External immune inhibitory efficiency of external secretions and their metabolic profiling in red palm weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

External secretions play a vital role in
external immune defense. However, the functions and components of these
exudates are largely unknown in the red palm weevil, <i>Rhynchophorus ferrugineus</i> (Olivier) (Coleoptera: Curculionidae). In
order to determine their role in external immunity, the immunosuppressive
efficacy of the secretions <i>in vitro</i> against
microbes, including bacteria and fungi, was clarified. In the present study, we
found that these secretions had antimicrobial activity <i>in vitro</i>, implying external immunizing potency against pathogens. Surprisingly,
all liquid phases of secretions could not significantly inhibit the growth of microbes <i>in vitro</i> compared to solid phases. To explain this phenomenon, the
composition and emission differentia of secretions from the exocrine glands
associated with different developmental stages, secretory regions and phases
were identified and analyzed based on metabonomics technique. A total of more than two hundred compounds, including quinines, phenols, aldehydes,
acids, alcohols, saccharides, ketones, esters,
amines, salts, ureas and heterocycles, were identified in the secretions of larvae and adults. The liquid phase shared a number of metabolites
with the solid phase, but the emission types and amounts were significantly
different in the two phases, resulting in differences in external immunological
activity. Tyrosine
and p-benzoquinone were the dominant metabolites
in all of the secretions, accounting for
approximately 11.29% of emissions, with the portion in the solid phase being
generally higher than the liquid phase. Moreover,
only
p-benzoquinone was entirely significantly up-regulated in the solid phase compared to the liquid phase. Therefore, metabolome analysis suggested
that p-benzoquinone,
which may potentially be developed to be a valuable
marker for determining external immunity, was considered to be the main
substance responsible for external immune functions. This hypothesis was further demonstrated by the antimicrobial activity of p-benzoquinone.