Changes in landscape structure decrease mortality during migration.

I examined the dispersal of the red milkweed beetle, Tetraopes tetraophthalmus, among patches of its host plant, common milkweed, Asclepias syriaca. Over a 5-year period, the number of patches in a landscape and their mean size increased, while the distance between patches decreased. Over the same period the proportion of beetles dispersing between patches increased from 0.48 to 0.62. Estimates from the virtual migration model showed that mean migration distance decreased from 158 to 72 m for male beetles and from 129 to 72 m for female beetles. Estimated mortality per migration event decreased as the landscape changed, but was low in all years. The estimated mean migration mortality per patch decreased from 1.45 x 10(-2 )to 3.70 x 10(-7 )for male beetles. Female migration mortality decreased from 5.48 x 10(-3 )to 3.88 x 10(-6). Increasing the size and number of patches and decreasing interpatch distance decreases migration mortality and may play an important role in the conservation of species, particularly where mortality during dispersal is high.