Biomass evaluation of concealed insect pests at parasitism based on allometric scaling laws

Abstract

In general, it is difficult to evaluate host resource utilization of parasitoids by examining the biomass of concealed wood borers at parasitism, because the larval tissues of parasitized hosts observed in field surveys are often almost completely consumed. This work explored a precise and convenient method for evaluating the biomass of concealed wood-boring insect pests after they are parasitized. Allometric scaling laws were used to determine mathematical relationships between larval weight and other size variables in two host insects: the emerald ash borer [EAB], Agrilus planipennis Fairmaire, and the oak long-horned beetle [OLB], Massicus raddei (Blessig). These insects are hosts of the parasitoid Sclerodermus pupariae Yang et Yao (Hymenoptera: Bethylidae). Our results showed that body weight among different instars was significantly different for both host species, with greater overlap between neighboring instars. Mean larval weight had a highly significant exponential relationship with instar number. EAB larval weight showed a highly significant power relationship to body length and width, width of the prothoracic plate, peristoma width, urogomphus length, and anteriormost width of the urogomphus. Significant power relationships were also observed between OLB larval weight and body size parameters (including body length, length of the mesothoracic spiracle, width of the prothoracic plate, distance between the main ocelli, and mandible length). These findings indicated that host biomass could be easily calculated if any one size variable could be measured at the time of parasitism. Allometric methods provide a precise means of evaluating time-specific biomass of concealed wood borers.