The expression of antimicrobial peptides (AMPs) as main humoral defense reactions of insects during infection by entomopathogenic nematodes (EPNs) and their symbiont is addressed herein. Three AMPs, attacin, cecropin, and spodoptericin, were evaluated in the fifth instar larvae of Spodoptera exigua Hübner when challenged with Steinernema carpocapsae or Heterorhabditis bacteriophora. The results indicated that attacin was expressed to a greater extent than either cecropin or spodoptericin. While spodoptericin was expressed to a much lesser extent, this AMP was induced against gram-positive bacteria, thus not expressed after penetration of Xenorhabdus nematophila and Photorhabdus luminescens. Attacin and cecropin in the larvae treated with S. carpocapsae at 8 hour post injection (hr pi) attained the maximum expression levels and were 138.42-fold and 65.84-fold greater than those of larvae infected with H. bacteriophora, respectively. Generally, the ability of H. bacteriophora to suppress attacin, cecropin, and spodoptericin was greater than that of S. carpocapsae. According to the results, the expression of AMPs by S. exigua larvae against S. carpocapsae was determined in the 4 statuses of monoxenic nematode, axenic nematode, live symbiotic bacterium, and dead symbiotic bacterium. The expression of attacin in treated larvae with monoxenic nematode and live bacterium at 8 and 2 hr pi, respectively, were increased to the maximum amount. Live X. nematophila was the strongest agent for the suppression of attacin. The expression of cecropin against monoxenic nematode and live symbiotic bacteria at 8 and 4 hr pi, respectively, reached the maximum amount, while the expression levels of attacin and cecropin for axenic nematode were less and stable. The results highlighted that P. luminescens was much more than X. nematophila. The results also showed that the effect of symbiotic bacterium in suppressing attacin and cecropin expression was greater than that of monoxenic nematode; this result provided deep insight into the expression pattern parallels and fluctuations of the main AMPs during nematode infection

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