ENTFACT-330: Flea Beetle IPM in Brassica Production 

Elaine Losekamp and Dr. David Gonthier
Entomology Graduate Researcher and Associate Professor of Entomology
University of Kentucky College of Agriculture 

Description 

Flea beetles are a family of small, leaf-eating beetles that often have a black, brown, bronze, or bluish coloration. In Kentucky, two introduced species, crucifer (Phyllotreta cruciferae) and striped (Phyllotreta striolata) flea beetles, are common and problematic pests of brassica crops. Also known as ‘cole crops’, this economically important group includes vegetables such as broccoli, Brussels sprouts, cabbage, and turnips, as well as leafy greens like arugula, bok choy, kale, and mustard greens.  Crucifer and striped flea beetles are specialist herbivores, feeding only on plants in Brassicaceae and a few closely related families. Both species have an elongate-oval body shape and are approximately 2 mm long. Crucifer flea beetles are shiny and solid black, while striped flea beetles are black with a thick amber or yellow line running down the length of each elytrum, or wing case (Figure 1).

Flea beetles can be identified by their distinctive jumping behavior and feeding patterns. Using the powerful enlarged femurs in their hind legs to evade potential predators, their movement is reminiscent of the flea beetles’ namesake. The insects leave characteristic ‘shot hole’ feeding damage on leaves (Figure 2, 3). These small holes result from the beetles chewing through the leaf surface and reduce the plant’s ability to photosynthesize. Shot-hole feeding damage can have negative financial consequences for farmers by killing seedlings, delaying crop growth, and rendering leafy greens unmarketable.

Biology

The life cycles of crucifer and striped flea beetles are similar. Crucifer flea beetles produce 1-2 generations per year, while striped flea beetles typically have only one. Adults overwinter in plant residues and emerge in early spring. Females lay eggs in the soil at the base of host plants. The eggs hatch within two weeks and the newly emerged larvae burrow into the soil, where they feed on the roots of their host plants and undergo development underground before emerging as adults 1-2 months later. Adults live aboveground and feed on leaves.

Flea beetles have multiple adaptations that have enabled their success. In addition to their characteristic flea-like jumping abilities, flea beetles are strong fliers, able to disperse widely in search of food or mates. The beetles also have a chemical defense. Glucosinolate, which the flea beetles sequester while feeding upon brassica plants, makes the insects unpalatable to potential predators.

Crucifer and striped flea beetles also use pheromones to their benefit. Males release aggregation pheromones when feeding, attracting other flea beetles of both sexes. Aggregation enables the beetles to locate mates and food, collectively overcome plant defenses, and feed more effectively.

 These adaptations, which protect the insects from predators and enable them to congregate where food sources are abundant, serve to make crucifer and striped flea beetles significant pest species. However, multiple management strategies can help growers mitigate their impact on brassica crops.

Management Strategies

Integrated Pest Management (IPM) emphasizes cultural practices that prevent insects such as flea beetles from establishing and becoming problematic pests. The IPM framework encourages growers to use chemical controls as a last resort to preserve the effectiveness of pesticides and mitigate harm to people, non-target insects, and the environment. Monitoring is essential throughout the growing season to help growers make informed management decisions. In the following sections, IPM recommendations for flea beetle management in brassica production are explored.

1. Cultural Controls

Field Sanitation

Proper sanitation practices can make a plot or field less hospitable for flea beetles. Weed management is crucial because weeds in the Brassicaceae family can provide a source of food and shelter before crops are planted in the spring. Without suitable food sources, newly emerged flea beetles may disperse elsewhere in search of food. Likewise, brassica crop residues can provide shelter for overwintering flea beetles and should be removed after harvest.

Cultivar Selection

Crop and cultivar selection can affect the chances that flea beetles become problematic later in the season. Some brassica varieties are less susceptible to damage from flea beetles than others. Like humans, flea beetles have distinct preferences for what they eat. Flea beetles have been shown to prefer leaves that are smoother, less waxy, and have higher levels of mustard oils. If flea beetles are a concern, growers may consider planting brassica varieties that have hairier or waxier leaves or lower levels of mustard oils. When growing crops that are highly attractive to flea beetles, it may be prudent to proactively use additional management strategies. 

Timing of Planting and Harvest

Flea beetles often cause the most damage in the spring following the emergence of the overwintering generation. Growers can avoid some of this damage by choosing planting dates that minimize overlap with high levels of flea beetle activity. Plantings of brassicas in the summer and fall months may experience lower flea beetle pressure than spring plantings. Alternatively, brassica crops could be grown early in the year and harvested before flea beetles emerge in spring. Season extension methods, such as row covers, cold frames, or high tunnels, can be used to protect early season crops from frost damage. Finally, transplanting rather than direct-seeding gives crops a head start: flea beetles generally cause the most damage to young plants, delaying plant growth or killing seedings. In later stages of plant growth, flea beetle feeding can damage leaves but is less likely to be fatal for plants. 

Trap Crops

Growers sometimes try to draw insect pests away from cash crops by intentionally planting a trap crop that the pest species finds more appealing. In some instances, growers find that simply providing a more desirable food source effectively prevents the pest insect from feeding on the cash crop. Often, growers use trap crops to efficiently concentrate insects in an area and destroy them using insecticides or other mechanical suppression tactics. This method can allow growers to apply less pesticide both overall and to the cash crop.

Multiple studies have found that planting trap crops around a brassica cash crop can be an effective tool for flea beetle management. Several strategies have been shown to increase the efficacy of this practice. These include planting trap crops 1-2 weeks prior to the cash crop, dedicating 10% of the total planting area to trap crops, planting trap crops in a border around the perimeter of the cash crop, and selecting trap crop varieties that are more appealing to flea beetles than the cash crop. Varieties with high levels of mustard oils, such as daikon radish and mustard, have been used successfully as trap crops around brassica plantings. 

Row Covers

Row covers can prevent flea beetles from accessing plants, reduce feeding damage, and increase yield. However, proper application of row covers is essential to increase the likelihood of success. If flea beetles infiltrate a row cover, they may become trapped in a sheltered environment without natural enemies and can cause extensive damage to crops. Crop rotation should be practiced to minimize the risk of flea beetles emerging from crop residue or the soil under a row cover. Row covers should be installed immediately after planting, before insects can access the plants.

Once a row cover is in place, it is important to ensure that there are no gaps through which insects can move to access the plants. The edges of the row cover should be buried or sealed with weights. Furthermore, the type of row cover material used is a crucial consideration. The material should have a small enough mesh size or weave to prevent flea beetles from passing through. Cloth-type row covers designed for frost protection will effectively exclude flea beetles but may cause plants to experience heat stress or bolting when used during the summer. For warm season crops, select a breathable fine-mesh row cover designed for insect exclusion with a very small mesh size. In some studies, these fine-mesh row covers were as effective as conventional insecticides. It should be noted that some netting products are expensive, though they can be used for multiple seasons (see Crops Under Cover Project website: https://cropsundercover.mgcafe.uky.edu/). 

2. Monitoring

Monitoring is vital for understanding how many flea beetles are present, where they are attacking crops, and whether it is time to pursue suppression strategies. It is important to record data when monitoring so that results can be compared to previous days, weeks, or seasons.

Sticky traps

Sticky traps placed within beds of crops can be a useful tool for monitoring flea beetles. Flea beetles are attracted to yellow and white, so sticky traps in these colors are recommended for monitoring the insects. Sticky traps should be replaced at regular intervals to show whether flea beetle populations have changed and to help growers understand whether management strategies have been effective.

Visual surveys

Scouting enables growers to track flea beetle abundance and damage, helping to inform decisions about management actions. Frequent monitoring is important because flea beetles can rapidly colonize plantings due to their high level of mobility and tendency to aggregate. Soon after planting, when crops are small and at their most vulnerable, monitoring is especially important. Growers should scout for flea beetles at least two to three times per week. 

Sampling a subset of plants is more feasible than inspecting every plant in a plot or field. Growers should establish a survey protocol and repeat that procedure each time to increase accuracy and reduce bias. One recommended surveying method is to walk through the field or plot and routinely stop after a predetermined number of steps to inspect random plants. Taking care not to startle the beetles, first count the number of flea beetles on the leaves before estimating leaf damage. The average number of flea beetles observed and percentage of leaf damage are used for economic thresholds.

Economic threshold

The level of flea beetle feeding damage at which treatment is merited depends on multiple factors, including the cultivar and age of the plants. A lower level of damage may be tolerated on crops sold as leafy greens. Mature plants can withstand higher levels of flea beetle feeding than seedlings. Additionally, the intended market for the crop may influence the acceptable level of damage: for instance, customers at a farmer’s market may have different expectations than a buyer for a restaurant. As a general rule of thumb, the University of Minnesota Extension recommends treatment when 10-20% of a crop shows damage. 

3. Biological Control

Biological control options for crucifer and striped flea beetles are limited. Some parasitic wasp species and a few predatory insects, including big-eyed bugs, damsel bugs, and lacewing larvae, are known to attack flea beetles. Unfortunately for growers, these natural enemies generally have limited efficacy in controlling flea beetle infestations because flea beetles are highly mobile in their adult life stage and are protected by living underground during their larval and pupal life stages. In research studies, some entomopathogenic nematodes have been effectively deployed against striped flea beetles during their larval stage. Application of commercially purchased nematodes may lower the survival rate of immature flea beetles.

4. Chemical Control

Insecticides should be used as a last resort to prevent the development of flea beetle resistance to the chemicals. Flea beetle resistance to pyrethroids and neonicotinoids has been documented in multiple studies. Growers may consider restricting the quantity of pesticide applied by spot treating areas of heavy infestation or applying pesticides to a trap crop where flea beetles have congregated.

Insecticides are available for both conventional and organic systems. Certain insecticidal products with active ingredients derived from plants, such as pyrethrin, and soil-living bacteria, such as Spinosad, are allowable for certified organic producers. Organic products typically have a relatively short persistence in the environment, with residues often becoming inactive within days. A list of pesticides approved for control of flea beetles in Kentucky and instructions for use can be found in the Cole Crops section of the most recent edition of ID-36: Vegetable Production Guide for Commercial Growers.

Conclusion

Crucifer and striped flea beetles are significant pests of brassica crops in Kentucky. A combination of multiple management strategies is likely to be most effective. Following Integrated Pest Management principles, growers should prioritize prevention through practices such as crop rotation, proper sanitation, and cultivar selection. Row covers can effectively exclude flea beetles from crops, but it is important to select row cover products with adequately small mesh size and ensure that row covers remain sealed. Growers should routinely monitor crops to inform management decisions. If flea beetle populations and damage reach an economic threshold, growers may choose to use biological or chemical controls to manage flea beetles. 

References

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Issued: 11/25
Revised: 3/26

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication. Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!