Unaspis citri Comstock: Citrus Snow Scale

California Pest Rating for
Unaspis citri Comstock: Citrus Snow Scale
Hemiptera: Diaspididae
Pest Rating: A

Initiating Event:

Unaspis citri is frequently intercepted by CDFA and requires a pest rating proposal to support its pest rating.

History & Status:

Background: Unaspis citri, citrus snow scale, is an armored scale insect that primarily feeds on the trunk and tree limbs of older citrus trees¹. Populations of the scale reduce tree vigor and fruit production¹. Prolonged high populations can cause bark to split, allowing other insect pests and pathogens to invade the tree¹. Known hosts of citrus snow scale include: Anacardiaceae: mango (Mangifera indica³), Mangifera sp.²,³; Annonaceae: custard apple (Annona muricata²,³); Arecaceae: Cocos sp.²,³; Bromeliaceae: Ananas sp.²,³, Spanish moss (Tillandsia usneoides²,³); Celastraceae: evergreen spindle (Euonymus japonicus²,³), broad-leaved spindle tree (Euonymus latifolia²,³), Fabaceae: Acacia sp.²,³, Inga sp.³; Malvaceae: Hibiscus sp.²,³; Musaceae: Musa sp.²,³; Myrtaceae: guava (Psidium guajava²,³); Oleaceae: Osmanthus sp.²,³; Pittosporaceae: Pittosporum sp.²,³; Rosaceae: avocado (Persea americana²,³); Rutaceae: Mediterranean mandarin (Citrus deliciosa²,³), Citrus sp.²,³, key lime (Citrus aurantifolia²,³), bitter orange (Citrus aurantium²,³), Citrus decumana²,³, sweet orange (Citrus sinensis²,³), mandarin (Citrus reticulata²,³), grapefruit (Citrus paradisi²,³), tangerine (Citrus nobilis²,³), citron (Citrus medica²,³), Citrus medica acida², pomelo (Citrus maxima²,³), lemon (Citrus limon²,³), Citrus grandis²,³, kumquats (Fortunella sp.²,³), Glycosmis parviflora²,³, orange jessamine (Murraya paniculata²,³), trifoliate orange (Poncirus sp.²,³), Severina sp.²,³; Sapindaceae: rambutan (Nephelium lappaceum²). Unaspis citri may be transported long distances when infested plants or fresh plant parts are moved.

Worldwide Distribution: Unaspis citri is believed to have originated in Asia, but has spread through many of the citrus growing regions of the world¹.

Official Control: Unaspis citri has listed as a harmful organism by 46 nations including Albania, Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Germany, Greece, Holy See (Vatican City State), Hungary, India, Ireland, Israel, Italy, Japan, Jordan, Korea, Latvia, Lithuania, Luxembourg, Malta, Monaco, Montenegro, Morocco, Netherlands, New Zealand, Poland, Portugal, Romania, San Marino, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Taiwan, Turkey, United Kingdom, Venezuela4.

California Distribution: Unaspis citri has never been found in the environment of California.

California Interceptions: Unaspis citri is commonly intercepted by CDFA’s border stations on citrus fruit from Mexico and Florida.

The risk Unaspis citri (citrus snow scale) would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Citrus is widely planted across much of California and Unaspis citri can be expected to establish wherever it is grown. It receives a High (3) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Unaspis citri is known to feed on 34 varieties of plants in 13 families. It receives a Medium (2) in this category.

Evaluate the host range of the pest. Score:

– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.

3) Pest Dispersal Potential: Scale insects are capable of rapid reproduction and can be transported long distances when infested plants or fresh plant parts are moved. They may also disperse locally by crawling, wind, or by hitchhiking on clothing, equipment, or animals. Unaspis citri receives a High (3) in this category.

Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.

4) Economic Impact: If Unaspis citri were to establish in California it is expected that it would lower yields and increase crop production costs in citrus nursery stock and fruit industries. Citrus snow scale is also considered a quarantine pest by many of California’s trading partners. The presence of the scale in the state would disrupt markets for fresh fruit and nursery stock. Unaspis citri receives a High (3) in this category.

Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.

5) Environmental Impact: Unaspis citri is not expected to lower biodiversity, disrupt natural communities, or change ecosystem processes. It is not expected to directly affect threatened or endangered species or disrupt critical habitats. It may trigger additional treatment programs in agriculture and by residents who find infested trees unsightly or diminished citrus yields unacceptable. Citrus snow scale may also significantly impact ornamental plantings. It receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
D. The pest could trigger additional official or private treatment programs.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Unaspis citri (Citrus Snow Scale): High (14)

Add up the total score and include it here.

– Low = 5-8 points
– Medium = 9-12 points
– High = 13-15 points

6) Post Entry Distribution and Survey Information: Unaspis citri has never been found in California and receives a Not established (0) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.

– Not established (0) Pest never detected in California, or known only from incursions.
– Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
– Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
– High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (14)


Management for other citrus pests may mitigate some of the damage from Unaspis citri.

Conclusion and Rating Justification:

Unaspis citri has never been found in California and it is expected to have significant economic and environmental impacts if it were to enter the state. An “A” rating is justified.


1 Buckley, Courtney R. and Amanda C. Hodges. 2013. University of Florida Featured Creatures: Common Name: citrus snow scale. http://entnemdept.ufl.edu/creatures/orn/scales/citrus_snow_scale.htm

2 Miller, Dug, Yair Ben-Dov, Gary Gibson, and Nate Hardy. ScaleNet. http://scalenet.info/validname/Unaspis/citri/

3 SEL Catalog. http://www.sel.barc.usda.gov/catalogs/diaspidi/Unaspiscitri.htm

4 USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). https://pcit.aphis.usda.gov/pcit/

Responsible Party:

Jason Leathers, 1220 N Street, Sacramento, CA, 95814, (916) 654-1211, plant.health[@]cdfa.ca.gov.

Pest Rating:  A

Posted by ls