SOURCES OF VARIATION IN SYSTEMIC UPTAKE BIOASSAYS OF NEONICOTINOID INSECTICIDES

Authors: Castle, Steven; BYRNE, FRANK - UCR, RIVERSIDE, CA; PRABHAKER, NILIMA - UCR, RIVERSIDE, CA; TOSCANO, NICK - UCR,RIVERSIDE, CA

Submitted to: Sweetpotato Whitefly Progress Review Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2002
Publication Date: 6/1/2002
Citation: N/A

Interpretive Summary:

Technical Abstract: Interpretation of insecticide bioassays is predicated on the assumption that all test subjects exposed to a dosage of insecticide actually received that dosage and not one greater or lesser. Mortality at a series of insecticide dosages can then be analyzed statistically and the pertinent statistics for different populations compared to assess relative differences in susceptibility to a particular insecticide. Most insecticides are effective as contact poisons that act either by direct exposure with a contact spray, or by indirect exposure to a residue deposited on a plant or other surface within the insect's environment. Accordingly, bioassays designed to test the susceptibility of a sample population to a contact insecticide have relied upon various devices to ensure equal exposure of test subjects either by topical application or by uniform deposition of a residue. Precise quantities of known concentrations of an insecticide could then be deposited upon a test insect or surface to assess the susceptibility of the sample subjects. Treatment replication helps to compensate for minor variability in test conditions and reduce experimental error. Neonicotinoid insecticides are active both as contact and as ingested insecticides. However, bioassays performed worldwide on imidacloprid, the longest available and most widely used of the neonicotinoids, have relied principally upon systemic uptake of imidacloprid solutions of known concentrations by severed plants or leaves. Interpretation of bioassay results have been made on the basis of the concentration of the imidacloprid solutions only, without regard for how much solution is actually taken up by the sample leaf or small plant. In many laboratories, a disk is punched from the uptake leaf and placed on agar within a petri dish or vial, to which test subjects are added and subsequently evaluated for mortality. In addition to substantial variation in uptake both within and between treatment concentrations of imidacloprid by the severed leaves or plants, variability in the distribution of imidacloprid within leaves can also affect mortality assessment of the test insects. We have measured uptake by detached leaves over a series of concentrations of imidacloprid and thiamethoxam ranging between 1 and 1000 ppm and found that volume uptake by leaves progressively declines with increasing concentrations. This is especially true for imidacloprid, a less soluble compound than thiamethoxam, at concentrations above c. 300 ppm. Variation in volume uptake within a concentration among individual detached leaves can vary by as much as 4-fold. This degree of variability among leaves often is carried over into variable mortality among replicates within a treatment concentration. However, volume uptake by individual leaves tends to be a poor predictor of mortality, suggesting that within-leaf distribution of active ingredients is non-uniform.