The expression of antimicrobial peptides by bollworm larvae feeding on maize and cotton from Texas and Mexican fields

Bollworm Helicoverpa zea (Boddie) is a major pest in corn and cotton crops in America; its control by Bacillus thuringiensis (Bt)-crops is about 88% efficient. We analyzed the expression of five antimicrobial peptides (AMPs) among bollworm larvae feeding on different host plants. Our aim was to demonstrate that different hosts, including Bt-crops, may change bollworm AMPs expression as fitness to survive insecticidal compounds. Larvae and plant samples collected from six areas in Mexico and Texas were compared with artificial-diet laboratory feeding larvae, as control. AMPs were analyzed using specific primers designed from Helicoverpa armigera reported sequences, using retro-transcription and semi-quantitative PCRs. Cry1Ab/Cry1Ac (consortium); Cry1F and Cry2A proteins expression in plant samples were analyzed by immunoassays. AMP expression was compared among 2nd, 4th and 5th instar larvae. In general, expression of galiomicin was found in 91.67% of larvae from maize samples, followed by cecropin (83.3%), lysozyme (58.33%), gloverin (41.67%), and PGRP (16.67%). Surviving larvae were only found in one Bt-cotton field in Mexico and one in Texas, were all AMP were expressed by Bt-cotton, whereas larvae from all cotton field did not amplify gloverin. Similarly, gloverin was only amplified in 5th instar larvae after 6 h feeding on cotton (Bt or not Bt), whereas PRGR was only detected in 5th instar larvae, and galiomicin  was not found in 4th instar larvae feeding on regular cotton in laboratory bioassays. AMP immune response and insect resistance to different hosts, Bt products or Bt-crops on insect survival is discussed.