Figure 1. Panicle rice mite on the inside of a rice leaf sheath.

The panicle rice mite (PRM), Steneotarsonemus spinki Smiley, (Figure 1) has recently been reported in the continental United States. The panicle rice mite is a pest of commercial rice, Oryza sativa L., and also completes its development on the invasive plant Oryza latifolia Desv. During the summer of 2007, breeding facility greenhouses and in some cases fields in Arkansas, Louisiana, Texas and New York were found to be infested by PRM. This included one commercial field in Vermilion Parish, which was infested with both PRM and bacterial panicle blight. This combination appeared to reduce the yield. Currently, the PRM is listed as a reportable and actionable pest by the USDA APHIS. The reason for this status is that it has been reported to cause from 5% - 90% crop losses in the Caribbean region. The mite is thought to have originated in Asia, first being reported in the 1930s. It then spread to India, Africa, Central America, the Caribbean and Mexico. Currently, it is found 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.

The PRM is not thought to have the ability to thrive in the temperate climate of the United States. Yet, southwest Louisiana has a sub-tropical climate marked by both high temperature and high humidity. The PRM thrives under both of these conditions. Furthermore, the PRM has been reported to cause economically significant losses when found in association with Burkholderia glumae (bacterial panicle blight) and Sarocladium oryzae (sheath rot) pathogens. Both of these pathogens are present in southwest Louisiana. For these reasons, we are launching a survey of the state to determine the distribution of PRM in rice-producing parishes of Louisiana. This article provides information on the worldwide distribution, lifecycle, biology and cultural management of this pest of rice.

Description, Life Cycle and Biology

The PRM is not visible to the naked eye, and a minimum 20X hand lens is required to observe PRM on the inside of the leaf sheath. The PRM are clear to straw-colored and are approximately 250 µm in length (Figure 2). Male PRM have elongated rear legs containing a pair of elongated spines (Figure 3). The legs are carried above the body. Males are very active and can be observed moving on the surface of the leaf. Female PRM are ovoid-shaped (Figure 4). Larval stages are about half the size of adults. Eggs are also half the size of adults (Figure 5).

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 lifecycle in the laboratory can vary from 3 days at 86º F to 20 days at 68º F. If held in the laboratory at 17.6º F for 72 hours, all PRM died. PRM may be transported in seed inside the hull. PRM in seed can be killed by extreme cold or hot temperatures or by chemical fumigation. Normal drying of rice for consumption may kill the mite inside the seed. Methods of controlling PRM both in seed and on plants are being investigated by LSU AgCenter scientists.

PRM feed behind the leaf sheath (Figure 6). The feeding lesion can be detected by cinnamon to chocolate-brown discoloration of the leaf sheath (Figures 7, 8, 9). 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. Thus, damage will often be observed on interior sheaths when the outer sheath is removed. This continues until the PRM reaches the leaf nearest the stem. They also feed on developing panicles from the boot stage to the milk stage of heading.

High temperatures and low amounts of rainfall are favorable for development of large populations of PRM in the field. Continuous rice culture, as well as the sharing of equipment between fields, is also conducive to building economically damaging populations of PRM. The PRM has many potential modes of movement between fields, including: 1) hitchhiking on insects, people and equipment, 2) floating on irrigation water, 3) blowing on wind and 4) transmission on green seed.

Damage Associated with Panicle Rice Mite Infestations

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. When feeding, they inject a toxic saliva. 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 results in sterility and deformed grains, parrot-beaking of grains and straight-head (Figures 10, 11, 12). Damage to the leaf sheath may have a negative effect on fertility.

Crop losses ranging from 5% - 90% have been attributed to PRM in countries where economically damaging populations have become established. It is very difficult based on the literature to determine what damage is caused directly by PRM feeding and how much damage is caused by pathogens often associated with the PRM. Research is needed to determine how much damage is caused directly by PRM feeding and how much is caused by PRM associated with disease. The largest and most economically damaging populations of PRM are often reported in late-planted and second-crop rice.

Monitoring and Control

Damage symptoms associated with PRM include light-yellow, cinnamon or chocolate-brown marks (feeding lesion) on the outside of leaf sheaths (Figures 7, 8, 9). It is necessary to open leaf sheaths and observe the inside of the leaf sheath in order to observe PRM on the plant (Figure 14). Because of the common association between PRM and sheath rot and bacterial panicle blight, it will be wise to begin sampling for these mites in fields that have a high incidence of these diseases. If you observe any of these diseases in your field, please contact your county agent.

Chemical controls are not normally efficacious because the PRM are present in a water-sealed area of the plant -- behind the leaf sheath and near the stem. Thus, systemic miticides will be our best option for chemical control. Predatory mites may have the ability to suppress populations. Fungal pathogens that kill the PRM may be another control option. Cultural controls include plowing stubble after harvesting crops 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, sampling 15 days after planting to catch mite populations at low levels, and avoidance of second-cropping. These cultural control methods in addition to breeding for resistance have successfully suppressed populations in some infested countries.

If PRM are present in the field, we will all benefit from identifying infested fields quickly in order to effectively treat infested fields before populations exceed treatable levels. There are a variety of miticides that may be used to treat these mites, but we will need to work toward registration of these chemistries in rice if necessary. If the PRM becomes an economically limiting pest, we will need to consider modifying cultural practices to limit the impact of this mite in the rice-producing parishes of the state.