Some Early Spring Insect Pests of Christmas Trees

The accumulation of heat units (Growing Degree Days (GDD) will soon begin to increase rapidly in New Jersey as we approach the early weeks of April. If past histories or present-day monitoring indicate the need for interventions, then the best control windows for numerous insect/mite pests are beginning to occur. The following is an incomplete listing of a handful of some of our early-season Christmas tree insect pests that may need to be scouted & possibly controlled. Those included in this blog are the European pine sawfly, Eastern spruce gall adelgid, Cooley spruce gall adelgid, Nantucket pine tip moth, Pales weevil, White pine aphid, & Pine bark adelgid.

 

European Pine Sawflies: (Neodiprion sertifer) (35-135 GDD = Egg hatch):

The most common sawflies on pines are the European pine sawfly (EPS) and the redheaded pine sawfly (RPS). They commonly attack the two-needle pines (mugo pine, Scotch pine, and Japanese black pine). EPS overwinters as eggs in the needles, while RPS overwinters as pupae. EPS may soon begin to hatch, so monitor for the young larvae feeding on the needles. This sawfly will do most feeding during April. By May, most larvae will complete their 1-generation feeding damage for the year. RPS larvae will not be active until May after adults and the eggs they lay begin to hatch.

 

 

 

 

 

 

 

 

 

When sawfly larvae are young, their mouthparts are small so they cannot initially eat the entire needle. Partial eating of the needles results in the remaining part curling, turning light brown & becoming straw-like. This appearance provides a highly effective monitoring tool, particularly on larger trees. The brown curled needles are easy to detect from a distance and indicate the presence of a cluster of sawflies feeding near that area. Large EPS populations are capable of completely defoliating large pines. When new candle growth begins to elongate, this sawfly has already dropped off the tree & moved into the pupation stage under the duff.

 

 

 

 

 

 

 

 

 

 

Remember, sawfly larvae are wasps within the order Hymenoptera & are not caterpillars. Therefore, B.t. (Bacillus thuringiensis) will not control them.  A 1% horticultural oil drench may control young larvae (not larger & older larvae). The uses of spinosad (Conserve), acephate (Orthene), carbaryl (Sevin), or any of the pyrethroids are some choices for the control of older larvae.

 

 

 

 

 

 

 

 

 

 

Eastern Spruce Gall Adelgids: (Adelges abietis) (22-91 GDD = nymphs/adults):

This spruce gall species forms at the base of the current year’s growth. The Eastern spruce gall adelgid (ESGA) is typically found on Norway spruce. Often the ESGA has the potential to be damaging to the aesthetic appearance of highly infested trees and can cause branch thinning & dieback. The other common spruce gall is the Cooley spruce gall adelgid which forms at the twig tip terminals & is primarily found on Colorado spruce.

The white, wax-covered, overwintering females, eggs & then nymphs are present at the base of buds on Norway spruce. The target to inspect for is small, white webbing located at the base of lateral buds (10X hand-lens might be useful). These are the protective sites where the adelgid stem mother will lay eggs within the webbing. Overwintering nymphs remain at the base of lateral branch buds waiting for stems to elongate.

 

 

 

 

 

 

 

 

 

 

Apply horticultural oil or insecticidal soap in April before the new bud shoots begin to swell and expand. Good results are achieved by targeting the females in late fall before eggs are laid or against nymphs in early spring with oils or soaps. Insecticidal soaps have dynamic controls against adelgids if good coverage is achieved.

Feeding nymphs will introduce a toxin within new expanding growth that eventually stimulates abnormal plant cell division and the formation of the protective galls. Once the green pineapple-like galls have formed at the base of new shoots during May, the only available control is to prune out and destroy the galls. Not even systemic insecticides will affect the nymphs within the galls.

 

 

 

 

 

 

 

 

 

 

Cooley Spruce Gall Adelgid: (Adelges cooleyi) (25-120 GDD)

The nymphs of the native Cooley spruce gall adelgid (CSGA) overwinters on Colorado spruce (especially the green forms) at the terminal bud tips. They will migrate to new growth as the twigs begin to elongate. The combined feeding and salivary secretions induce the development of a bright green, oval “pineapple” gall on the branch terminal tips (as opposed to galls from Eastern spruce gall adelgid that are at the base of new growth on Norway spruce). During June, the green-colored galls may grow to two inches in size. By midsummer, the gall will turn brown and crack open, as the mature adelgids emerge as adults. The CSGA winged adults will then migrate to Douglas fir (or another spruce) and spend the summer feeding on the needles. The pruning out and disposing of the “pineapple” shaped galls before they open in July is relatively easy & can provide effective physical controls. Once the galls have opened they will no longer be of concern. For aesthetic reasons, they can be easily removed by hand.

 

 

 

 

 

 

 

 

 

 

The CSGA has a complex multigenerational life cycle. Douglas fir is the alternate host & will often be found growing nearby. They feed on the needles of Douglas-fir for the summer before a percentage will fly back to spruce, where they will overwinter. Feeding causes the needles on Douglas-fir to become discolored, bent, and misshapen, however, galls are not produced. Individual tufts of wax (“cotton”) are noticeable on individual needles protecting the adelgid. This pest may go unnoticed in the landscape when infestations are light. Christmas tree growers will treat Douglas fir during the spring when the new growth is 3-4 inches in length. By the next fall, another winged generation flies back to spruce and lays eggs for the complicated life cycle to repeat itself. The CSGA has one of the most complex & confusing life cycles of any of our key insect pests. Blue forms of Douglas-firs are more susceptible to attack than green forms.

 

 

 

 

 

 

 

 

 

The damage done to Douglas-firs from the Cooley Spruce Gall Adelgid (CSGA) is typically a major concern within nurseries, especially Christmas tree farms. The distortion and white webbing on infested needles of the Douglas-fir are aesthetically unacceptable to the buyer. Nevertheless, studies have shown that this pest does not affect the growth and vigor of the trees. Also, the feeding injury by the CSGA does not cause premature needle drop. In production settings, the appearance of the ornamentals to be sold is not critical until the year of sale. Therefore, it only becomes necessary to apply control materials against the CSGA until a season or two before the projected year of sale. Two seasons of undamaged needles will more than adequately mask the damage done to the inner foliage during previous years.

Controls:

Place sticky traps out in late summer (on Douglas-fir) (1850-1950 GDD) to determine the timing to spray adults with insecticidal soap (+ sticker). Future control windows will occur during the fall or early spring months, against the overwintering females located just beneath terminal bud tips. Acetamiprid is a reduced-risk material that is labeled for use on adelgids. Other possible treatments include horticultural oil, insecticidal soap, or carbaryl. Dormant oil sprays on spruce in early April (not blue forms) before stem elongation can control over-wintering nymphs. The systemics imidacloprid & dinotefuran are also labeled.

 

Nantucket Pine Tip Moth: (Rhyacionia frustrana) (121-448 GDD = 1^st generation larvae): 

The native Nantucket pine tip moth is a caterpillar pest that overwinters as brown-colored pupae within branch tips. With small trees & low populations, the tips containing the pupae can be pruned out before adults emerge next month. Pheromone traps will detect the initial emergence of the male moths. Adults in central NJ first emerge during mid to late April to lay their eggs on the tips of new growth of two and three-needle pines. Scotch pine is a common Christmas tree species that can be attacked. Newly hatched larvae feed in the needles first and then enter the buds and stems. This feeding causes the growing tips to turn brown & die back. Pines less than 6 feet tall that are growing in full sun will receive the most damage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For small populations, prune out the dying tips and destroy. For larger populations, hang out the pheromone traps to monitor for the adult flight period. Adults are ¼ inch long or less & have gray wings that are mottled with rust-colored patches. When the first moth is caught, wait 10 to 14 days before applying controls. Spinosad is a reduced-risk insecticide that is labeled for use in Christmas tree farms. Traditional control materials include acephate & the pyrethroids cyfluthrin or fluvalinate. The systemic imidacloprid is effective when proper lead time is provided (4 to 8 weeks with soil injections or drenches). Imidacloprid is usually not effective against caterpillars (i.e., therefore the NPTM is an exception to the rule). Be prepared for a second generation in July.

 

 

 

 

 

 

 

 

 

Pales Weevil: (Hylobius pales) (7-121 GDD)

The native Pales weevil is a conifer weevil that prefers to feed on the tender bark of seedling white pine and other conifers up to 18 inches tall. Occasionally they can be found feeding on spruce & Douglas-fir. The night-feeding adult weevils may also infest smaller twigs of larger pines, creating holes in the bark, girdled twigs, and dead needles. These insects are attracted to freshly cut pine stumps and sawdust and are considered a serious pest of young pines, especially in nurseries and Christmas tree farms. They are only occasionally a problem in the landscape because they need dead stumps or recently killed trees to perpetuate their life cycle.

Adults overwinter in duff under pines and become active in March/April to feed on the bark of branches. The small, chewed areas create wounds that cause a flow of clear sap. This sap eventually crystallizes into a hard white substance and is usually the first symptom of Pales weevil feeding activity. Eggs are laid on stumps during April/May and larvae feed under the bark during the summer. There is one generation a year. New adults emerge during September & will feed briefly before overwintering in the duff under conifers.

Setting out freshly cut pine blocks can monitor adult weevil activity. The adults are attracted to these blocks and will hide beneath them during the day. Applications of long residual materials such as pyrethroids & carbaryl to the branches of infested trees can control the feeding by adults. Treat the branches in the spring (April) & then again in early fall (September). The removal of dead trees and stumps is an important sanitation practice. Otherwise, to suppress larvae, treat stumps with a residual insecticide in March & April for two consecutive years to reduce future damage.

White Pine Aphid: (Cinara strobi) (7-246 GDD = Pre & Post Egg Hatch):

This native aphid feeds only on eastern white pine. Overwintering oval black eggs laid in rows along the needle length is easily visible when monitoring. Adult aphids are large (3/16 inch) and brownish-black in color with long legs. Adults also have a central white stripe and white spots on each side of the abdomen. They suck phloem sap through the bark of branches and may cause some branch dieback if populations are high. Their presence is commonly discovered when aphid honeydew excrement turns the needles black with the sooty mold fungus growth. The sugary honeydew may also attract ants and swarms of hornets and yellowjackets (the latter two are good predators in the spring but become a nuisance in the early fall). Feeding aphid populations live in dense clusters on white pine twigs. There are several generations per year.

 

 

 

 

 

 

 

 

 

When numerous natural enemies are present, attempt to use less harmful controls. With good coverage, horticultural oil can be used to treat both overwintering eggs as well as nymph & adult aphids when heavy populations warrant control. Acetamiprid is a reduced-risk material that is labeled. Numerous insecticides are labeled for use against white pine aphids. Insecticidal soaps will typically only provide suppression against aphids & may not provide satisfactory results.             

 

 

Pine Bark Adelgids: (Pineus strobi) (22-58 GDD)

These common aphid-like insects (they are adelgids) form a white, cottony coating on the bark of white pine and Scots pine. Immatures are presently overwintering on the bark. With heavy infestations, copious amounts of white wax can be found on trunks, branches & small twigs. Damage is usually cosmetic & light to moderate populations will not be a plant healthcare concern to infested pines. However, customers may react negatively to their presence.

 

 

 

 

 

 

 

 

 

Reduce the first of several generations of adelgids with a strong blast of water, horticultural oil, or insecticidal soap. Insecticidal soaps are highly effective in controlling adelgids. These treatments should help conserve beneficial insects such as flower fly maggots & lady beetle larvae that are commonly associated with this pest. They can often be found feeding on adelgids & their eggs under the white wax.

 

Reference: Managing Insects and Mites on Woody Plants: an IPM Approach, John A. Davidson & Michael J. Raupp, Bulletin 350, Maryland Cooperative Extension, 1999.