Physical and biotic effects of straw mulch applied at two different times to potatoes and watermelons were assessed in field trials over 2 yr. Plots with straw mulch generally had lower soil temperatures and higher soil moisture than control (weedy, no straw) plots. When straw was applied at planting weeds were suppressed, whereas straw applied after cultivation 4 wk after planting had less effect on weeds. In 2000, potatoes with straw at planting had fewer potato leafhoppers, Empoasca fabae (Harris) (and less associated plant damage) and more colonizing Colorado potato beetle, Leptinotarsa decemlineata (Say), adults than the other treatments. Subsequent Colorado potato beetle egg mass and larval numbers, however, were not higher in this treatment, possibly because of the higher numbers of predators in these plots as assessed by pitfall trapping. In 2001, there were no differences in numbers of colonizing potato leafhopper or Colorado potato beetle adults, but by midsummer there were more Colorado potato beetle larvae in the control plots than in the straw plots, again possibly due to differential predation. The few pests observed in the watermelon plots were not affected by the straw treatments. Potato yields were similar in all treatments, but melon yields were higher in plots with straw at planting (in which weeds were suppressed) in 2000. In 2001, when melons were planted earlier in the spring, melon plant biomass at 4 wk was lower in plots with straw at planting than in the other treatments, probably due to reduced soil temperatures. Thus, for potatoes, straw may be useful to control weeds and enhance predator numbers, but insect pests may still require control. In melons, straw should be used only if the crop is planted after the soil is sufficiently warm.