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June 2, 2000

Christmas Tree Scouting Report

Welcome to the Christmas Tree Scouting Report for the week ending June 2. The next report will be available after 4 PM on Friday June 9.  To receive a FAX of this week’s message, please call (814) 865-1636. If you have pest activity at your location to report, or would like to receive email reports, please leave a message at 717 772-5229.

Cooler temperatures during the last week have delayed the hatch of cryptomeria scale eggs at monitoring locations in Berks, Lancaster, Lebanon, and York counties. Growers of true firs are advised to continue watching for the minute yellow crawler stage of this troublesome armored scale.

Bagworm larvae in Lancaster and York counties have constructed their protective bags using bits of host material held together with strong strands of spun silk. The 1/4 inch long bags are easy to overlook and the feeding damage is still insignificant. However, this is the best time to control these moth larvae by using a biological insecticide, Bacillus thuringiensis, or Bt. Spruce, Douglas fir, and pine are all susceptible hosts and can be defoliated by these voracious caterpillars.

Spider mites continue to build populations on spruce and fir at many locations. Judging from the calls received recently, growers are really starting to notice the mites because their damage has become apparent. The best time of year to control the mites is when the population is building, usually in early spring or fall. At these times, controls can be applied to prevent damage and some of the newer materials that only require a single application can be effective. However, at this time, the populations are reaching their spring peaks and a material that is capable of quick knockdown is the choice. Many traditional miticides will serve the purpose but only if a total program of two applications 7-10 days apart are included in the plan. Single miticide applications frequently miss molting mites and eggs, and as a result the population quickly builds back to the original level.

Populations of balsam twig aphid are starting to dwindle on true firs. This is partly due to predation by lady beetle and syrphid fly larvae. The other factor is the end of the aphid’s seasonal activity. Females will soon begin to deposit large, light-colored, overwintering eggs on the new growth. These eggs are covered with waxy rods rubbed from the abdomen of the female during oviposition. After several days, the light-colored eggs will darken, looking more like the overwintering eggs everyone recognizes. Balsam twig aphid will remain in the egg stage until early next spring, when growers will again have an opportunity to control this pest.

A block of Fraser fir in York County has been diagnosed as having botrytis. This disease affects newly emerging shoots may frequently be mistaken for frost damage since infected shoots turn browned and droop. Botrytis occurs during rainy periods in spring and is not serious since most infected tissue is removed during shearing later in the year. Good airflow is helpful in preventing the disease because trees can dry more rapidly between showers.

Most pine needle scale eggs have hatched and crawlers have settled at locations in the Harrisburg area. If you missed your chance to control this pest of pines, a second generation will occur in July and you can apply a registered insecticide at crawler emergence.

Growers have asked about controlling Cooley adelgid on Douglas fir at this time of year. Frequently they see clutches of eggs on the new growth and want to apply a control to prevent further damage. This is not recommended for two reasons: first, most of the damage is already done. White spots and bends on needles have resulted where the adelgids fed on the tender, elongating growth. Additional feeding will not cause distortion of significant chlorosis. The second reason is the complicated life cycle of this pest. Not all of the adelgids on the tree are in a controllable stage at this time of year. Some nymphs will die after an insecticide application, but the majority of individuals will not be killed and control will be less than needed to prevent damage next spring. However, all is not lost, since fall applications are often highly effective in controlling the overwintering immatures.

In Lebanon and York counties, the fruiting bodies of rhabdocline needlecast on last year’s infected Douglas fir needles are turning dark brown. This signifies the end of the sporulation period and also the end of the time when preventive fungicides must be applied. Growers wishing to remove heavily infected trees can do so whenever needles are not sporulating without fear of spreading the disease. The needles that erupted this year to release spores will be cast by the tree during the summer, but will not be capable of producing spores next season.

Large tan or brown Cinara aphids are starting to appear on new growth of spruce. These aphids feed only on new growth and may cause damage resembling chemical burn. They prefer the leader or lateral branches near the top of the tree and are frequently tended by large black ants. The ants, feeding on the honeydew produced by the aphids, are usually easier to spot than the aphids themselves and can be useful in detecting the aphids.

Eriophyid mites continue to feed on needles of spruce at many locations. Populations will decrease sharply with the approach of warm weather but rebound later in summer. These “cool season” mites can cause significant chlorosis or silvering of the needles.

The next report will be available after 4 PM on June 9.