Fitness Costs of Two Maize Lepidopteran Pests Fed on Bacillus thuringiensis (Bt) Diets Enriched with Vitamins A and C

Abstract

Serious malnutrition problems occur in developing countries where people’s diets are mainly based on staple crops. To alleviate this, high-production crops are being developed that are better adapted to climate change, enriched in micronutrients and vitamins, or resistant to pests. In some cases, new varieties have been developed with several of the characteristics mentioned above, such as biofortified and pest-resistant crops. The development of biofortified Bacillus thuringiensis (Bt) crops raises the question of whether vitamin enrichment of Bt crops can in any way favor those pests that are not very susceptible to the Bt toxin that feed on these crops, such as Helicoverpa armigera (Hübner) or Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae). In this study, the response to a Bt diet enriched with vitamins A (β-carotene) and C (ascorbic acid) was somewhat different between the two species. M. unipuncta was less sensitive to the toxin than H. armigera, although the ingestion of the Bt diet resulted in oxidative stress (longer larval development and lower pupal weight) which was not mitigated by the vitamins. However, the two vitamins reduced the mortality of H. armigera larvae fed on a Bt-enriched diet; in addition, ß-carotene reduced the activity of the antioxidant glutathione S-transferase (GST) of both species, suggesting it has an antioxidant role. The results obtained here indicate that biofortified Bt crops will not favor the development of H. armigera very much and will not affect M. unipuncta’s development at all, although the effect of the increase in vitamins may be very variable and should be studied for each specific phytophagous.

Parental effects of Bt toxin and vitamin A on Helicoverpa armigera

“…The results confirmed that feeding on a Bt toxin diet throughout the larval instar prolongs development, producing lighter pupae from the larvae that survive. These findings have also been observed in a previous work on the same species, with a similar procedure (larvae fed on lyophilized Bt diet) [ 8 ] and on other related species, such as H . zea [ 28 ] but with larvae from field conditions, fed on sublethal amounts of the toxin.…”

“…The pupae were maintained at 15°C and 16:8 LD photoperiod to synchronize adult emergence. When the number of females and males was considered sufficient to full a mating cage (6 females and 5 males per cage, [ 8 ]) the pupae were placed in the general rearing conditions in pots with daily moistened soil. Adults (F0) from these pupae were placed in the mating cages (cylindrical methacrylate tubes of 25 cm diameter and 40 cm height, closed at the base with an expanded polystyrene lid, and sealed at the top with white gauze) and given a drinker of honey solution and a cotton ball for egg laying.…”

“…The amount of lyophilized Bt was chosen according to our previous study [ 22 ] in order to obtain a low toxin concentration in the diet (approximately 160 ng Cry 1Ab g -1 fresh weight) in the range of commercial transgenic maize [ 23 ]. The β-carotene concentration was decided according to our previous works [ 6 , 8 ]. The resulting diets ( Fig 1 ) were: diet made with non-Bt leaves (non-Bt), non-Bt leaves with 0.6% β-carotene (non-Bt+β), Bt leaves (Bt), and Bt leaves with 0.6% β-carotene (Bt+β).…”

“…For instance, Lopez et al . [ 8 ] determined that the response to a vitamin A- and C-enriched Bt diet was somewhat different between two secondary maize pests, Helicoverpa armigera (Hübner) and Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae). Ingestion of the Bt diet caused oxidative stress in both species and resulted in longer larval development and lower pupal weight.…”

“…armigera larvae that feed on Bt transgenic maize survives and pupates. These pupae are usually smaller in size and weight than those fed a non-Bt diet [ 8 ]. Because the weight of lepidopteran pupae has been associated with the fertility of the resulting adults [ 18 ], it is conceivable that adults from heavier pupae (larvae fed non-Bt maize) will present with higher fertility than adults from lighter pupae (larvae fed Bt maize).…”