On those days when you think your kids will never grow up, you’re experiencing what it’s like to grow a mugo pine (Pinus mugo). Horticulturist Michael Dirr reports a 25-year-old "Gnom" mugo pine cultivar (Pinus mugo "Gnom") standing 15 inches high and 3 feet wide. Their slow growth rate, pyramidal, spherical or prostrate forms, and striking foliage make these shrubs ideal small-space accents. Too often, however, needle-munching worms shred them like scissors-happy kindergarteners.
Wherever mugo pine varieties grow in U.S. Department of Agriculture plant hardiness zones 2 through 8, they incubate European sawfly larvae. The adult sawflies -- small wasps with serrated appendages -- carve pockets in mugo pines' needles and deposit their eggs during summer. Their larvae hatch the following spring and feed for about a month. They eat like babies moving from milk to solid food, with newly hatched worms concentrating on the older needles' soft surface layer and exposing their dying rust-brown cores. Mature worms consume the entire needles. By the time they pupate, the shrubs' bare branches adorned with terminal tufts of young needles resemble poodles' tails.
Curving, brown branch tips on your mugo pines signal Nantucket pine tip moth or European pine shoot moth caterpillar infestations. The creamy-white tip caterpillars feed at the base of the needles before burrowing inside. They eventually invade cones and shoots, leaving dying, resin-dotted needles and webs in their wake. Black-headed, brown shoot moth caterpillars immediately tunnel into the needles, eating their way to the branch tips by late summer. Heavy shoot-caterpillar infestations may kill the plants. Both pests pupate within the damaged tissues.
If you're not squeamish, handpick small sawfly infestations from your pines. If you are, try dislodging them with a strong blast of water instead. Pruning and destroying the sawfly- or caterpillar-infested branches reduces heavier populations, but takes an additional cosmetic toll. If you decide to clip them anyway, do so before June. You might have to trim some healthy branches to reshape the shrubs.
Insecticidal soap spray controls sawfly populations without endangering your kids, pets or other plants. Apply a ready-to-use product according to its label specifications or mix 5 tablespoons of liquid dish detergent with 1 gallon of soft water to make your own. Saturating the plants suffocates young larvae before they do serious harm. Spraying to eliminate older larvae has little benefit because the damage is nearly done.
To treat tip and shoot moth caterpillars, monitor your pines closely for eggs at the bases of the needles or on the new shoots. As soon as the eggs begin hatching, saturate the branches' ends with a solution of 1 to 2 teaspoons of wettable Bt (Bacillus thuringiensis var. kurstaki), or the label's specified amount, in 1 gallon of water. Repeat as needed. The Bt microbes starve the pests by disrupting their digestion. Spraying in early evening protects the UV-sensitive organisms from sun damage.