Adults of A. karli are approximately 3 mm in length, have a dark brown thorax and abdomen and distinct yellow heads. The flies also have light-yellow halteres (reduced hindwings) and dark brown legs with narrow, yellow ends of the femur and tibia.
Larvae are white and measure around 4.5 mm when fully grown. Pupae are brown and approximately 2.5 mm in length. The eggs of A. karli have not yet been observed, but the eggs of other closely related species are white, oval shaped and laid in clusters.
• This stem-boring fly was recently (2021) discovered in quinoa in the United States.
• In 2021, infestations were present in 100% of quinoa grown in San Luis Valley in Colorado, and either caused severe yield loss or destroyed the crop entirely.
• There are no management recommendations for this pest, however, there are monitoring protocols that can help growers determine whether an infestation is present.
Adult A. karli collected from quinoa grown in Alamosa, Colorado. Distinguishing features include a yellow head and dark brown body with light-yellow halteres. The legs are dark brown with yellow ends of the femur and tibia. Image credit: Tim McNary, Colorado State University
Larva of A. karli collected from quinoa stems grown in Colorado. These larvae feed within the plant stem of quinoa and cause plant death or severe yield losses. Image credit: Tim McNary, Colorado State University
Ventral view of an A. karli pupa. Most other stem-boring agromyzids pupate outside of the stems, and while pupation habits have not yet been observed in A. karli it is likely that the larvae exit the stems before pupating in the soil as well. Image credit: Tim McNary, Colorado State University
Exit hole in quinoa stem likely caused by larvae of A. karli. Image credit: Adrianna Szczepaniec, Colorado State University
Examples of field damage to quinoa resulting from an outbreak of A. karli. Image credit: Patrick O’Neil, San Luis Valley, Colorado
Life history and habits
Little is known about the life history and habits of A. karli. Many closely related species lay eggs by repeatedly thrusting an egg-laying structure, called an ovipositor, into plant tissue until the eggs are deposited. After hatching, A. karli larvae feed within the stems of quinoa and exit the stems to pupate outside of the plant in the soil. The flies likely have several generations each year.
Extensive feeding of A. karli larvae destroys the pith and disrupts nutrient transport which causes lodging, reduced yield, and death of the plant. This feeding injury has caused significant declines in quinoa production in Colorado. Upon exit, the larvae create exit holes in the stems.
Monitoring populations of A. karli in quinoa is effectively accomplished through several techniques that help determine the onset of an infestation and the extent of an infestation after A. karli is confirmed. To monitor the adults, yellow sticky traps can be used to capture flies and then identify and count them using a hand lens or a dissecting microscope. Fields can also be surveyed using a sweep net. Once A. karli adults are detected using sticky traps or sweep netting, quinoa stems can be destructively sampled by cutting plants at the base and splitting the stems to examine for the presence of larvae in the pith.
This is an emerging pest, and there are no research-supported recommendations for suppression of A. karli. However, host plant resistance and biological control are likely to emerge as the most effective management tactics. Stem boring insects are difficult to control using insecticides, and combining chemical control with other integrated pest management tactics is likely to be key to suppressing the impact of A. karli on quinoa.
Szczepaniec, A., and G. Alnajjar. 2023. New Stem Boring Pest of Quinoa in the United States. Journal of Integrated Pest Management. Available https://academic.oup.com/jipm/article/14/1/5/7032941