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June 29, 2007

White Pine Shoot and Top Dieback—Insect or Environmental?

Gail Ruhl, Sr. Plant Disease Diagnostician, Botany & Plant Pathology and
Cliff Sadof
, Insect Diagnostician, Department of Entomology

Top and shoot dieback of white pine as well as dieback of several other pine species including Norway and Colorado blue spruce, has recently been reported in many places throughout Indiana and other Midwest states. In some cases, the tops of these conifer species are being attacked by the white pine weevil (Pissodes strobi (Peck)), however, in other cases there is no external evidence of holes in the branches nor evidence of internal feeding injury below the dead shoot area.  In these situations, a combination of various environmental stress factors is most likely responsible for the symptoms observed (Figures 2, 3, 4). An abrupt change from high temperatures to freeze conditions in mid-April this year caused immediate, as well as insidious, injury to plant tissues. Excessive temperatures, hot and dry winds, and drought conditions, of late, have added further stress to injured plants, resulting in the expression of visible dieback symptoms. Variation in the amount of damage is likely a result of a number of factors including variability in genetic vigor, growing site, and microclimate. 

When white pine weevil (Pissodes strobi (Peck)) is the culprit responsible for the dieback noted, insect larvae can be found feeding inside the shoot, below the affected area (Figure 1). Infested trees have the leader curled into a shape that resembles a shepherd's crook. Lateral branches from the infested tree's first whorl of branches may also be curled. The top 2 to 3 years of growth can be affected.

Adult weevils winter in leaf litter and fly to treetops to mate when the weather warms in the spring.  Females lay many eggs in terminals that hatch into grubs that bore into shoots between late-March to late-April. In late-May, legless (1/4") white c-shaped grubs can be found in stems, beneath the bark surface or in the stem. By early to mid-July, chip-bark cocoons are formed by larvae inside the base of injured stems. Adults will leave the twigs in early August and chew on twigs. 

July presents a very good opportunity for controlling these insects without insecticides. Simply removing the affected shoots and destroying them will keep the next generation of adults from chewing on the twigs in August and laying eggs on the twigs next spring. Mixed species plantings of pines are less likely to build damaging numbers of this pest than pure stands of susceptible species. 

In the event that it is impossible to reach the twigs, control of this year’s adults can be achieved by spraying the twigs with a broad spectrum insecticide, like Sevin, or a pyrethroid directed at the top of the tree. Look for active ingredients labeled for white pine weevil such as bifenthrin, cyfluthrin, lambda-cyhalothrin or permethrin. A repeat application in early April can also kill adults.

Also see: Cold Temperature Injury



Click image to enlarge

White pine weevil

Figure 1. Symptoms of damage from white pine weevil

White pine dieback

Figure 2. Symptoms of damage from environmental stress

White pine dieback

Figure 3. Symptoms of damage from environmental stress

White pine dieback

Figure 4. Symptoms of damage from environmental stress


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