Association of declining red pine stands with reduced populations of bark beetle predators, seasonal increases in root colonizing insects, and incidence of root pathogens

Abstract

Declining red pine, Pinus resinosa (Aitman), stands in the Great Lakes region appear to arise from a complex of biotic and abiotic factors. We monitored stem and root colonizing beetles in declining and healthy plantations in Wisconsin from 1997 to 1999. We also conducted systematic field excavations and laboratory isolations of various root pathogens. Multiple funnel traps were baited with the synthetic aggregation pheromones of the two most common bark beetle (Coleoptera: Scolytidae) species in the region, Ips pini (Say) and Ips grandicollis (Eichhoff). Lower-stem flight traps were baited with 1:1 (+)-α-pinene:75% ethanol, and pitfall traps were baited with 1:1 (−)-α-pinene:75% ethanol. Healthy stands had higher populations of predators, particularly Thanasimus dubius (F.) (Coleoptera: Cleridae), Platysoma cylindrica (Paykull) (Coleoptera: Histeridae), and Platysoma parallelum Say than declining stands. Seasonal abundance patterns of predators and Ips also varied between declining and healthy stands. Declining stands had higher numbers of Ips and lower numbers of predators early in the season, whereas healthy stands had higher predator populations in the early season. Declining stands had more lower stem infesting bark beetles Dendroctonus valens LeConte (Scolytidae) and weevils (Curculionidae) Pissodes spp. Surprisingly, healthy stands had more root weevils, Hylobius pales (Herbst) and Hylobius radicis Buchanan, and root bark beetle Hylastes porculus Erichson, over the entire season. However, spatial by temporal patterns again were important. Populations of root colonizing insects were higher in healthy stands early in the season, but higher in declining stands throughout much of the ovipositional period. These results suggest dispersal patterns and overwintering behaviors are important in the interactions among herbivores, predators, and host plant condition in Red Pine Decline. The principal fungi isolated from roots were Leptographium procerum (Kendr.) Wingfield and Leptographium terebrantis Barras & Perry. These were more prevalent in declining than healthy stands. Staining fungi were more frequently isolated from roots of trees at the pocket margin than from trees in the asymptomatic portion of declining stands. Heterobasidion annosum (Fr.) Bref. was not isolated from root samples or stem disks, despite 40% recovery in positive controls using the latter method. Likewise, Armillaria spp. were not associated with declining plantations. Our results further support the view that forest declines are due to complex interactions among multiple biotic and abiotic stresses, and exhibit particular spatial and temporal patterns.