Macrosaccus neomexicanus

The combination of minute size (forewings 2.7–3.5 mm), geographic restriction to Arizona and New Mexico, and exclusive association with Robinia neomexicana distinguishes this species from congeners. The larval leaf mine is diagnostic: beginning as a short serpentine track that expands into an elongate-oval whitish blotch on the leaflet underside, eventually becoming strongly tentiform with silk deposition that causes the upper surface to roll over. Similar Macrosaccus species occur on different host plants or in different regions.

Adults are minute moths with forewings measuring 2.7–3.5 mm in length. The wings and body exhibit typical gracillariid proportions for the genus. Larvae are internal feeders within leaf tissue and are not typically observed externally.

Associated with stands of its host plant, New Mexico locust (Robinia neomexicana), which grows in canyon bottoms, riparian zones, and moist slopes in montane and foothill regions of the southwestern United States. The moth has been collected in the Chiricahua Mountains and other sky island ranges of southeastern Arizona.

Southwestern United States: Arizona and New Mexico. Documented from the Chiricahua Mountains in Arizona and presumably from New Mexico based on host plant distribution and the species epithet.

Larvae feed exclusively on Robinia neomexicana (New Mexico locust, Fabaceae). Adults do not feed; their mouthparts are reduced or non-functional as in most gracillariids.

• Robinia neomexicana - larval hostNew Mexico locust; larvae mine leaves

Larvae develop through feeding stages inside leaf mines. The mine progresses from an initial serpentine track to an elongate-oval blotch, then becomes tentiform as the larva deposits silk. Pupation occurs within the mine. Specific details regarding number of generations per year, egg-laying behavior, and adult emergence timing are not documented.

Larvae are endophagous leaf miners that progressively modify their feeding site. As they develop, they lay down silk that contracts the mine into a tentiform structure, causing the leaf upper surface to roll over. This behavior creates a sheltered microhabitat within the leaf.

As a specialized herbivore, larvae act as a foliar parasite on Robinia neomexicana, potentially affecting leaf function and plant fitness. The species contributes to the diversity of leaf-mining insects in southwestern riparian and montane ecosystems. No information is available regarding parasitoids or other trophic relationships.

No direct economic or agricultural significance has been documented. The species is not known to be a pest of cultivated plants. Its narrow host specificity on a native tree species suggests limited potential for economic impact. The species was described relatively recently (2011), indicating ongoing documentation of regional biodiversity.

• Macrosaccus morrisellaAnother North American Macrosaccus species, but associated with different Fabaceae hosts including soybean and American hog peanut; occurs in more northern and eastern regions (Canada, Minnesota, northeastern United States). Larval mines are blotch-type but not described as becoming strongly tentiform with leaf rolling.
• Other Gracillariidae on RobiniaThe genus Macrosaccus appears to be the primary gracillariid lineage associated with Robinia; other leaf miners on this host would likely be in different families (e.g., Agromyzidae) with different mine morphologies.

• leaf miner
• Gracillariidae
• Robinia
• Fabaceae
• sky islands
• endemic
• recently described

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Grasshopper Collections and Survey
• Leaf-Mining Moth May Be New Pest of Soybean
• 2018 Arizona Insect Collecting Trip “iReport” | Beetles In The Bush