If you haven't grown beans before in your soil, you may need to add a rhizobial inoculant, which supplies the particular bacteria that helps the bean to fix nitrogen in its roots. Google "lima bean inoculant" to find sources. Some garden centers sell legume inoculants that contain a range of rhizobia for different kinds of legumes.

Plants can turn yellow for several reasons: too much water, not enough nutrients, etc. The combination of mold on the leaves and insects probably means the plants were not healthy. (Unhealthy plants have less resistance to disease and pests.) Did you plant them at the right time of year for your climate? Have you grown other crops in the soil?