Environment and Local Substrate Availability Effects on Harem Formation in a Polygynous Bark Beetle

Many forms of polygyny are observed across different animal groups. In some species, groups of females may remain with a single male for breeding, often referred to as “harem polygyny”. The environment and the amount of habitat available for feeding, mating and oviposition may have an effect on the formation of harems. We aimed to determine how the surrounding environment (a harvested or unharvested pine plantation) and availability of local substrate affect the harems of the bark beetle, Ips grandicollis (Coleoptera: Curculionidae: Scolytinae). In a harvested pine plantation with large amounts of available habitat, the population density of these beetles is much higher than in unharvested plantations. We found the number of females per male to be significantly greater in the harvested plantation than the unharvested one. Additionally, the amount of substrate available in the immediate local vicinity (the number of logs in replicate piles) also influences the number of beetles attracted to a log and size of individual harems. We also examined how females were distributing themselves in their galleries around the males’ nuptial chamber, as previous work has demonstrated the potential for competition between neighbouring females and their offspring. Females do not perform clumping, suggesting some avoidance when females make their galleries, but they also do not distribute themselves evenly. Female distribution around the male’s nuptial chamber appears to be random, and not influenced by other females or external conditions.

Respuesta de insectos descortezadores de pino y de sus depredadores a semioquímicos en el sur de México

Se documenta la atracción en campo de insectos descortezadores (Coleoptera: Curculionidae, scolytinae) de pino y sus depredadores, a trampas multiembudos cebadas con formulaciones comerciales que contenían una de tres combinaciones de a-pineno con frontalina, ipsenol o con ipsdienol en el sur de México. El descortezador Dendroctonus frontalis Zimmermann fue atraído únicamente a frontalina + a-pineno. El descortezador Ips grandicollis (Eichhoff) (Coleoptera: Curculionidae, Scolytinae), los depredadores Enoclerus ablusus Barr (Coleoptera: Cleridae) y Elacatis sp. (Pascoe) (Coleoptera: Salpingidae) fueron atraídos a las combinaciones de ipsenol + a-pineno e ipsdienol + a-pineno; el depredador Temnochila chlorodia Mannerheim (Coleoptera: Trogositidae) respondió a todos los tratamientos. Los depredadores Tenebroides corticalis (Melsheimer) (Coleoptera: Trogositidae) y Temnochila virescens (F.) fueron capturados en números muy bajos. Los resultados de este estudio indican que el depredador T. chlorodia, reconoce y se orienta a los semioquímicos producidos tanto por las especies de descortezador, D. frontalis e I. grandicollis, mientras que los depredadores E. ablusus y Elacatis sp son atraídos exclusivamente por los producidos por las especies de Ips. Las diferentes respuestas de los insectos atrapados son discutidas a la luz de trabajos similares, enfatizando las variaciones encontradas en la asociación descortezador-depredador y la respuesta a los semioquímicos entre las poblaciones de Estados Unidos y las del sur de México para el manejo de los mismos.

Positive and negative effects of phoretic mites on the reproductive output of an invasive bark beetle

When multiple species coexist upon a single host, their combined effect on the host can be unpredictable. We explored the effect of phoretic mites on the reproductive output of the five-spined bark beetle, Ips grandicollis. Using correlative approaches and experimental manipulation of mite numbers we examined how mite load affected the number, size and condition of bark beetle offspring produced. We found that mites have both negative and positive consequences on different aspects of bark beetle reproduction. Females from which mites were removed were more fecund and produced larger offspring than females with mites, implying a cost of mite loads. However, when mites were present on females, those bearing the highest mite loads produced offspring that were larger and in better condition, indicating a beneficial effect of mites. These data suggest that phoretic interactions between mites and bark beetles differ over the course of the host’s lifespan, with either the mites interacting in different ways with different life stages of the host (parasitic on adult, mutualistic with larvae), and/or the beetles being host to different mite assemblages over their lifetime.