We think you are referring to Cabbage White butterfly caterpillars (their other name is Imported Cabbageworm, even though they're not a worm), which are green and have a fondness for members of the cabbage plant family. (Slugs can be several colors, but out local species aren't really green, and they leave behind opalescent dried slime trails while caterpillars do not. Cabbage Looper is the caterpillar of a moth that might be another culprit.) The linked pages have more information and images for comparison.

The simplest approach is to cover the plants next time you put in new cabbage-family crops, especially since they don't require pollinator access to the flowers where the cover would get in the way. Row cover or insect mesh netting will suffice. Covers need to be in place as soon as they're planted, so won't provide much benefit now unless you can first remove the existing caterpillars, because otherwise it will trap the current caterpillar population alongside their buffet while keeping out the predators you want to eat them.

You can pluck the caterpillars off by hand (toss them somewhere for birds to enjoy, if you want), or spot-treat them with a low-toxicity insecticide like insecticidal soap or horticultural oil. Be aware that either will potentially cause plant tissue damage if applied while temperatures are above 85 degrees. Oils can also remove some of the wax layer on plant leaves that gives them their bluish color, so it might turn a sprayed plant greener. As with any pesticide (even these, which can be organic), always follow label directions for use carefully, and make sure the product is labeled for use on edible plants as well. If you do not wish to use even these low-impact pesticides, then physical removal is the only recourse.

Natural predators like paper wasps, yellowjackets, and tiny wasps that parasitize the larvae will help to kill a few of the caterpillars, plus any birds that find them in the meantime, but they probably won't get them all. Growing a diversity of flowering plants in the yard, like native perennials with varying bloom times, can help attract these beneficial wasps so they help to keep the vegetable plants more free of pests like caterpillars and aphids. Plant species in the aster and mint families (both large and diverse plant families) are both very popular with pollinators; examples include asters, perennial sunflowers (Helianthus and others), mountain-mint (Pycnanthemum), and Eupatorium appeal to beneficials.

Head formation (or lack thereof) on broccoli can depend on environmental conditions, including temperature. The University of Minnesota Extension notes that: "Hot weather (86°F in the day and over 77°F at night) can cause broccoli to grow no crown. Plants may produce a crown if it cools off." Soil moisture levels and the timing of when they were planted back in spring may also play a role in a plant stubbornly not forming a crown.

Miri

Thank you!

You're welcome!