Question
I suddenly noticed these groups of bright yellow caterpillars on my Mungo pine.  They are eatting all the needles on each branch.  What can I do to eradicate them?  They seem to be eating a lot every day.  Regular pesticide with pyrethiam seems to make a small dent in the clusters of them

Answer
Sounds like Red Headed Pine Sawfly. It is not a caterpillar so some of the insecticides like Bt will not effect the larvae.

There are several species of sawflies that attack pines, but the redheaded pine sawfly, Neodiprion lecontei (Fitch), can be especially troublesome. Because it has two generations per year, all the needles on a pine can be eaten in a single season. The spring generation can strip trees of their older needles and the summer generation can strip off the current year's needles. This total defoliation can result in death of the pine. This sawfly is found from Southeastern Canada through the eastern United States.


Redheaded pine sawflies prefer two and three needled pines including: Scotch, jack, shortleaf, loblolly, slash, red, mugho. It has also been rarely found feeding on white pine, Norway spruce, Deodar cedar and larch, especially if near its preferred pine hosts.

Mechanical Control - If the trees are small enough to reach, colonies of larvae can be easily removed by clipping off the infested branch. Place these branches in a plastic bag and destroy. Colonies can also be knocked off by sharply striking the infested branch. Crush the larvae or knock into a pail of soapy water. If few colonies are present, they can be controlled using these methods but large infestations are better controlled by general spraying.


Some of the insecticides that can be used for sawfly control are listed below by the common name of the active ingredient (in parentheses# followed by an example brand name. Recognizing the pesticide common name allows you to find comparable products that contain the same active ingredient. Be sure to read the product label carefully before purchasing any pesticide.

 

#acephate# Orthene Turf, Tree %26 Ornamental Spray
#azadiractin# Neem ?various products
#bifenthrin# Ortho Houseplant %26 Garden Insect Killer
#carbaryl# Sevin
#cyfluthrin#  Bayer Advanced Garden Multi-Insect Killer Concentrate
#esfenvalerate#  Ortho?Bug-B-Gon Garden %26 Landscape Insect Killer Concentrate
#permethrin# Ortho Mosquito B Gon Tree, Shrub, and Lawn Spray  
#spinosad) Captain Jack抯 Deadbug Brew, various products


I would use the Orthene first it is an all around insecticide that works on most insects feeding on trees.