Native to South America, it first established around Mobile, Alabama in 1947 and has since spread throughout the Southeast, where it is found from Florida and North Carolina west through Texas and Oklahoma. Recently, it has also been reported from southern Illinois (2013) and California (2011).
Figure 1. Microtheca ochroloma. First-, second- and fourth-instar larvae, pupa, and adult. Photo: Michael Skvarla, Penn State
Models based on cold hardiness of the eggs, which are the stage most resistant to freezing temperatures, predict the beetle can colonize southeastern Pennsylvania (Figure 2). In October 2017, a grower in Montgomery County contacted Penn State extension due to an unknown beetle causing substantial defoliation to a crop of napa cabbage. The beetle was identified as YMLB and is the first known occurrence of the pest in the state.
Figure 2. Predicted cold-tolerance limits of yellowmargined leaf beetles in the northeastern United States with inset showing the predicted limits across the continental U.S. Predictions are based on the eggs, which are the most cold-hardy stage. At 32°F (0°C), 50% of the eggs died after 13 days and 90% died after 38 days. Figure modified from larger map (upper left) by Manrique et al. (2012); used with permission.
Adult YMLB are 0.2 in (5 mm) in length and resemble other leaf beetles, such as striped cucumber beetles, in shape, but with colors and patterns that make they fairly easy to distinguish. . They are black to dark brown with a conspicuous yellow to brown or red border along the edge of the elytra (hardened wings). Adult beetles can live up to 105 days and can mate and lay eggs multiple times.
The elongate, bright orange eggs (Figure 3) are laid in clusters of 15–60 on the undersides of leaves or on dry plant material or the soil surface. The egg clusters resemble those of lady beetles, except that lady beetle eggs are laid upright in tightly packed clusters while YMLB are loosely packed, irregular clusters. Female YMLB can lay up to 1400 eggs, although usually average 400–500; additionally, the number of eggs laid is significantly affected by the host plant (e.g., more on preferred hosts like turnip, fewer on less preferred hosts like cabbage) and adult longevity.
Figure 3. Yellowmargined leaf beetle eggs. Photo: Michael Skvarla, Penn State
Larvae are yellowish to nearly black, with conspicuous hair-bearing plates. They progress through four instars and development typically lasts between 7 days (at 85°C) to 23 days (at 60°C). Young larvae aggregate to feed while older larvae disperse and become solitary.
Prior to pupation, the larvae spin a loose brown web or net, which can make the pupae resemble dirt or other debris. Pupae are usually found on the underside of leaves, but can also be found on the soil surface or in mulch and other debris.
Yellowmargined leaf beetle is a serious pest of cruciferous crops and can be the most serious pest to organic cruciferous crops where it occurs. It is strongly attracted to and causes the most damage to turnips and napa cabbage. It is less attracted to but has been recorded on cabbage, collard, mustard, mizuna, arugula, radish, and watercress. Other brassicas, such as broccoli and Brussel sprouts, are presumably fed upon as well. Additionally, YMLB can be found on uncultivated and wild Brassicaceae, such as wild mustard and turnips, which can serve as alternate hosts and sustain beetle populations between crop plantings.
The beetles feed on the leaves and can cause severe defoliation (Figure 4), resulting in total loss of leafy crop; when leaves of radishes and turnips are totally consumed, the beetles and larvae will continue to feed on the exposed tubers, which can result in crop loss as well.
Figure 4. Defoliation by yellowmargined leaf beetle to Yokatta-Na, a type of Asian cabbage (Brassaca rapa subsp. narinosa), in southeastern PA. Photo credit: Stephanie Jones, used with permission.
Handpicking beetles and larvae off infested plants can be an effective control method on small scales. The adults and larvae readily drop off the plant when disturbed and can be easily collected by lightly shaking plants over a container of soapy water.
A number of cultural practices that can help manage YMLB if it becomes established on a farm. The presence of mulch and other plant debris improves the survival of overwintering beetles and eggs, which is likely very important in Pennsylvania as it is at the expected northern limit of the species. Therefore, cleaning up and disposing of unharvested plants, plant debris, and mulch may reduce the number of suitable overwintering sites and therefore reduce the beetle population the following growing season.
Wild brassicas, such as wild or charlock mustard (Sinapsis arvensis) and field mustard/wild turnip (Brassica rapa), are common weeds that are likely utilized as alternate hosts. They likely serve as sources of re-infestation after pesticide applications. Eliminating such weeds from fields and nearby areas is an important factor in reducing on-farm beetle populations, although this area has not been researched in depth yet.
Unfortunately, no egg parasitoid wasps have been recorded in North America despite collection and rearing of thousands of eggs and no investigations have been made to find such wasps in South America that could be introduced for biological control. One study found spined soldier bugs (Podisus maculiventris) to be effective biocontrol agents and suggested releasing 10 first-instar P. maculiventris if plants have 7 or more leaves, and 4 first-instar P. maculiventris if plants have 6 or fewer leaves. Other generalist predators feed on YMLB, but none are known to exert enough control to stop economic injury. A few fungi have been recorded to attack YMLB and are detailed below under chemical control.
Turnips are especially attractive to YMLB and can be utilized as a trap crop when planted along the perimeter of other, less attractive (to the beetles) cash crop crucifers, such as cabbages (see Damage above for other less attractive crops). This method relies on a number of factors, including that YMLB generally colonizes a crop by migrating into a field or greenhouse rather than emerging from within, the beetles tend not to move between plants once a suitable host plant is found, and the fact that they exhibit strong edge effect behavior, that is, they colonize the field margins first before moving into the center as pest density increases. A recent study found that turnip planted along the field perimeter ~2 weeks before the cash crop reduced the YMLB density in the cash crop by 2–8x.
The addition of an appropriately timed application of pesticide was adequate to prevent the beetles from spreading into the cash crop and reduced or eliminated the need for a pesticide treatment of the cash crop. However, because YMLB has higher reproductive rates on turnips compared to other brassicas, caution should be used in late season as beetle densities increase and the trap turnips could serve as a source of infestation to the cash crop rather than act as a sink as they do during the early season.
Organic chemical control
The number of OMRI-approved chemical control options is rather limited. One study found that weekly sprays of spinosad (Entrust: Dow AgroSciences LLC, Indianapolis, IN) or alternating sprays of spinosad and pyrethrin (PyGanic: Valent BioScience Corporation, Libertyville, IL) or the entomopathogenic fungi Isaria fumosorosea Wize (NOFLY: Natural Industries, Inc., Houston, TX) gave the best suppression of adults and larvae and reduced crop damage.
Conventional chemical control
Yellowmargined leaf beetles are not generally pests in conventionally grown crops. If YMLB does become a problem, pesticides labeled for flea beetle control will be sufficient.
This article summarizes and expands upon an article that has been submitted for peer-reviewed publication: Skvarla, M.J., and S.J. Fleischer. First report of yellowmargined leaf beetle, Microtheca ochroloma Stål, 1860 (Coleoptera: Chrysomelidae), in Pennsylvania.
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