How to Manage Pests
UC Pest Management Guidelines
Frosted Scale and European Fruit Lecanium
Frosted scale: Parthenolecanium pruinosum
European fruit lecanium: Parthenolecanium corni
In this Guideline:
The frosted scale is
the most important soft scale pest of walnuts. Its frostlike cover and copius
honeydew secretion in spring make it easy to recognize. This scale has one
generation a year. It overwinters as
a nymph on twigs and small branches. In spring it grows rapidly, becomes
convex, forms a frostlike waxy
and secretes large amounts of honeydew. In late spring females lay many eggs,
which fill the entire space beneath their cover, and die after egg production.
The white waxy substance weathers away, leaving oval, dark brown covers that
may be present for a year or more.
Newly hatched nymphs or crawlers emerge from beneath the scale cover from late May through June and settle mostly on the underside of leaves. Here
they feed for the rest of the summer. In fall, the nymphs molt and move back to
The European fruit
lecanium has essentially the same life cycle. The immature stages closely resemble those
of the frosted scale, but the adults do not form the thick, frostlike cover in
spring. Instead, the cover is domed, shiny brown, and about 0.25 inch (6 mm) in
diameter with several ridges along the back.
Soft scales suck plant juices from leaves and twigs. Low to moderate
populations apparently are not damaging, but heavy populations reduce terminal
growth and vigor, resulting in smaller nuts and poor kernel quality. The
secreted honeydew may cover nuts and favor the growth of sooty mold, increasing
the chances for sunburn damage.
Parasites play an important role in controlling these soft scales.
If this natural control is disrupted by adverse weather or by insecticides
applied for other pests, a treatment may be required.
The most important of the parasitic wasps that attack these soft scales
Encyrtus, and Metaphycus spp. The Metaphycus wasps produce several generations a year, compared
with one generation of the scale, and parasitize all stages of the frosted
scale except the eggs. Parasitized
nymphs are almost black and have
convex covers; unparasitized nymphs are flat and opaque. Several parasites
commonly emerge from a single parasitized adult scale, leaving a perforated cover.
Organically Acceptable Methods
Biological control and sprays of narrow range oils can be used in an
organically certified crop.
Monitoring and Treatment Decisions
Monitor scales during the dormant period. For details on how to monitor
these scales with other pests
MONITORING. Examine the previous
season's growth on randomly selected trees throughout the orchard. If you find
more than five nymphs per foot of last year's wood throughout the orchard and
parasitism is not significant, a treatment is warranted. If a treatment is
necessary, apply it during the delayed dormant period before rapid scale growth
begins in early spring. Treatments applied to first instar scales on leaves are
also effective. High populations of soft scales often result from the use of
chemicals that are disruptive to parasites and predators. If a high degree of
parasitization is observed, treatments may be delayed until after crawlers
emerge in late spring. Set out double-sided sticky traps in early spring to
monitor for crawlers.
||Amount to Use**
|The following materials are listed in order of usefulness in an IPM program,taking into account efficacy and impact on natural enemies
and honey bees. When choosing a pesticide, also consider information relating to environmental
impact. Not all registered pesticides are listed. Always read label of product being used.
|CAUTION: Oils are not recommended for use during the dormant season on walnut trees.
||MODE OF ACTION GROUP NUMBER1: 1B
||COMMENTS: Do not combine with oil or severe phytotoxicity may
occur. Do not graze livestock in treated orchard. Make no more than one
application during the dormant period or more than 2 cover sprays each
season. Take mitigating measures to prevent this pesticide from entering
surface water with orchard runoff. Restricted entry interval is 48 hours when 8 or less lb per acre used and 14 days when more than 8 lb per acre applied.
||MODE OF ACTION GROUP NUMBER1: 1B
||COMMENTS: Make no more than 2 applications/season. Levels in
surface waters of this material that are high enough to be toxic to certain
aquatic invertebrates have occurred following rains; avoid runoff into surface waters.
||NARROW RANGE OIL#
||MODE OF ACTION: Contact including smothering and barrier effects.
||COMMENTS: Moderately effective on these scales during the
delayed dormant period. Apply only after buds begin to swell as a dilute
application in at least 300 gal/acre. An application in summer will suppress
low-to-moderate populations. In most areas, oils can be applied to walnuts
during the delayed dormant period (as buds begin to swell) and in summer.
However to avoid injury, the trees must not have suffered from a lack of
adequate soil moisture or other stressing factor (insects, disease damage,
etc.) at any time during the year and the temperature must not exceed 90°F
at or shortly after time of application. If in doubt, check with your farm
advisor. In any case, do not apply oils to walnuts during the dormant season
or between bud break and shoot elongation. Check with certifier to determine which products are organically acceptable.
UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471
Insects and Mites
C. Pickel, UC IPM Program/UC
Cooperative Extension, Sutter/Yuba counties
J. A. Grant, UC Cooperative Extension, San Joaquin County
W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
W. W. Coates, UC Cooperative Extension, San Benito County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
W. H. Olson, UC Cooperative Extension, Butte County
L. C. Hendricks, UC Cooperative Extension, Merced County
G. S. Sibbett, UC Cooperative Extension, Tulare County
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