Effects of host switching on gypsy moth ( Lymantria dispar (L.)) under field conditions

Effects of various single and two species diets on the performance of gypsy moth ( Lymantria dispar (L.)) were studied when this insect was reared from hatch to population on intact host trees in the field. The tree species used for this study were red oak ( Quercus rubra L.), white oak (Q. alba L.), bigtooth aspen ( Populus grandidentata Michaux), and trembling aspen ( P. tremuloides Michaux). These are commonly available host trees in the Lake States region. The study spanned two years and was performed at two different field sites in central Michigan. Conclusions drawn from this study include: (1) Large differences in gypsy moth growth and survival can occur even among diet sequences composed of favorable host species. (2) Larvae that spent their first two weeks feeding on red oak performed better during this time period than larvae on all other host species in terms of mean weight, mean relative growth rate (RGR), and mean level of larval development, while larvae on a first host of bigtooth aspen were ranked lowest in terms of mean weight, RGR, and level of larval development. (3) Combination diets do not seem to be inherently better or worse than diets composed of only a single species; rather, insect performance was affected by the types of host species eaten and the time during larval development that these host species were consumed instead of whether larvae ate single species diets or mixed species diets. (4) In diets composed of two host species, measures of gypsy moth performance are affected to different extents in the latter part of the season by the two different hosts; larval weights and development rates show continued effects of the first host fed upon while RGRs, mortality, and pupal weights are affected strongly by the second host type eaten. (5) Of the diets investigated in this study, early feeding on red oak followed by later feeding on an aspen, particularly trembling aspen, is most beneficial to insects in terms of attaining high levels of performance throughout their lives.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47802/1/442_2004_Article_BF00323144.pd

Determination of larval melanization in the moth, Mamestra brassicae, and the role of melanin in thermoregulation

Variability in the colour of lepidopteran larvae has been recorded in a broad range of species, yet little is known of its evolutionary significance, or whether it has a genetic basis. I assess the role of genes and environment in determining the degree of larval melanization in the moth, Mamestra brassicae, and examine functional aspects of larval pigmentation. In particular, whether melanization is of importance in thermoregulation, and whether larvae differing in melanization exhibit concurrent differences in size, rate of development, and fecundity. In the fourth and fifth instars, larval M. brassicae exhibit a continuous range of colour from pale green to black: a classification scheme is described to quantify this variation. Heritability (h2) was measured using regression of brood means against mid-parent values for 36 broods (2339 offspring), and was estimated to be 0.237 0.07 (SD) for fourth instar larvae and 0.421 0.10 (SD) for fifth instar larvae. However, environmental factors mediated development of larval colour: larvae were darker when reared at low temperature (12°C) compared to high (24°C). Direct measurement of larval temperatures using thermocouples inserted into the alimentary canal indicated that dark larvae absorbed more radiant heat, and thus under illumination consistently maintained a higher body temperature than pale larvae. Hence dark larvae are presumed to be at a selective advantage at low ambient temperatures, and increased melanization of larvae reared at low temperatures may be adaptive. I suggest that variation within natural populations may be maintained by fluctuating weather conditions. Dark larvae were found to be smaller, but developed more quickly than light larvae so that weight at pupation, time to pupation, and fecundity (measured by the number of fertile eggs produced) did not differ according to colour. Further studies are suggested to examine the influence of larval colour on thermoregulation, growth rates and predation in the field