Plasticity in life-history traits of the bark beetle Ips calligraphus as influenced by phloem thickness

Abstract

Reproduction and development of Ips calligraphus (Germar) (Coleoptera: Scolytidae) occur primarily in the phloem (inner bark) tissue of its pine hosts. In slash pine (Pinus elliottii Engelm. var. elliottii), phloem thickness can vary widely from tree to tree. We compared adult residence time and fecundity and progeny development time, body size, and sex ratio of this beetle when reared in slash pine bolts and bark slabs with phloem thicker or thinner than average adult body width. Most studies were conducted at 20°, 25°, and 30° C. Residence time of parent males and females decreased with increasing temperature but it was not affected by phloem thickness. Residence time of parent males was about 2 days shorter than that of parent females at each temperature. Reproductive fitness was greater in thick phloem at all temperatures as measured by faster larval development, earlier emergence of progeny (F ) adults, and emergence of more progeny adults per parent female. Progeny survival was similar between phloem those from thick phloem, and their sex ratio was female biased (1:2 vs 1:1, male:female). When allowed to inhabit thick phloem, the progeny (F) of adults from thin phloem were just as large and of similar sex ratio (1:1) as were progeny of F adults from thick phloem. The high degree of plasticity in the life-history traits of this beetle may enable it to successfully colonize pines that vary widely in phloem thickness.

Death of vigorous trees benefits bark beetles

“…These results contrast with other studies that have emphasized the positive eects of phloem thickness for bark beetles (Amman 1972;Berryman 1976;Haack et al 1987aHaack et al , 1987b. Moreover, phloem quantity is expected to have a direct eect on reproduction.…”

“…Moreover, phloem quantity is expected to have a direct eect on reproduction. The mean thickness of phloem in our focal stand (1.07 mm, Table 1) was less than the body width of pine engravers (ca 1.45 mm), requiring beetles to also consume less nutritious outer bark and xylem (Haack et al 1987a). The lack of an eect of phloem thickness in our study cannot be attributed to greater variation in the measurement of phloem thickness for each tree (due to either intrinsic variation or measurement error), as the four measurements taken for each tree were not more variable for phloem than for MAI or BAI for the past 5 or 10 years (analyzed following Sokal and Braumann 1980;data not shown).…”

“…The nutritive quality of tree phloem likely increases with host vigor (Hodges and Lorio 1969;Waring and Pitman 1985;Sundberg et al 1993), but an inverse relationship has also been proposed (Cates and Alexander 1982;Rhoades 1983;Mattson and Haack 1987). The quantity of tree phloem (phloem thickness) is consistently related to bark beetle success (Amman 1972;Berryman 1976;Haack et al 1987aHaack et al , 1987b, and tends to increase with tree size and growth rate (Cole 1973;Hawksworth et al 1983). …”

Effects of host quality parameters on fitness of the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae).

“…Mountain pine beetle success has been linked to phloem thickness (Amman 1972b;Berryman 1976;Haack et al 1987). Several studies have indicated that larger-diameter trees with thicker phloem and bark produce brood beetles that are larger in size (Amman & Pace 1976;.…”

“…Thicker phloem provides mountain pine beetles with access to a larger quantity of the resource, and a higher quality food source Amman 1972b;Berryman 1976;. Reproductive fitness, measured by short larval development time, early emergence, and large quantity of offspring per parent female, was greater in Ips calligraphus (Germar) beetles developing in trees with thick phloem (Haack et al 1987). Mountain pine beetles from small trees have been shown to produce broods with a slightly different genotype, a sex ratio strongly skewed towards females, and slower development, than broods from larger trees ).…”