Rice Farming

Mighty Mite

The panicle rice mite – a new pest in the United States

By Natalie Hummel

During the summer of 2007, breeding facility greenhouses and a limited number of fields in Arkansas, Louisiana, Texas, and New York were found to be infested by the panicle rice mite (PRM), Steneotarsonemus spinki Smiley. This included one commercial field in Vermillion Parish, which was infested with both PRM and bacterial panicle blight.

Currently, the PRM is present in all rice-producing regions of the world, with the exception of Brazil. In the tropical climate of the Caribbean, it has caused the most significant crop losses. Fortunately, the damage from this mite can be minimized by breeding programs and proper cultural management practices.

Panicle rice mites are clear to straw-colored and are approximately 1/100 of an inch in length. Male PRM have elongated rear legs containing a pair of elongated spines. The legs are carried above the body. Female PRM are ovoid-shaped.

Larval stages are about half the size of adults. Eggs are also about 1/3 the size of adults. The PRM is parthenogenetic, which means that virgin females can produce male offspring. The female will then mate with these male offspring and produce eggs. A mated female PRM can produce an average of 55 eggs in her lifetime.The life cycle in the laboratory can vary from three days at 86 degrees F to 20 days at 68 degrees F. If held in the laboratory at 17.6 degrees F for 72 hours, all PRM die.

PRM damage to plant tissue and grains
Panicle rice mites cause damage to plants both directly by feeding on leaf tissue in the leaf sheath and developing grains at the milk stage and indirectly by transmitting fungal pathogens. PRM have been associated with sheath rot and bacterial panicle blight. PRM can carry sheath rot spores on their body.

It is thought that feeding by the mites causes damage to plant tissue, which may facilitate entry of fungal pathogens into developing grains and the leaf sheath. Damage to grains can result in sterility and deformed grains, parrot-beaking of grains and straight-head. Damage to the leaf sheath may decrease yield. Research is needed to determine how much damage is caused directly by PRM feeding and how much is caused by PRM associated with disease.

PRM scouting guidelines
Scout for panicle rice mite by looking for symptoms associated with bacterial panicle blight and sheath rot. In affected plants, look for a cinnamon, yellow or chocolate-brown discolored lesion on the leaf sheath that does not have a distinct edge.

To find mites, pull the leaf sheath back and examine the underside of the leaf sheath with a minimum 20X hand-lens. A 30X hand-lens is preferred for identification purposes.

The PRM feeds on plant material on the inside of the flag leaf sheath. Once a new leaf begins to develop, a female PRM will move to the new leaf sheath, produce male offspring and establish a new feeding lesion. Damage will often be observed on inner sheaths when the outer sheath is removed. This continues until the PRM reaches the leaf nearest the stem. They also feed on developing panicles.

The PRM – where it flourishes and how it moves
High temperatures and low rainfall amounts are favorable for development of large populations of PRM in the field. Continuous rice culture, as well as sharing equipment between infected and uninfected fields is also conducive to building economically damaging populations of PRM.

The PRM has many potential modes of movement between fields including: hitchhiking on insects, people and equipment; floating on irrigation water and blowing on the wind.

USDA-APHIS, in cooperation with local state regulatory agencies, is currently developing a plan to survey the United States to determine the distribution of the panicle rice mite in commercial rice fields. LSU AgCenter staff are also exploring options to manage this pest. There are currently no management guidelines. Contact your county agent if you suspect that this mite may be present in your field and to obtain the latest information on this pest.

Additional information about the PRM can be obtained at www.lsuagcenter.com.

Natalie Hummel is an entomologist, LSU AgCenter, Baton Rouge, La. Contact her at (225) 578-7386 or nhummel@agcenter.lsu.edu.

Scientists search for PRM control options

Chemical control for field infestations of the panicle rice mite (PRM) has been difficult in other countries because PRMs are present in a protected area of the plant – behind the leaf sheath. Research is being conducted to determine if systemic miticides and/or insecticides with trans-laminar activity could be a good option for control.

Predatory mites may have the ability to suppress populations. Fungal pathogens that kill the PRM may be another control option.

Cultural controls often recommended to reduce PRM infestations in fields include:
• Plowing stubble after harvesting the rice crop and ensuring no re-growth of plant material for winter.
• Fallowing fields.
• Rotation with an alternate crop.
• Cleaning machinery before using in an un-infested field.
• Avoidance of second cropping.

In newly planted fields, scouting should begin 15 days after plant emergence to detect early PRM infestations. These cultural control methods – in addition to breeding for resistance – have successfully suppressed PRM densities in some infested countries.

Where do we go from here?
At this time, it is very important to begin surveying for the PRM in rice fields in the United States. The rice industry will benefit from identifying infested fields quickly to effectively treat them before densities exceed treatable levels.

Currently no miticides are labeled for field management of the PRM infestation in the United States. Scientists are in the process of developing possible chemical options to manage the infestation.

If the PRM becomes an economically limiting pest, it may be necessary to modify some cultural practices to limit the impact of the PRM in rice-producing areas of the United States.