Paddy panicle/sheath mite (Steneotarsonemus spinki)

Steneotarsonemus spinki Smiley is a tarsonemid sheath and panicle mite of rice. Feeding inside the developing flag leaf sheath and on reproductive parts causes grain discoloration, sterility and sheath-rot-like symptoms. It is included on USDA-APHIS's pest-alert list as a quarantine-significant invasive in several regions.

Identification and symptoms

The diagnostic damage is blackened or empty grains at maturity combined with brown sheath-rot symptoms on the flag leaf sheath. Affected panicles exsert partially or unevenly and carry a high proportion of chaffy spikelets. Because the mite itself is microscopic (under 0.3 mm), diagnosis usually relies on the combination of symptoms and microscopic examination of sheath scrapings.

Host crops and life cycle

The pest is essentially monophagous on cultivated rice. Generations are short and overlapping under warm, humid conditions, with populations building rapidly inside the boot before panicle exsertion. The species is also implicated as a vector or facilitator of sheath fungal pathogens such as Sarocladium oryzae (Paddy Sheath Stem Rot).

Damage and economic impact

Reported yield losses range widely, from about 5% in light infestations to 45% in severe outbreaks. Quality losses through chaffy and discoloured grains add to the economic impact and depress head rice recovery during milling.

Management

Cultural: avoid late and dense planting; manage stubble between crops to break population continuity.
Chemical: published Indian and global trials confirm efficacy of spiromesifen 22.9% SC, propargite 57 EC, fenpyroximate and combinations such as diafenthiuron + propiconazole when applied preventively at booting (Paddy Panicle Stage Management).
Quarantine awareness: under USDA-APHIS rules the species is flagged for trade in seed and unmilled rice.

References

Steneotarsonemus spinki: Rice panicle mite. USDA APHIS NPAG.
Bio-efficacy of pesticides against rice panicle mite. Biochem Journal.