The tree root structures stabilizes the unstable soil. Remove the trees and the resulting erosion deposits the soil as sediment into the rivers that crisscross the border, choking off river bottoms where fish feed and breed. Declining fish populations and fish kills are the result.
Any insect infestations that exist will move from the deforested areas to forested areas to feed and breed. carried by the wind or vehicle transition between the areas. As far as I know, the DR government doesn‘t conduct aerial forestry insecticide spray programs. They rely on natural processes which isn’t a bad thing, given their poor control over pesticide applications on farmland, industrial and residential areas.