Midgut juice plays an important role in food digestion and detoxification in insects.  In order to understand the potential of midgut juice of Cnaphalocrocis medinalis (Guenée) to degrade Bt proteins, the enzymatic activity of midgut juice and its degradation of Bt proteins (Cry2A, Cry1C, Cry1Aa, and Cry1Ac) were evaluated in this study through protease inhibitor treatments.  The activities of total protease in midgut juices were significantly inhibited by phenylmethylsulfonyl fluoride (PMSF), tosyl-L-lysine chloromethyl ketone (TLCK), pepstatin A and leupeptin.  The enzymatic activity of chymotrypsin was significantly inhibited by PMSF, and enzymatic activity of trypsin was significantly inhibited by ethylenediaminetetraacetic acid (EDTA), PMSF, tosyl phenylalanine chloromethyl ketone (TPCK), TLCK and trans-epoxysuccinyl-L-leucylamido-(4-guanidino) butane (E-64).  EDTA could significantly inhibit the degradation of Cry2A by C. medinalis.  EDTA, PMSF, TPCK, and TLCK could inhibit the degradation of Cry1C and Cry1Aa.  EDTA, PMSF, TPCK, TLCK, and E-64 could inhibit the degradation of Cry1Ac.  Our results indicated that some protease inhibitors hindered various enzymatic activities in the larval midgut of C. medinalis, which may reduce the insect’s ability to degrade Bt toxins.  These findings may aid the application of protease inhibitors in the management of this insect pest in the future.

Midgut extracts play crucial roles in food digestion and detoxification.  We evaluated the effect of pH on the profiles of the midgut extracts in rice leaffolder, Cnaphalocrocis medinalis and the degradation of activated Bt-toxins by the midgut extracts under different pH conditions.  Total protease activity increased slightly with the increase with the simulated pH in the midgut extracts and the maximal protease activity was observed at pH 10.5, while an upward trend was observed as the pH of reaction buffer increased.  Activity of chymotrypsin-like enzymes increased with pH, both in the buffer and midgut extracts, while the activity of trypsin-like enzyme was unaffected.  Degradation of the activated Cry2A by the midgut extracts enhanced as the pH increased.  Cry2A was fully degraded into smaller segments at pH 9.0–10.5.  Activated Cry1C protein at pH 9.0–10.5 was partially degraded by the midgut extracts.  Activated Cry1Aa and Cry1Ac were partially degraded into fragments by the midgut extracts at high pH.  These results will facilitate our further understanding of the interactions between C. medinalis and the Cry toxin.

 

Rice is an important staple food in China and it is at risk of attack by rice striped stem borer Chilo suppressalis, which occurs in most rice growing areas.  In recent years, severe C. suppressalis outbreaks have been observed in China mainly due to changes in the rice cultivation systems, wide adoption of hybrid varieties and resistance to the dominant insecticides.  Management relies primarily on chemical insecticides and resistance is an important contributing factor in these outbreaks.  As a result, food safety of agricultural produce is reduced and the ecological and environmental integrities are threatened as well.  Recently, environmentally friendly pest management measures, such as trap plants have been introduced for C. suppressalis management and this method can greatly reduce insecticide use.  Our previous results indicated that the vetiver grass (Vetiveria zizanioides) is a dead-end trap plant that can effectively attract the adult females of C. suppressalis to lay eggs on it but where larvae are unable to complete their life cycle.  This paper further explored the application of vetiver grass as a trap plant to manage C. suppressalis in the paddy fields.  This environment-friendly tool can not only reduce C. suppressalis populations, it can also increase the diversity and abundance of natural enemies that can provide better environmental conditions for rice production.