Question
At our state park in Capitan, NM,our bristlecone pines are heavily infested with scale. What pesticides are effective and at what time of the year do we treat?

Answer
The first thing to do is get an Entomologist to ID the scale.  Several species of scale insects develop on conifers in the West ,sometimes causing serious damage as they feed on plant sap. Pine-needle scale and striped-pine scale are particularly damaging. Several additional species are of minor or occasional importance

As scales feed on bark and needles, they remove sap and may damage cells. This can lead to decreased vigor, needle drop and dieback,and increased susceptibility to other insects or disease. Some scales also excrete sticky honeydew, which further detracts from plant appearance and attracts nuisance bees and wasps.

The pine-needle scale (Chionaspis pinifoliae) feeds on the needles of most species of pines, spruce and fir. During outbreaks, needles may be nearly covered with the bodies of the scales, giving an appearance that the plant is spattered with white paint. Heavily infested needles drop prematurely. Sustained outbreaks cause dieback of the plants. Diagnosis of a pine-needle scale infestation should be made only after close inspection. Certain pines, notably bristlecone pine, naturally exude resin from their needles, resulting in white flecking. This can easily be mistaken for pine-needle scale from a distance.

Striped-pine scale (Toumeyella pini) has greatly increased in importance. It is a soft scale (Coccidae) that is primarily damaging to Scotch pine. Pinyon, Austrian and lodgepole pine also have been affected. Infested trees can decline rapidly in vigor. An increase in bark-infesting woolly aphids (Pineus spp.) also appears to be associated with striped-pine scale infestation. Striped-pine scale excretes large amounts of honeydew as it feeds,which results in a sticky covering on twigs and needles. Dark gray sooty mold fungi grow on the honeydew, further degrading the appearance of infested trees. Nuisance bees and wasps are attracted to the honeydew.

The controls will depend on which scale is present, the extent of the problem  AND the local regulations of the public agencies involved. There are insecticides that will control the scales either by a timed spray or through the use of a systemic insecticide applied to the soil. The sprays are timed when the "crawler" stage of the insect is present--the time of the year is generally the spring but will vary with temperature so an Entomologist should be consulted if sprays are to be used. The  systemic insecticides are applied to the soil and the plant takes them up into and they are in the juices of the plant and when the scale insect sucks the plant juices it kills the scale.

Here is a contact for an Entomologist in NW with the US Forest Service that should be able to help. Her name is Debra Allen-Reid.
You can tell her I sent you she a friend --she worked in Alabama  when she first started with the USFS. She can help or direct you to the proper folks who can.
http://www.fs.fed.us/cgi-bin/address_detail_net?bMpLI4no4pWweu2j