Dealing with Flea Beetles on Brassicas and Nightshades

Young turnips (with flea beetles!) in need of thinning for cooking greens.
Photo Pam Dawling

This article will mostly be about brassica flea beetles, as our current struggle is getting fall brassica seedlings established. Brassica flea beetles are a different species from the ones that plague eggplant, and if you have one, you won’t necessarily have the other. Most methods for managing flea beetles are the same, regardless of the type of flea beetle and the crop they are attacking.

Unlike some cool-weather vegetables such as spinach, brassicas do germinate very well at high temperatures: the ideal is 77°F–85°F (25°C–29°C), but up to 95°F (35°C) works. Some people find refrigerating the damp seeds for 24 hours gives a better germination rate. Given enough water, summer seedlings will emerge in only three days. Once they have emerged, the bigger challenge begins. As well as temperature and moisture in the right ranges, the seedlings need protection from pesty bugs. We deal organically with flea beetles, harlequin bugs and, sometimes, cabbage worms. Our main defenses are farmscaping and insect netting.

Systems for growing transplants

The same systems you use for growing brassica transplants in spring may also work well for fall. Flea beetles can only fly a few hundred yards (meters). If you use flats, it can help to have them on benches in an enclosed shade-house, or outdoors, above the height of flea beetles. Ira Wallace of Southern Exposure Seed Exchange tells me 3′ (1 m) is high enough. Richard Jones, in his book Beetles, says flea beetles can jump at least a hundred times their body length.

Brassica seedlings under Proteknet.
Photo by Bridget Aleshire

We use an outdoor nursery seedbed and bare root transplants, because this suits us best. The nursery bed is near our daily work area, so we’ll pass by and water it. We’re always relieved to close the greenhouse transplant season in May, as we no longer need to water little plants many times each day to keep them alive. Having the seedlings directly in the soil drought-proofs them to some extent; they can form deep roots and don’t dry out so fast.

If you have no small troublesome pest insects, you can use shadecloth (up to 50%), which also keeps out large insects if well anchored to the soil. If you have flea beetles you will need covers with no sizeable holes, and you will need to close any gaps, either by burying the edges of the fabric, or weighting them down with sticks, jugs of water, or rocks. You can use clothes pins or binder clips to close any accidental holes (or sew them up with synthetic thread).

Rowcover can make the seedlings more likely to die of fungal diseases in hot weather — good airflow is vital. In warm weather, I prefer netting to keep bugs off, because it lets in lots of air. We use insect netting (ProtekNet) on wire hoops.

Biointensive integrated pest management (IPM)

This is a step by step approach that tailors strategies for each situation. The goal is to reduce pest damage of crops to an acceptable level, while minimizing the effects of pesticides on crops, workers and the wider environment. The focus is on restoring and enhancing natural balance and resilience to create healthy plants and soil better able to withstand attacks. Bio-intensive IPM includes controlling pests by physical and mechanical means (preventing the pest from reaching the crop, or removing the pest from the crop).

Start with preventative methods, monitor for pest levels, and if needed, take action via physical and mechanical methods, only turning to biological controls when other methods are insufficient.

 

  1. Prevention (cultural controls)

In your plans, minimize opportunities for pests to get out of control. Include such strategies as caring for the soil, minimizing soil compaction and hardpan layers, adjusting soil pH to suit the crops being grown, planting resistant, pest-tolerant, regionally adapted varieties, growing strong plants, maintaining good sanitation, promptly removing and destroying pest-infested plants, ensuring that transplants are pest-free before planting out.

 

  1. Avoidance (physical controls)

After growing the healthiest plants possible, the next stage is taking actions to reduce the chances of a specific pest taking over. All these methods reduce problems without adding any new compounds into the soil:

Crop Rotation Pinwheel

Use good crop rotations so that it’s hard for pests in the soil to find new host crops; plant successions of the same crop distant from each other, and each harvest day, pick the newer crop before the older one to minimize transfer of pests to the new planting; remove pest habitat; grow transplants in a protected greenhouse and plant out once the starts are large enough to withstand most pests.

Netting with small holes is better than rowcover for physically excluding pests in hot weather, as airflow is better. ProtekNet Pest Control Netting from Dubois Agrinovation is made of clear high-density polyethylene, polypropylene or knitted nylon, with UV resistance and a lifespan of eight to ten years (a lot longer than rowcover). Although it is not cheap, it is durable, given moderate care, and is easy to drape. Its light transmission is 90%. The 25g, 47g, 56g, or 70g mesh will protect against flea beetles and bigger insects. Netting also lets water through and protects crops against weather damage.

Bare root transplants growing outdoors under insect netting.
Photo Pam Dawling

Netting is best held on hoops above the plants so bugs can’t lay eggs or feed on the leaves through the holes. We remove netting about three weeks after transplanting brassicas, once the plants are big and sturdy. We also use this netting against flea beetles on eggplant transplants, removing it when the plants flower and need pollination. As we transplant, we hose the eggplants with a jet of water to dislodge any flea beetles already there, while someone else follows immediately behind to spread the cover and batten it down quickly with sticks. Collars around each transplant can be used to repel cutworms and cabbage root fly.

Mixed crop plantings (intercropping), hedgerows, field borders and farmscaping (intentional planting of suitable flowers that attract beneficial insects) can help maximize the diversity of insects and other beneficial organisms and avoid problems. We plant sweet alyssum in our spring broccoli patch to attract predators of aphids and caterpillars. Expect to plant 5% of the crop area to insectaries.

An insectary circle in early June. The flowers will attract beneficial insects.
Photo Pam Dawling

Provide habitat for bats, insectivorous birds, spiders, birds of prey and rodent-eating ground predators (snakes, bobcats).

Physically remove pests by handpicking, spraying with a strong water spray, flaming, vacuuming or by using a leaf-blower to blow bugs into a collecting scoop. Covering the soil with plastic mulch does reduce the problem.

  1. Monitoring

Just because a pest appears does not mean you have to kill it. The action level is the point at which the losses from the pest warrant the time, money and ecological disruption put into applying control measures. If flea beetles get your radish leaves, no one need know about them!

Make a habit of walking around weekly to scout for trouble as well as for natural enemies of pests. Scout vulnerable crops daily at critical stages. Learn to recognize bugs of all kinds and understand their lifecycles and enemies. Take a hand lens. Use a red LED headlamp for nighttime scouting. Photograph or capture insects you are unsure about and identify them from websites or books such as Whitney Cranshaw’s Garden Insects of North America.

Garden Insects, Whitney Cranshaw. Stock Image

Good record keeping helps improve effectiveness. Record pest names and numbers, date, crop, location, which input (if any), relative success of your action, comparison with methods used previously and overall evaluation.

Use weather forecasting to help determine when pest outbreaks are more likely. Use any available online pest-specific forecasting sites. These sites combine scouting and reporting of outbreaks with weather forecasts of temperature and wind direction to make a forecast map.

Keep a phenology chart to predict the likely arrival of key pests. Phenology is the study of seasonal and yearly variations in climate by recording regular plant and animal life cycle events each year. A visible change can be used as an indicator that another species is also likely to be changing. For example, we expect brassica flea beetles when the redbud blooms.

Traps

Set traps and lures. Although these may not significantly reduce the pest population, they are useful for indicating whether a certain pest is on your crops, so that other measures can begin as early as possible.

 Flea beetles can be trapped with a vacuum cleaner, or inside a bucket coated with Tanglefoot paste (hold the inverted bucket over the plant, tap it and catch the jumping beetles in the goo). A flea beetle trapping method I have not yet tried is to fill clear or white plastic bottles with water, cap them, coat the outside with sticky Tanglefoot compound around the middle two-thirds and set the bottles every 15′ (5 m) along the row under hooped covers. The water warms during the day and attracts the flea beetles at dusk. In four days you can expect to have caught almost all of them.

 Plant perimeter trap crops to lure pests from the food crop. Cleome attracts harlequin bugs, which can then be shaken off the tall flower heads into a bucket of soapy water. A row of pungent mustard greens can be used to lure brassica flea beetles. Once your trap crop is successful, you need to deal with them before you create a flea beetle breeding ground. Flaming the mustard plants is one possibility. If you have poultry that likes eating flea beetles, you could cut off some of the leaves and carry them to the chicken run.

Push-pull pest control
  1. Suppression

When the established action level for a particular pest has been reached, and prevention and avoidance strategies have been exhausted, biological, microbial, botanical, mineral (and for non-Organic growers, chemical) control measures can be used to reduce or eliminate that pest or its impact while minimizing environmental risks.

Design a series of measures that work together, to ensure that one action does not mess up another. Choose the least toxic materials and the least ecologically disruptive methods. Be aware of sensitive areas, such as waterways and habitat of vulnerable species. Be aware of better times to spray, eg for materials that kill honeybees, spray in the very early morning or at dusk when bees are not flying. For compounds that break down in sunlight, spray at dusk for maximum effectiveness and minimum amounts.

Bad damage from Mexican bean beetles on these bean leaves. Beetles will also attack the pods after a while, making them inedible.
Photo Wren Vile
  1. Biological control involves either introducing beneficial predators or parasites of the pest species, or working to boost populations of existing resident predators and parasites. Before we started buying the Pediobius wasp to deal with Mexican bean beetle, we used to plant more successions of beans, and flame the old plants when the pest count got too high. Most years we buy the Pediobius wasp parasite of the Mexican bean beetle. Some years populations are not high enough for us to need the parasites. Hb nematodes will control flea beetles, as will the braconid wasp Microctonus vittatoe Muesebeck.
  2. Microbial controls include fungi, bacteria, and viruses that kill pests. Bt (Bacillus thuringienisis), a species of bacteria, is the best-known microbial control. One strain kills caterpillars, another mosquitoes, another Colorado potato beetles. Spinosad is a fermentation product (no longer live material) of a soil microbe containing an enzyme that kills thrips, leaf miners, most caterpillars and some other insects. It is effective at one-eighth the recommended strength, if you don’t need to annihilate every last bug. It breaks down in sunlight, so spray at dusk. We use it against Colorado potato beetles on our March-planted crop. The June-planted potatoes don’t need any CPB control beyond the hay mulch we roll out to keep the soil cool and moist. We have also used Spinosad against the vegetable weevil larva when they took over our winter hoophouse turnips and greens.
  3. Botanical controls use plant-based products for pest control. If no selective method works, flea beetles can be killed with neem oil (it degrades in UV light in four to eight days and must be reapplied if the organisms are still around), or with Spinosad, a broad-spectrum insecticidal enzyme produced by a natural, although rare, soil organism. Pyrethrum is a botanical insecticide effective against flea beetles as well as beneficial insects. It is slightly toxic to birds and very toxic to fish so must not be allowed to contaminate waterways. Pyganic is one brand name. Rotenone is a strong broad-spectrum insecticide with short-term results that I do not recommend. In Canada, rotenone products are no longer registered for any agricultural use. It is toxic to cold-blooded animals including fish, and other animals including pigs. Research found that exposure to rotenone caused Parkinson’s disease-like symptoms in rats and correlated with a higher incidence of Parkinson’s disease in humans.
  4. Inorganic (mineral) controls, also known as bio-rational disease controls, include oils and soaps. Insecticidal soaps are made from potassium salts of fatty acids. They are effective only on soft-bodied insects such as aphids, not beetles. Kaolin clay, besides being a protective layer on crops, can suppress feeding of striped cucumber beetles, flea beetles, thrips and grasshoppers.

A useful resource is Kuepper, G. Flea Beetle: Organic Control Options (IP389). ATTRA, 2015.

Article on resilience in the September Growing for Market magazine

September 2012 issue of Growing for Market

The September issue of Growing for Market magazine is out, and with it, my article Building resilience into farm systems. I’ve embarked on a four-part fall and winter series of articles aimed at helping growers thrive under varying situations, some of which we have no control over.

This first article is about being prepared for whatever Nature throws at you, expecting to adapt, and building in options. I’ve sent in the second article, about  understanding and predicting conditions,for the October issue. It covers weather forecasting, frost prediction, Growing Degree Days and phenology. The next one after that will include using soil temperatures, scouting and monitoring for problems and something about on-the-spot decision-making. The last one will deal more with decision-making, reviewing results and learning from mistakes.

To read the articles, get a subscription to the magazine.

How Buildings Learn, by Stewart Brand

For those who like inspiring background reading, I recommend Stewart Brand in How  Buildings Learn: What Happens After They’re Built (Penguin 1995). He advocates for
constructing buildings that are easy to modify later, in gradual or drastic ways to meet the changing needs of the people inside. Farms can be looked at similarly. Keep as many options as possible (for crops, cover crops, crop layout) open for as long as possible. Brand’s current main activity is through The Long Now Foundation

The Art of the Long View, by Peter Schwartz

It can be helpful to do some scenario planning, which I learned about when I read The Art of the Long View, by Peter Schwartz (Doubleday, 1991). Scenario Planning is a method of making flexible long-term plans, using stories (scenarios) to help us visualize different possible futures that include not only factors we don’t control, like the weather or the market’s enthusiasm for bulb fennel, but also intangibles such as our hopes and fears, beliefs and dreams.

No time to read books? Very sad! Maybe see you at the Heritage Harvest Festival at Monticello, near Charlottesville, this Friday and Saturday.