Long-Term Scale Herbivory Studies

The pinyon needle scale (Matsucoccus acalyptus) begins feeding on the needle mesophyll of juvenile trees prior to the emergence of the current year’s needles. Scale feeding causes chlorosis and early senescence of needles, resulting in only 1 to 3 years of foliage rather than the usual 6 to 8 years present on scale resistant trees.

Long-Term Scale Monitoring Program

In 1985, we established a one-hectare plot at Sunset Crater National Monument for a long-term census of herbivory and tree performance. All trees were mapped and tagged, which includes 152 juvenile trees (40 yrs old), 41 intermediate-aged trees (70 yrs old), and 23 mature trees (160 yrs old) that have been observed since 1985. For the last 10 years we have annually censused the scale, moth and sawfly populations on the juvenile trees.The intermediate-aged and mature trees have been monitored periodically for all three herbivores. We are continuing to monitor all three age classes of pinyons for the three major herbivores. With the sessile pinyon needle scale, we have annually collected needles and quantified scale density, stage-specific mortality, degree of needle chlorosis, and sex ratios. We will continue monitoring scale demographics.

Scale Herbivory:
Scale populations vary significantly from year to year (Fig.1).

Annual Stem Growth:
Trees infested with scale exhibit consistently lower annual stem growth than trees without scale (Fig.2).

Cone Production:
Scale herbivory has little effect on cone production, even during mast years (Fig.3). This is in contrast to the results from the scale removal experiment (see below), where susceptible trees produced fewer cones in mast years. Since we are dealing with juvenile trees that produce few cones to begin with, variations like this are expected.

Strobili Production:
Male reproductive function is lower on trees with scale (Fig.4).

Cascading Effects On Other Species:
Scale presence has no effect on sawfly presence (Fig.5), but trees with scales have consistently fewer stems killed by moths than do trees without scale (Fig.6).

Long-Term Scale Removal Experiment

Beginning in 1985, we have manually removed scale egg masses from a set of trees, preventing colonization of those trees. We have monitored these trees, along with control groups of resistant and susceptible trees for reproduction, herbivory, and general tree performance. Some of the results from this study can be found below:

Cone Production:
Scale herbivore removal increases cone production during high cone production years (Fig.7). There is very little cone production on any trees during other years.

Conelet Production:
Scale herbivore removal increases conelet production in high conelet production years (Fig.8).

Strobili Production:
Scale herbivore removal increases male strobili production in most years (Fig.9).

Aboveground Net Primary Productivity:
Beginning in 1990, six years after scale removal started, scale-removed trees showed higher annual stem growth than scale susceptible trees, and have done so every year since, equaling or surpassing growth by scale resistant trees (Fig.10). Resistant trees exhibit higher needle litterfall (a proxy for needle production) but removal of scale does not have much effect (Fig.11). The total aboveground biomass for scale resistant trees is four times that of scale susceptible trees (Fig.12).

Moth Herbivory:
Trees with scale removed are more susceptible to moth attacks than scale susceptible trees (Fig.13).