Utah Pests News Winter 2008-09

click here for pdf version

Common Spruce Insect Problems 

Spruce health starts with proper watering and fertilizing.  Become familiar with your trees through visual inspection on a regular basis (once a week) from February to November.  Early detection is critical.  The difference of a month or two could be the difference in your tree’s survival.  Using the information in this article combined with a regular scouting routine, you will be able to recognize arthropods that may be causing damage to your spruce.

Cooley Spruce Gall Adelgid

  Fig. 1. The feeding of immature cooley spruce gall adelgids at the base of needles causes galls to form.
  Fig. 2. An active gall (l) and a dried gall showing cracks where adults have emerged (r).
  Fig. 3. Spruce spider mites are hardly visible to the naked eye.
Adelgids look very similar to aphids, and are sometimes covered with a white, wax-like substance (Fig. 1).  You may have seen the cone-like galls formed by cooley spruce gall adelgid (Fig. 2).  When these galls dry out and open, adults may migrate to their alternate host, Douglas-fir, to lay eggs, although the adelgid can complete its life cycle on spruce alone.  Spruce trees are capable of tolerating damage from this insect, and usually no treatment is warranted.  Light infestations can be controlled by pruning the green galls in early summer (before they crack open).

If desired, insecticides should be applied before the galls form.  Horticultural oil is effective on overwintering adelgids, applied in fall or in spring prior to bud-break.  It is very important to note, however, that oil can remove the white coloration of blue spruce needles for one season or more.  Imidacloprid (Merit) is a systemic insecticide that should be applied as a soil drench.  Because this chemical takes several months for roots to absorb, apply in fall.

Spuce Spider Mites

Spruce spider mite is a small, brown to red mite that feeds on many different species of spruce (Fig. 3).  A spider mite is not an insect, but an arachnid, having 8 legs. Mites feed by rupturing plant cells with their mouthparts, and then sucking up the chlorophyll from within the cell.  As such, their feeding causes a yellow “stippling” on the needles. In severe infestations, needles can turn brown and drop.  Because of their small size, detection is difficult.  The easiest way to sample for spruce mite is to hold a sheet of plain white paper under a branch and whack the branch about three times.  Look closely at the paper for little period-sized black spots moving around.  Crush one of these moving spots and it should leave behind a brown/red smear.

Spruce mites are a cool season mite.  They cause most of their damage in spring and fall, so it is vital to scout for this pest in late winter to early spring.  When the temperatures become too hot in the summer their activity slows drastically, but resumes again in the cooler fall months.  Damage from spring and fall feeding is usually not visible until early summer when the affected needles begin to dry.

To control spider mites, first make sure there are mites present by scouting and conducting “beat surveys.”  Spruce spider mite populations can sometimes be reduced by spraying a strong jet of water into the tree twice a week.  Other low toxicity methods are horticultural oil sprays, and, to some degree, insecticidal soaps (when webbing is not present).  When selecting a chemical for mite control it is critical to understand that mites build resistance to insecticides/miticides very quickly.  Any single product should only be used once or twice per year.  If subsequent sprays are needed for control, it is vital to select a product from a different chemical family.  The following is a list of chemicals from which to choose: avermectin, spinosad, bifenazate, spiromesifen, hexythiazox, acequinocyl, dicofol, and etoxazole.  Spinosad is a low toxicity product that can be tried first.  Some chemicals can actually make mite outbreaks worse, so it is good to choose a product that controls both the adult and egg stages of development.

Bark Beetles

 Fig. 4. Don’t let the small size of ips bark beetles fool you; they are voracious feeders.  

  Fig. 5. Ips beetles that attack spruce often kill from the top down.
  Fig. 6. Typically, galleries of ips beetles have a primary egg-laying chamber (shown above) created by the adult female. These chambers can occur in an X, Y, star, or wavy line pattern.
In the landscape, ips bark beetles (also known as engraver beetles) commonly attack spruce trees. Ips species are small, brown/black beetles (Fig. 4) that emerge from trees in late April or May, seek a new host tree, and bore beneath the bark where they mate, construct parent galleries, and lay eggs.  After egg hatch the new larvae eat their way through the phloem, girdling the tree.  Often ips beetles will kill the top of a tree first (Fig. 5), and then kill the rest of the tree later in the year, or the following year.  Ips beetles can have 3 generations per year.

To control ips beetles it is critical to maintain proper tree health.  Bark beetles, in general, attack stressed or injured trees.  It is also important to monitor the surrounding area for bark beetle-killed trees, because when the beetles emerge from those trees, they may come looking for yours.  If you notice beetles in the area, apply a preventative trunk spray.  The standard recommendation for bark beetle control is a trunk drench of carbaryl (Sevin) before adults fly in early spring.  Spraying your trees after the beetles have made it through the bark will not provide effective control, and is a misuse of chemicals.

If you only have one tree, keep it healthy.  Predicting bark beetle attack is impossible, and it is not feasible or recommended to apply preventative sprays for no reason.  Scout regularly for signs of bark beetle attack, including: frass (sawdust) around the base of the tree or on the bark, pitch tubes (noticeable globs of resin that are exuded from beetle entrance holes), or discoloration of the needles.  Once beetles infest a tree it is recommended to immediately remove and debark the tree if you are saving firewood.  Debarking the tree will kill the young beetle larvae and adults present under the bark.  While you can’t save an infested tree, you can reduce/remove the population of beetles from your property and save the rest of your trees.

-Ryan Davis, Arthropod Diagnostician