Nitrogen Fertilization of Hemlock Increases Susceptibility to Hemlock Woolly Adelgid

Methods

An experiment was conducted during 1988 and 1989 in a forest plantation of eastern hemlock at the Connecticut Agricultural Experiment Station, Lockwood Farm in Mount Carmel, Connecticut. The plantation consisted of 30, 10 year old trees 2 m tall obtained from a common stock and planted in 1986 in 6 rows of 5 trees each spaced 2 m apart. On 18 October 1988 10 randomly selected hemlocks were fertilized with nitrogen by spreading 0.25 kg of Homogenite^r 41N on the soil in a circular band 0.5 m from the trunk. Ten other hemlocks were fertilized in the same manner on 19 April 1989 at which time all 20 fertilized trees and the remaining 10 unfertilized ones were intentionally infested with hemlock woolly adelgid. Four infested hemlock branches (0.5 m long) collected from a forest in East Hampton, Connecticut were placed at the four cardinal directions on each of the 30 trees in the plantation and left in place for 2 weeks so that an ample number of hatching nymphs could colonize the trees.

On 28 June 1989 after most adelgids had completed their development and had laid eggs, each of the 30 trees were sampled by removing one branch from each of the four cardinal directions. Branches were examined and the number of living and dead adelgids present on 25, 2 mm wide by 10 mm long segments of the preferred youngest growth on each branch was counted. Fecundity was also determined by counting the number of eggs and chorions from hatched eggs in each of 10 ovisacs per branch (n = 40 per tree, 400 per treatment). Only ovisacs with dead adult females alongside were examined to ensure that oviposition had been completed. These methods have been described in more detail elsewhere (7, 9, 10).

The growth and foliage color of fertilized and unfertilized trees was appraised by removing 25 new leaders from each of the four branches sampled on 28 June, noting their color, and then drying them at 40 ° C for 3 days and then weighing them.

Analysis of variance and Duncan’s (3) multiple range test were used to appraise differences in adelgid performance and hemlock growth on fertilized and unfertilized trees.

Results

Fertilizing hemlocks with nitrogen significantly improved the performance of resident adelgids for all three parameters measured (Table 1). After a single adelgid generation, population densities were more than five times higher on fertilized hemlocks than on unfertilized ones. In addition, percent survival of nymphs and numbers of eggs produced per adult were more than twice as high on fertilized hemlocks than on unfertilized trees. These trends did not differ between hemlocks that had been fertilized 6 months prior to infestation by A. tsugae and those that were fertilized at the same time that trees were infested (Table 1). These data indicate that nitrogen fertilization causes a swift and persistent improvement in the nutritional quality of hemlock for the adelgid.

Hemlock growth, measured as the biomass of each new leader was not significantly enhanced by nitrogen fertilization on either date (Table 1). Instead, the average biomass of new growth was significantly greater on unfertilized hemlocks than on fertilized ones. The foliage of fertilized hemlocks also had far inferior color than the unfertilized trees; the characteristic gray-green foliage color of hemlocks under attack by this insect (10) was far more prevalent among branches from fertilized trees (88%) than among those from unfertilized hemlocks (12%). The higher adelgid densities (and feeding intensity) on fertilized hemlocks more than offset any potential advantage in hemlock growth and foliage color that otherwise might have been gained through nitrogen fertilization. This inverse relationship between adelgid density and hemlock growth and injury has been documented previously (10).

Discussion

Fertilizing with nitrogen produced no obvious advantages for hemlocks under seige by A. tsugae. Fertilization did not improve hemlock growth and appearance, and neither increased host resistance to adeglid attack nor repressed population growth once the adelgid had become established. The disadvantages of fertilization, however, were obvious. Tree feeding with nitrogen significantly enhanced adelgid performance and stimulated adeglid population growth, such that by the end of a single growing season, fertilized hemlocks were more heavily infested and were far less vigorous than unfertilized ones. Nitrogen fertilization may be of some value in helping hemlocks to recover once adelgids have been successfully controlled, but this was not examined in this study.

These findings may have widespread application to many other insect pests of trees and shrubs, especially those which feed on sap. Indeed, there is substantial evidence that piercing and sucking insects such as adelgids, aphids, leafhoppers and scales are generally favored by an increase in the soluble nitrogen component of their food (1, 2, 12). It is also well established that fertilization with nitrogen can significantly increase the concentration of soluble nitrogen in host sap where it can be readily intercepted by feeding nymphs. Consequently fertilizing trees with nitrogen may undermine programs for managing populations of piercing and sucking insects.