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Organic management recommendations for Colorado potato beetle

Author: Natalie Hoidal, University of Minnesota Extension - Horticulture

Colorado potato beetle (CPB) seems to be hitting Minnesota farmers especially hard this year. This article provides an overview of organic management approaches - for some background reading on the Colorado potato beetle, Leptinotarsa decemlineata (Say), and recommendations for growers using synthetic pesticides, please consult the VegEdge page on CPB. 

Colorado potato beetles in Forest Lake, Minnesota. All four larval stages are represented. Photo: Natalie Hoidal

Biology and life cycle

Adapted from VegEdge
Colorado potato beetles are an important economic pest of potato and eggplant, though they can feed on all plants in the Solanaceae family. CPB overwinters in the soil in Minnesota, most often in field margins, but also directly in fields. The first generation of adults emerges from the soil around the time that potatoes are emerging. They feed briefly on young potatoes, and then lay eggs in clusters of 10-30. An average female can lay around 350, and up to 500+ eggs in her lifetime.

CPB lifecycle, image by Natalie Hoidal

Eggs hatch and undergo four larval stages. With each molt, they become larger and more destructive. Their color also changes with each molt from dark red to a light salmon color. Younger larvae are more susceptible to most insecticides (including those approved for organic systems).

Cultural and preventative management 

Crop rotation

Colorado potato beetles can travel fairly vast distances, with reports of up to 100 kilometers. More typically, a beetle will travel anywhere from a few hundred yards to a mile or two from its overwintering site (Alyokhin et al., 2014). While crop rotation is an excellent strategy to prevent CPB infestations, small-scale growers may not have sufficient land to adequately rotate away from CPB populations. That said, rotating potatoes as far away as possible from previous plantings can help to slow the spread of CPB and reduce populations. A 2003 organic CPB management bulletin indicated that population levels are inversely related to the distance between potato plots from year to year (Kuepper, 2003). If populations of CPB reach damaging levels for multiple consecutive years, consider taking potatoes and eggplant completely out of your rotation for a year.

Early maturing varieties

By planting early maturing varieties, you do not altogether avoid damage from CPB. However, you may be able to either harvest before significant damage has occurred, or allow your plants to reach a mature life stage where potato development is well underway before defoliation reaches damaging levels. Check with your seed dealer for early maturing options, taking into consideration things like disease resistance, storage life, etc.

Early maturing varieties can also be used as a trap crop around plots with later maturing varieties.

Mulch and plastic trenches

Straw mulch has been shown to reduce populations of CPB, in part due to a higher abundance of natural enemies in mulched fields (Brust, 1994). Alternatively, plastic mulch between trenches can serve as traps for beetles. When plastic trenches (Univerisity of Wisconsin recommends 50cm deep x 30cm wide) are covered in a light dusting of soil, beetles become trapped and are not able to crawl up the sides of the trenches. Overwintering beetles tend to crawl rather than fly, so plastic trenches can also be set up around field margins to serve as a barrier between overwintering sites and the field.

Potatoes grown in mulch. Photo: Flickr, Local Foods Initiative

Habitat for beneficial insects

Insects from many different genera including lady beetle (Coleomegilla maculata), the ground beetle Lebia grandis, the parasitic wasp Edovum puttleri, and predaceous stink bungs (Perillus spp.) have been shown to prey upon and / or parasitize CPB (Olle et al., 2014). Many of the studies identifying predation and parasitism occurred in the lab rather than under real-world conditions. However, planting habitat for beneficial insects such as beetle banks and flower strips could potentially help to manage CPB populations.

Mechanical management strategies including exclusion

Row cover

As long as you are not planting in an area where adults may be overwintering in the soil (see above for the importance of crop rotation!), placing row cover over your potatoes just before emergence can help to prevent adult overwintering beetles from accessing your potato plants. Row covers can be removed later in the summer after the threat of egg laying has passed.

Hand picking

If you’re growing on a small enough scale, don’t write off hand-picking. Scout for eggs in the spring and squish them, put larvae and adults into soapy water. Be persistent; scouting and squishing / picking every couple of days is necessary for this to be a viable management option. CPB populations may exceed a level on which this is practical - one farmer reported collecting 10 pounds of CPB in one bed. However, if populations are not excessive, this is a good prevention strategy. If you do decide to pick the eggs, larvae, or adult beetles by hand, wear gloves, as they contain chemicals that can cause skin irritation.

Trap cropping

Plant a couple of extra sacrificial rows of potatoes near overwintering sites. This works best if you are able to plant your trap potatoes before your main crop, ideally two full weeks before. As populations start to build, use methods like flame weeders or vacuum suction to remove the adult beetles. After the first generation of adults has laid their eggs, you can remove the trap crop and plant something else.

Biological and chemical management

This section covers various spray products including those containing naturally occurring bacteria. Spray programs are most effective when used in concert with the cultural and mechanical methods described above. Holistic management in which multiple strategies are employed is the key to success with difficult to manage pests like the Colorado potato beetle.

Image: Teagasc

CPB has developed resistance to multiple classes of insecticides; growers must be careful and vigilant to prevent resistance. This is challenging in organic systems due to the limited number of products available. The graphic below from the UMN Private Pesticide Applicator Manual depicts how resistance can develop in populations. By using multiple types of pesticide products rather than relying on the same product year after year helps to reduce the potential for resistance. The term “pesticide products” includes products approved for organic systems.

The following is a list of products that can be used in organic systems on CPB, including considerations for when and how these products will be most effective. Remember that while these products are organic, they are still pesticides. Always read the label for information regarding the proper personal protective equipment, re-entry intervals, pre-harvest intervals, and environmental considerations. Check with your organic certifier to find products with these ingredients that align with your organic systems plan.
  • Beauveria bassiana sprays - Beauveria bassiana (Bb) is a naturally occurring soil fungus that parasitizes soil-dwelling insects. This fungus can be applied to CPB larvae with a standard sprayer. While experimental results show mixed success for controlling larvae, this product can be effective at the pupation stage. Wraight & Ramos (2015) found foliar applications of Beauveria bassiana result in moderate to high inoculation rates. Once inoculated, larvae do not tend to succumb to the bacteria until they pupate in the soil. Once they’re in the soil, which is a cooler and more moist environment where the pathogen can flourish, pupae will die in the soil. Thus, when using this product, you can expect to suffer defoliation damage from the larvae, but it can help to prevent egg laying and future generations.
    • Follow degree day models and spray when larvae are reaching 4th instar. They also tend to be higher in the canopy and more exposed at this point. 
    • This product works best when used with another pesticide such as Spinosad, Bt, or neem earlier in larval development - these other products can help to reduce larval populations, while Bb can help to control adults. 
    • Emulsifiable oil formulations may be more effective than wettable powders. Wraight & Ramos (2017) found that a 40% higher dose of the WP formulation was needed to achieve a comparable level of control. 

For all of the products listed below, Colorado potato beetles are most susceptible to insecticides early on in the first instar larval stage (approx. ⅛” in length). Waiting until larvae are full-grown will result in very poor control; adults are also difficult to control. So, timing sprays at the early instar stage is critical. Scout regularly for eggs and use degree days to predict larval development. Consider spraying every 5-7 days early in the spring as beetles emerge.

Table from VegEdge - Once you start to see eggs, you can use this table and the forecast to predict development and plan your applications. 
  • Bacillus thuringiensis - Bacillus thuringiensis is a soil bacteria that produces toxins impacting the digestive system in various insect larvae. Bt can provide good control of CPB if applied early on in development. Keep in mind that there are many strains of Bt, and using the same strain over and over can lead to resistance. Bacillus thuringiensis subspecies kurstaki (e.g. Monterey and Dipel) is the most commonly used strain in organic systems. Other strains are on the market, including Bacillus thuringiensis subsp. Tenebrionis and Bacillus thuringiensis subsp. Aizawai. Check with your organic certifier for specific product names. The Tenebrionis strain is recommended in the 2019 Midwest Vegetable Production Guide for Commercial Growers - it is sold under the brand name “Trident” and is OMRI approved. Note: many Bt products require respirators for safe use.
  • Neem and pyrethrins - Neem has been shown to be less effective than Bt and spinosad (Kuepper, 2003), but it will provide limited control. Products containing a mix of neem and pyrethrins have shown some promise (Wantuch et al., 2016). Keep in mind that pyrethrins are very sun-sensitive and break down very quickly in the environment. While this has benefits for pollinators and other beneficial insects, you may need to re-apply these products more frequently than products such as Spinosad or Bt. Always consult the label of each specific product for this information, as breakdown time will depend on the formulation. 
  • Spinosad - Spinosad has been the go-to treatment of CPB for many organic growers in Minnesota for years. It’s a naturally occurring substance made by soil bacterium (Saccharopolyspora spinosa) that can be toxic to a wide variety of insects. While Spinosad can be highly effective in controlling CPB (Wantuch et al., 2016), we have anecdotally heard of Entrust working less and less well for certain growers. Resistance to Spinosad was determined at a Minnesota farm in 2018. We always advise follow-ing up with applications and keeping good records. If you find that a product is not working as well as it used to, you may have resistance developing in your insect population. If you've applied Bt, neem, pyrethrins, or a combination of the three already and the population remains uncontrolled, a follow-up application of Spinosad is recommended. Note: the Entrust formulation of spinosad requires a respirator for safe use.

A. Alyokhin, D. Mota-Sanchez, M. Baker, W.E. Snyder, S. Menasha, M. Whalon, G. Dively, W.F. Moarsi, 2014. The Red Queen in a potato field: integrated pest management versus chemical dependency in Colorado potato beetle control. Pest Management Science. 71(3): 343-356.

G.E. Brust, 1994. Natural enemies in straw-mulch reduce Colorado potato beetle populations and damage in potato. Biological Control. 4(2): 163-169.

G. Kuepper, 2003. Colorado potato beetle: Organic Control Options. Appropriate Technology Transfer for Rural Areas: Current Topic.

M. Olle, A. Tsahkna, T. Tähtjärv, I.H. Williams, 2014. Plant protection for organically grown potatoes - a review. Biological Agriculture & Horticulture. 31(3): 147-157.

H.A. Wantuch, T. Kuhar, A. Hessler, Hl. Doughty, 2016. Evaluation of organic insecticides for the control of Colorado potato beetles in potatoes in Virginia, 2014. Arthropod Management Tests. 41(1), tsv128.

S.P. Wraight, M.E. Ramos, 2015. Delayed efficacy of Beauveria bassiana foliar spray applications against Colorado potato beetle: Impacts of number and timing of applications on larval and next-generation adult populations. Biological Control. 83: 51-67. 

S.P. Wraight, M.E. Ramos, 2017. Effects of inoculation method on efficacy of wettable powder and oil dispersion formulations of Beauveria bassiana against Colorado potato beetle larvae under low-humidity conditions. Biocontrol Science and Technology. 27 (3): 348-363.

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