Apple Leaf Gall Midge | Pest Rating Proposals and Final Ratings

California Pest Rating for

Dasineura mali (Kieffer): Apple Leaf Gall Midge

Diptera: Cecidomyiidae

Pest Rating: A

PEST RATING PROFILE

Initiating Event:

In August 2014 USDA released a draft pest list for apples and pears from the European Union. Dasineura mali, which is present in Europe, is not on this list. Because the fly presently has a temporary rating of “Q” a pest rating proposal is needed to determine future direction.

History & Status:

Background: Dasineura mali is a small fly that feeds only on species of apples. Adult females lay eggs in leaf folds of immature apple leaves¹. After eggs hatch, larval feeding causes the margins of leaves to curl¹. Curled leaves become brittle and may fall from the tree¹. Heavy infestations may reduce shoot growth, reducing photosynthetic leaf area⁵. This may stunt growth of nursery stock and newly planted trees. Larvae of the midge sometimes pupate in the calyx of fruit. They can therefore spread long distances by the movement of infested fruit, trees, or other apple plant material.

Worldwide Distribution: Dasineura mali is native to Europe¹. It has spread to New Zealand, Argentina, and Canada². In the United States it has been present in New York and Massachusetts since the 1960’s and was found in Washington State in 1994¹. The midge has since spread through much of western Washington, but it is not known if it occurs in the major apple production areas east of the Cascades.

Official Control: Dasineura mali is listed as a quarantine pest by China, Taiwan, Japan, Republic of Korea, and Peru³.

California Distribution: Dasineura mali has never been found in the environment of California.

California Interceptions: Dasineura mali has been intercepted by CDFA’s border stations 89 times since 2000, typically as pupae on apple fruit from New Zealand or the Pacific Northwest.

The risk Dasineura mali (apple leaf gall midge) would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Apple trees are commonly grown as ornamentals throughout California and apples are produced commercially in nearly every county in the state⁴. Dasineura mali is likely to establish wherever apples are grown and 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: Dasineura mali is only known to feed on species of apple (Malus spp.). It receives a Low (1) 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: Dasineura mali has high reproductive potential. There are 2-4 generations per year^{1, 5}, and each female lays many eggs. Although midges are not strong fliers, they may be spread long distances by wind or commerce in infested fruit or plants. Dasineura mali 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: California is the 2nd largest exporter of apples in the United States and the 5th largest producer⁴. California growers ship apples to 27 countries⁴. 14,000 acres are dedicated to apple production and yield a crop valued at over $105,000,0004. If Dasineura mali were to enter California it is probable that it would disrupt export markets due to its habit of pupating in the fruit calyx. It is also likely to increase production costs by triggering new chemical treatments since biological control is often not effective in commercial apple orchards due to the use of broad spectrum insecticides targeting other pests⁵. Dasineura mali larvae are protected from such applications inside leaf curls⁵. Fruit yield of mature orchards is usually not reduced by this midge, but they may sometimes have reduced fruit size and fruit bud formation⁵. Newly planted trees and nursery stock are vulnerable to reduced shoot growth⁵. Dasineura mali 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: Dasineura mali 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 habitat. The midge may trigger additional treatment programs in nurseries, orchards, and residential settings. Apple trees are popular ornamentals and may be disfigured by the presence of this midge. Dasineura mali 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 Dasineura mali (Apple Leaf Gall Midge): High (13)

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: Dasineura mali has never been found in the environment of 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 (13)

Uncertainty:

Dasineura mali was accidentally introduced into New Zealand in 1950 and was fortuitously controlled in orchards by existing treatments⁶. However, by the mid 1980’s midge populations and damage increased considerably to the point where new chemical treatments were required⁶. It is possible that damage to orchards in California will be limited until the species adapts to current integrated pest management practices.

Conclusion and Rating Justification:

Dasineura mali has never been found in California. Its entry to the state may have significant economic and environmental impacts to California’s apple industry and ornamental apple trees. An “A” rating is justified.

References:

¹Antonelli, Art and Jenny Glass. 2005. Apple Leaf Curling Midge. WSU Puyallup Plant & Insect Diagnostic Laboratory. http://puyallup.wsu.edu/plantclinic/resources/pdf/pls18appleleafcurlingmidge.pdf

²CAB Direct. Distribution Maps of Plant Pests 2008. http://cabdirect.org/abstracts/20083279226.html;jsessionid=39C077A8B4AABFDE351642AF7450D0FD

³USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). https://pcit.aphis.usda.gov/pcit/

⁴California Apple Commission website. http://www.calapple.org/faq.php?n=14&id=7#q3

⁵Apple Best Practice Guide, Department for Environment Food and Rural Affairs, UK. Horticultural Development Company. http://apples.hdc.org.uk/apple-leaf-midge.asp

⁶Tomkins, A.R., D.J. Wilson, C. Thomson, S. Bradley, L. Cole, P. Shaw, A. Gibb, D.M. Suckling, R. Marshall, and C.H. Wearing. 2000. Emergence of apple leafcurling midge (Dasineura mali) and its parasitoid (Platygaster demades). New Zealand Plant Protection 53:179-184.