2.4. Effect of GHQ (9) on growth and development of H. zea larvae

Since gossypol (1) and related terpenoids are important components in the plant’s arsenal of terpenoids that protect it from herbivorous insects, it seemed appropriate to test the toxicity of GHQ (9) to insects. Thus, racemic gossypol (1) and racemic GHQ (9) were individually incorporated into a soybean artificial diet using the non-nutritive additive, Alphacel, at concentrations of 0.00% (control), 0.06%, 0.12% and 0.18%. To determine if GHQ (9) has a synergistic effect with gossypol (1), diets were also prepared with a total concentration of terpenoids of 0.06%, 0.12% and 0.18%, and a 15.5:84.5 ratio of GHQ (9):gossypol (1). One-day-old H. zea larvae were placed in plastic cups containing the various diets described above. After pupation, the pupae were allowed to harden for one day and then weighed. Days-to-pupation, pupal weights and percent survival to adulthood were recorded.

The mean number days-to-pupation increased as the concentration of gossypol (1) and GHQ (9) increased compared to larvae fed the control diet (Fig. 2). However, the difference in days-to-pupation between the larvae fed the gossypol (1) and GHQ (9) were not significantly different. When the larvae were fed the 15.5:84.5 mixtures of GHQ (9) and gossypol (1), an extension in days-to-pupation was not observed at the 0.06% and 0.12% concentrations. However, at the 0.18% concentration, the days-to-pupation were significantly longer compared to all other diets.

The mean pupal weights (Fig. 3) were significantly less for diets containing 0.12% and 0.18% gossypol (1), GHQ (9), or the 15.5:84.5 mixture of the two terpenoids compared to those fed the control diet. However, pupal weights for larvae fed gossypol (1) were significantly less than those fed an equivalent concentration of GHQ (9). No synergistic effect in reducing pupal weight was observed with the mixture.

The percent survival to adulthood of larvae fed 0.06% gossypol (1) or the 0.06% 15.5:84.5 GHQ (9):gossypol (1) mixture was greater than the control (Fig. 4). This may be due to a hormetic effect (i.e., a biphasic dose–response exhibited by some toxic substances when fed at low concentrations) that has been demonstrated in other gossypol (1) feeding studies (Stipanovic et al., 1986; Celorio-Mancera et al., 2011). Survival of larvae fed 0.06% or 0.12% GHQ (9) or the 15.5:84.5 mixtures were not affected. At 0.18%, gossypol (1) and GHQ (9) reduced survival with gossypol (1) showing mortality>50% compared to larvae fed the control diet. However, the survival to adulthood of larvae fed the 0.18% 15.5:84.5 GHQ (9):gossypol (1) diet was less than one-third those fed the control diet indicating a potential synergistic effect.