Impact and Role of Monosodium Methanearsonate on Attack and Progeny Production by the Douglas-Fir Beetle (Coleoptera: Scolytidae) in Lethal Trap Trees

Abstract

Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, trees located in interior British Columbia were baited with frontalin and treated with monosodium methanearsonate (MSMA) on two dates. The first treatment was 2 wk before initial Douglas-fir beetle, Dendroctonus Pseudotsugae Hopkins, flight, and the second coincided with flight. Each application was subjected to three felling regimes (2 and 4 wk, and unfelled) after treatment, as were the controls. MSMA significantly reduced the number of Douglas-fir beetle progeny in all cases but was most effective (greatest attacks, 80% reduction in progeny) in trees treated at the time of initial beetle flight and felled 4 wk later. The unfelled trap trees were least effective (fewest attacks, greatest survival). Samples of phloem and xylem were subjected to arsenic compound (speciation) analysis which identified significant concentrations of inorganic arsenic (arsenate plus arsenite), monomethylarsonic acid (MMAA), and cacodylic acid (DMAA) in treated and untreated trees. The control tree contained mainly inorganic arsenic, but the presence of MMAA and DMAA was consistent with in situ biomethylation. Treatment with MSMA resulted in much higher levels of all arsenicals, with concentrations generally decreasing with height and radially from the phloem to the heartwood boundary. Inorganic arsenic appears to be translocated differently than MSMA, but the elevated levels of inorganic arsenic compounds in the treated tree can be attributed to an impurity in the commercial herbicide. Water content was not correlated with distribution of arsenic. The role of arsenic in the development of lethal trap trees is considered in terms of the translocation of the herbicide, its subsequent chemical fate, and the toxicities of the various arsenic compounds identified in the treated tree. These data strongly suggest that arsenic has a direct insecticidal impact on bark beetle survival.