Controlling Cucumber Beetles Organically
Cucumber beetles are a diverse group of beetles that infest many different crops in the cucurbit family. Susceptible species include summer and winter squashes, cucumbers and muskmelons. There is a range of susceptibility in these species, and there are varieties of each that have tolerance or resistance. To add to the problem for the grower, these beetles also transmit other diseases like bacterial wilt and a virus.
Controlling cucumber beetle infection includes using plant resistant and tolerant species, but sometimes the gardener is faced with a situation that requires some sort of intervention. Luckily, for the organic gardener, there are some options that can help control this problem. These strategies include using diatomaceous earth, kaolin clay, a botanical insecticide, several cultural control methods and biological control.
Preventing or Reducing Larval Infection
Overwintered adults lay eggs on the plants and larvae emerge in the late spring and early summer. There is also another generation of larvae later in the summer. A dust with diatomaceous earth (silicon dioxide) helps control the larvae and prevent them from feeding on the roots and stems of the plant. Diatomaceous earth damages the epidermal layer of the larvae, absorbs oil and fats, and the larvae dry up and die. Application is done after the plant surfaces have been watered. Dust it on all surfaces, especially the stem at the soil interface.
When applying diatomaceous earth, avoid breathing in the dust as it can irritate nasal passages and respiratory tissue. Also avoid applications when it is very windy.
If there is rain or if you overhead irrigate, the diatomaceous earth residue will wash off, so you will have to reapply it. Brisk winds may also whisk away residues and cause the need for reapplications.
A brief, one-year study at the University of Massachusetts indicated that applications of kaolin clay (Surround) help reduce the incidence of cucumber beetle. The presumed mechanism of lowering the incidence is that the clay makes the plant less appealing to the beetle. Kaolin clay applications are sprayed on the plant and they leave a white residue. This residue also helps prevent sunburn on many different plant varieties.
Because this study has not been repeated, the effectiveness of kaolin clay for the control of the cucumber beetle is in question. Further down the road, more studies will reveal if this is an effective control. It may be worth a try for individual organic gardeners to see if it helps. There is evidence that it helps with other insect pest problems.
Apply it in a sprayer as soon as seedlings emerge and repeat sprays to maintain a residue on the plants. Several applications are more effective as an insect deterrent versus a single application.
Pyrethrin Botanical Insecticide
Check you plants regularly to determine if any beetles are present, even if you employ both of the previous strategies. If you see the beetles, then a spray with pyrethrin-type insecticide can help knock them down. Pyrethrins are derived from Chrysanthemum plants, so they can be used by organic-minded gardeners.
Pyrethrins can be applied as either a dust or in a liquid formulation. Once beetles are observed, weekly applications for at least two weeks afterwards should be done to ensure the populations have been eliminated. But keep a lookout for more beetles and spray if more are present. Note that pyrethrins don't leave a residue.
A Pyrethrin Product
Spinosad is made from a bacterium known as Saccharopolyspora spinosa. This product works against a variety of insects and it attacks the larval stage of the cucumber beetle. It functions as a poison and a feeding deterrent. Some formulations of spinosad have been approved for organic growers. Spinosad applications are sometimes combined with pyrethrins.
Spindosad in Horticultural Oil
The fastidious gardener can choose to remove beetles by hand and do other cultural methods that help reduce the possibility of beetle infestations. In commercial organic farms, they sometimes even use vacuums to remove the beetles.
The beetle overwinters within plant debris. So, cleaning the planting site after the season is finished helps remove any adults that can survive until the next spring. Tilling the soil in the spring also helps kill overwintering adults by burying the debris and exposing it to degradation by soil organisms.
Straw and organic mulches are also helpful for reducing the beetle populations. Why? There are at least a couple of reasons for this. First, they are great environments for harboring predators of the beetle larvae, like spiders and ground beetles. Second, the mulch provides a barrier between plants to limit movement of the beetle larvae throughout the planting.
Conclusion and Overview of Organic Control Measures
There are several options available to the organic grower for the control of the cucumber beetle. The foremost option is to use varieties that are less susceptible. These varieties have less of a chemical that the beetle is attracted to, cucurbitacin.
Second, cultural controls need to be employed to help reduce overwintering adults, foster predators and prevent spread from plant to plant.
Third, preventative measures like kaolin clay and diatomaceous earth should be applied regularly during the season to help prevent the build up of insect populations and plant damage.
Lastly, if beetle populations become evident, remove them manually and/or apply pyrethrins or spinosad to kill them. Early season applications will help prevent build-up of crop-devastating populations later in the season.
References and Further Reading
Extension.org. Managing Cucumber Beetles in Organic Farming Systems.
National Pesticide Information Center. Diatomaceous Earth.
Pennsylvania State University. Kaolin Clay (Surround).
Purdue University. Alternative Control Guide for Spotted and Striped Cucumber Beetle.
Utah State University Extension. Western Striped Cucumber Beetle and Western Spotted Cucumber Beetle.