We have seen a lot of the four-lined plant bug causing damage this spring. Upon first look the damage resembles a disease. The bugs are hard to spot because any movement causes them to scatter under the leaf and drop to the ground. Fortunately the damage they cause is mainly aesthetic, the plant looks unattractive but the pest will not likely cause serious damage. The insect emerges and feeds April through June. Once the adults have matured and mated they insert their eggs into the stem of their host plant. There is only one generation per year and it is unnecessary to treat your plant with an insecticide at this point but it is important to give your plant a good trim now. This will clean up appearances and remove the eggs. Be sure to do another harder trim in late fall, after plants have gone into dormancy, and also rake away any leaf litter and debris.
For more information please visit the University of Minnesota article here.