Authors: Buyung Hadi and Jeffrey Bradshaw
There are three leafmining leaf beetle species that may occur on soybean. Soybean leafminer, (Odontota horni), locust leafminer (Odontota dorsalis) and Sumitrosis rosea. Soybean leafminer is a more common leafminer species occurring on soybean compared to the other two species.
Identification
The adults of soybean leafminer are small squarish beetle (about 6 mm long and 2.5 mm wide) with black head, brick red upper body and black underside. A narrow black stripe that doesn't extend to the end of the body can be seen on the middle of the upper side of the body. On locust leafminer, this stripe extends to the back end of the body. Adults of S. rosea are wedge-shaped, tan with brown spots, about 4 mm long and 2 mm wide. Larvae of all species are cream or reddish with dark colored head and lateral skin protrusions on each segment of the body. The full grown larvae of soybean and locust leafminers are 7 mm long and the larvae of S. rosea are about 3.5 mm in length.
Life Cycle and Seasonal History
Soybean leafminer and other associated leafminers are believed to overwinter as adult beetle. The beetle find soybean in early summer and feed before laying eggs. Mated female beetle lay the eggs singly or in loose group of up to eggs on the underside of soybean leaves. The hatching larvae burrow into the leaves, feeding on the mesophyll tissues between the epidermal layers. The larvae develop through three stages and pupate in the 'mine' within the leaves. It takes about 35-40 days for the eggs to develop into adult beetles. The emerging adult vacate the mine and start feeding on the host leaves. Adult beetles move into overwintering sites in late summer.
Plant Injury and Damage
The larvae feed within the leaves producing mines that look like blotches of light colored areas on the leaves. The adult beetles feed on leaf tissues and avoiding leaf veins producing skeletonized appearance. A larva of soybean leafminer and S. rosea mines an average of 4.8 and 1.4 cm2 of soybean leaf tissue, respectively. Newly emerged beetles of soybean and locust leafminers consume an average of 15.4 cm2 of leaf tissue over 45 days. Adult S. rosea consume about 6 cm2 over the same period of time.
Soybean pod is not a preferred food source of soybean leafminer. When feeding occurs, it results in superficial scraping of the pod surface.
Pathogen vectored
Soybean leafminer has been reported to successfully transmit Bean Pod Mottle Virus (BPMV) with 60% transmission efficiency. Soybean leafminer can feed on and acquire the virus from a wild host (Desmodium spp.). Since the beetle usually present in abundance in the beginning of soybean season, soybean leafminer maybe a part of the vector complex that introduce BPMV into soybean fields.
Management Approaches
Scouting and Threshold
The feeding activity of leafminers larvae are not deemed economically damaging for soybean. Adult beetles feeding on early season soybean (VE-V2) may result in economic damage, especially since field observations showed that the adult soybean leafminer population is the highest at the time of soybean emergence. Prolonged feeding period of adults on later stage soybean (R4-R6) may pose a certain risk, although the adult beetle population do not usually attain economically damaging level. At these stages, the injury due to leafminer adults should be considered together with the injuries imposed by other defoliating pests.
A preliminary observation during an outbreak year in Iowa showed that soybean leafminer is somewhat regularly distributed within rows and highly concentrated on the upper part of the canopy. Sampling using sweep nets on predetermined row-lengths on 10 random sites throughout the entire field should be sufficient in estimating the number of adult beetles per plant. For soybean seedlings, the economic injury level for soybean leafminer is 9 adults per row-ft (29 adults per row-m). For soybean in the development stage V6 to maturity, the economic injury level for soybean and locust leafminers is 125 adult per row-ft (410 adults per row-m) and for S. rosea is 325 adults per row-ft (1066 adults per row-m). Insecticidal control of soybean leafminers focuses on adult beetles since the leafmining larvae are protected within the leaf tissues.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com
