The Mexican Rice Borer

There is a new pest in town affecting the small farms in Fresno County. In November of 2023, the Mexican Rice Borer (MRB)(Eoreuma loftini) was  noted by Hmong lemongrass growers and brought to the attention of the previous Fresno Small Farms Advisor Dr. Ruth Dahlquist-Willard. The MRB has caused economic damage in rice and sugarcane in the southern and southeastern US, particularly Louisiana and Texas. Lemongrass growers in the Fresno area have said the borer has caused large yield losses though this has not yet been quantified. Continue reading to learn more about this insect and its' impact on local farmers.

This is the larva of a Mexican Rice Borer inside a rice stem (Photo J. Saichuk)

What is it?

This insect is a moth with four life stages. The adult prefers to lays eggs within folds of dry plant material but they will lay on fresh green tissue too. They can lay as many as 400 eggs at a time! The eggs take 5-14 days to mature before they hatch. The reason there is a big range is because the moths grow faster when it is warmer and slower when it is cooler.

Next is the larval or caterpillar stage. They feed on leaf parts that are easily available at first but eventually burrow inside the plant stem. The insects do the most damage in this part of their life and take 21-78 days to complete this stage. They can prolong this part of their life to live through winter inside grass stems. This means a cold winter is not likely to rid us of this pest.

When the larva has fed enough to make a bigger change it will pupate, or make its cocoon. it does this while still inside the plant's stem. It will remain in this stage for another week or two before it makes its transformation into an adult moth. They will live as adults up to two weeks.

This is the adult Mexican Rice Borer (Photo Credit: A Meszaros)

Where is it?

This cream colored moth is native to Mexico and no stranger to Riverside and Imperial counties in California. However, it has previously not been a serious enough pest in southern California to warrant longterm research inquiries. The UCCE Small Farms Team in Fresno has been tracking the insect since May of 2024 with pheromone traps. Thus far it appears to be infesting lemongrass and rice in a small area east of Fresno. The team has traps in all directions outside of this range and collaborators in the UC system have traps much further north and south so we hope to catch it, if it continues to expand from this area.

This map shows the general area where the insect has been caught in pheromone traps.

What Should Growers Do?

As stated above, the pest does not seem to have infested too large of a range so far. It will be important to scout for the pest by checking stems of affected crops for entrance holes or larvae. In rice, the presence of the larvae result in a "whitehead". This is when the grains do not fill in and the empty pannicle (or rice head) takes on a white appearance.

There are not many recommendations at this point for California growers. As the pest has previously not been an economic problem there are no registered insecticides for lemongrass or rice in the state that are expected to be effective. Cultural and physical management methods are currently the best options and are based off of studies in Louisiana and Texas.

One tactic may include planting early for rice. Late planted and ratoon rice provide abundant host plants when MRB populations are high. However, as lemongrass can span multiple seasons this may not be as effective in Fresno, as there will be host plants continuously present. Reducing the habitat for the moth when crops are not in production should reduce the overall pest population. This includes reduced stubble height for rice and sugarcane as well as stubble destruction after crop harvest for all crops. Removing lemongrass stems from the field by tilling or mowing can reduce latent habitat as well. Finally, removing dry tissue from fields and watering well to prevent drought like conditions can reduce the favorable dry leaves that adults like to lay eggs on.

Crops are not the only host of this pesky moth. The insect has been referred to as living in any grass with a wide enough stem to host the larvae. Johnsongrass, a pest itself, is a favored weed host of the MRB. Keeping fields and neighboring plots clear of grasses can reduce habitat.

This is a "whitehead" caused by the feeding damage of the larva.

Summary

The UCCE Fresno and Madera Small Farms Team will continue to work with our collaborators to track this pest and alert growers to any further expansion. Small farm producers of lemongrass, rice and sugarcane are allowing us to place pheromone traps on their farms to monitor for the pest. There will be further updates on scouting, types of damage in various crops, and management suggestions through this blog and our Hmong radio hour on Tuesdays at 2:00 PM PCT. Additionally, the team will begin small scale research trials to determine economic management strategies and alternatives for California lemongrass growers. If you have questions or concerns about this pest please reach out to me,  Emily Kraus, CE Small Farms Advisor.

This is a pheromone trap. The UCCE Small Farms Team has set these up across Fresno and as far north as Merced to track the pest.

References

Wilson, B. E., et al. "Expansion of the Mexican rice borer (Lepidoptera: Crambidae) into rice and sugarcane in Louisiana." Environmental entomology 44.3 (2015): 757-766

Showler, Allan T., and Thomas E. Reagan. "Mexican rice borer, Eoreuma loftini (Dyar)(Lepidoptera: Crambidae): range expansion, biology, ecology, control tactics, and new resistance factors in United States sugarcane." American Entomologist 63.1 (2017): 36-51.