Defoliating insects damage trees by eating leaves or needles, removing the photosynthetic tissue critical for plant maintenance and growth. A significant loss of leaves or needles results in growth loss, increased susceptibility to attack by other insects and pathogens, and sometimes tree mortality.

The impact of defoliation on individual trees is dependent on a variety of factors. Tree species vary in their tolerance to defoliation. In general, hardwood species can sustain repeated defoliation over several years because they store large food supplies and can refoliate in the same year. The timing of the defoliation relative to seasonal growth phenology is also important. For example, late season defoliation of hardwoods has a lower impact than does late season defoliation of conifers. A single late season defoliation of pines often results in tree mortality. Trees that are healthy and growing vigorously will generally survive defoliation better than stressed trees.

Historically, the two most destructive defoliators in the Southwestern Region were the western spruce budworm and the Douglas-fir tussock moth. Both of these defoliators can cause severe growth loss, top-kill, increase susceptibility to bark beetles, and cause outright tree mortality. More recently, three other defoliators have been causing heavy defoliation in higher elevation forests. Beginning in the early 1990s, the exotic spruce aphid and a native looper, Nepytia janetae, are causing high levels of mortality in the spruce-fir forest type in the White Mountains and Pinaleño Mountains of Arizona. A New Mexico fir looper outbreak began in 2002 in the Sacramento Mountains of southeastern New Mexico causing elevated levels of fir mortality.

Many other defoliating insects that have the potential to cause localized problems are not included in this guide. Consult Federal or State forest health personnel for assistance on insects not described in this guide.

References:  40, 66, 72, 108