55 to 65% of the (+)-form and 35 to 45% of the (-)-form. Gossypol also occurs in the leaves and cotton bolls. It helps protect the plant from attack by insects. In fact, plants that do not have gossypol in the leaves and cotton bolls cannot be grown because of aggressive feeding by various insects. There is a type of cotton plant from Brazil whose seed contains less than 5% of the (-)-form of gossypol. Seed from these plants can be fed to chickens and pigs. However, it is not known if insects would aggressively attack a plant that made mostly the (+)-form of gossypol in the leaves and cotton bolls. To answer this question, we studied the young worms of an insect called Heliothis virescens that normally feeds on cotton plants. We allowed the worms to feed on a diet that contained various amounts of either the (+)-form, or the (-)-form of gossypol, or one that had an equal mixture of the two forms. We allowed the worms to go through their life cycle until they formed a cocoon. We measured how many worms lived to that stage, how long it took them to reach that stage, and how much the cocoon weighed. Among the three diets, worms were less likely to survive on diets containing either the (+)-form or the (-)-form compared to the diets with an equal mixture of the two forms. The differences between the three diets were small with respect to the time it took to reach the cocoon stage and the cocoon weight. These results suggest that cotton breeding programs that increase the (+)- to (-)-gossypol ratio in foliage will not significantly affect susceptibility to such insect pests, and such plants could be grown successfully in the United States.
Technical Abstract: Gossypol is a constituent of the lysigenous foliar glands of cotton and is also found in glands in the cottonseed. Gossypol protects the plant from insect herbivores. Gossypol exist as enantiomers due to restricted rotation around the binaphthyl bond. The biological activities of the enantiomers differ. For example, experiments show that (+)-gossypol can be safely fed to non-ruminants such as chickens. Most commercial cottonseed contains a (+)- to (-)-gossypol ratio of approximately 3:2. Efforts are underway to develop cottonseed that contains greater than 95% (+)-gossypol. However, the effect of (+)-gossypol on herbivorous specialist insect pests of cotton is unknown. In the current work, three levels (0.16%, 0.24% and 0.32%) of racemic, (+)- and (-)-gossypol were fed to larvae of the specialist herbivorous insect Heliothis virescens. All 0.24% and 0.32% gossypol diets significantly lengthened days-to-pupation and decreased pupal weight compared to the control. Percent survival was significantly less for larvae raised on the (+)- and (-)-gossypol diets containing 0.24% and 0.32% gossypol as compared to the control, but percent survival was not significantly different for the racemic diets. Notably, for purposes of this study, (+)-gossypol was superior or equivalent to racemic gossypol as measured by the three parameters studied. As expected, higher concentrations of all gossypol diets were required to reduce survival and pupal weights and increase days-to-pupation for larvae of the specialist herbivore H. virescens compared to the generalist Helicoverpa zea. The results reported here and those from our earlier study with H. zea suggest that cotton breeding programs that increase the (+)- to (-)-gossypol ratio in foliage will not significantly affect susceptibility to either generalist or specialist insect herbivores.