Submitted to: Journal of Chemical Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/23/2006
Publication Date: 5/19/2006
Citation: Stipanovic, R.D., Lopez, J., Dowd, M.K., Puckhaber, L.S., Duke, S.E. 2006. Effect of racemic and (+)- and (-)-gossypol on the survival and development of Helicoverpa zea larvae. Journal of Chemical Ecology. 32(5):959-968. 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 commercial cottons grown in the United States has 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 and helps protect the plant from insect attack. In fact, plants that do not have gossypol in the leaves and cotton bolls cannot be grown because of heavy feeding by various insects. There is a type of wild cotton plant from Brazil whose seed contains almost none of the (-)-form of gossypol (less than 4%). Seed from these plants can be fed to chickens and pigs. However, it is not known if insects would heavily attack plants that contain mostly the (+)-form of gossypol in the leaves and cotton bolls. To answer this question, we studied the young worms of an insect called Helicoverpa zea that normally feed 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, there were no real differences between the time it took to reach the cocoon stage, and how much the cocoon weighed. The only real observable difference was in the number of worms that survived to form a cocoon, and these differences in survival were very small. We found that the diet with the mixtures of forms most effectively reduced worm survival. This was followed by the (-)-form. The (+)-form was the least effective. However, the differences between all diets were very small. Thus, it appears that cotton plants that had high levels of the (+)-form of gossypol in the seed, leaves and cotton bolls would not be more susceptible to attack by insects, and they could be grown successfully in the United States.
Technical Abstract: Gossypol is a sesquiterpene that occurs naturally in seed and other parts of the cotton plant. Because of restricted rotation around the binaphthyl bond, it occurs naturally as an enantiomeric mixture in (+)- to (-)-enantiomeric ratios that vary from 97:3 to 31:69 with commercial Upland varieties normally exhibiting an approximate 60:40 ratio. The (-)-enantiomer is significantly more toxic than the (+)-enantiomer to non-ruminant animals. Thus, cottonseed containing high levels of (+)-gossypol might be safely fed to non-ruminants. However, gossypol is an important component in the cotton plant’s defense against insect herbivores, and it is not known how cotton plants that exhibit only high levels of (+)-gossypol might be affected by insect herbivory. To answer this question, one-day-old Helicoverpa zea larvae were fed diets with 0.16%, 0.20% and 0.24% racemic, (+)- and (-)-gossypol. Larval pupal weights, days-to-pupation and survival were all adversely affected by all gossypol diets compared with the control diet. Statistical differences were determined by comparing the compounds among themselves at the three doses and between racemic and (+)- or (-)-gossypol at the same level. Within these parameters, no differences in pupal weights were observed except for the following. Pupae weighed significantly less for larvae raised on the 0.24% (+)-gossypol diet versus the 0.20% and 0.16% (-)-gossypol, and the same was true for pupae from larvae raised on the 0.24% (+)-gossypol diet versus the 0.16% (+)-gossypol diet. These differences were significant at P<0.001. Among the various gossypol diets, moderately significant different delays in pupation were noted between (-)-gossypol at the 0.16% and 0.24% levels (P=0.03), and between the 0.20% and 0.24% levels (P=0.02), and the (+)-gossypol diets between the 0.16% and 0.24% levels (P=0.06). The diet most effective at reducing survival was the diet containing racemic gossypol followed by the diet containing (-)-gossypol. These differences although significant, were small. The results indicate that (+)-gossypol is inhibitory to H. zea larvae, and cotton plants containing predominately this enantiomer in foliage should maintain significant defense against insect herbivory.