Japanese beetles: Do four o’clock flowers help? 

Partially due to the mild winter, Japanese beetles (Popillia japonica) emerged unscathed in southern Minnesota. Once out and about, they quickly went to work scathing our plants. Hollyhock, basswood, and rose leaves turned into lace as the beetles chomped away. Folks scrambled to protect their gardens, bringing out buckets of soapy water and spraying insecticides. Soon, a rumor emerged that there existed a flower which could not only halt the beetle’s garden invasion but kill them in the process.

After some investigation, this mythical plant people talked about was the four o’clock, also called marvel-of-Peru (Mirabilis jalapa). Unfortunately, it appears a four o’clock would rather hit the “snooze” button than do battle with Japanese beetles.

What are four o’clocks?

While perennial in their native habitat in South America, many Minnesota gardeners grow four o’clocks as an annual. Their unusual name likely relates to their blooms, which tend to open in the afternoon and evening. Soils that are well fertilized and drained are good spots to place them.

Four o’clock flowers provide many clues to pollinators, with bright colored petals, a sweet smell, and nutritious nectar. You may also see more moths visit four o’clocks. Indeed, back home in South America, hawk moths do most of the pollinating. You may even see some relatives of these insects in Minnesota.

Is the four o’clock really toxic to beetles?

The origin of this myth is hard to pinpoint. Sites such as Pinterest and other gardening blogs often mention four o’clocks as a control to Japanese beetles. Several state that university research proves this, but they do not provide links to these resources.

Since Japanese beetles arrived in the U.S. around the 1900s, old books and journal articles could be a possible source of this idea. In a 1940 USDA pamphlet titled “The Japanese Beetle and Its Control,” the four o’clock was listed as “never or rarely fed upon.” A later bulletin in 1963 upgraded the plant to one “lightly or moderately” eaten by Japanese beetles.

In that same 1963 publication, the author briefly discussed plants that could be poisonous to Japanese beetles. These included geraniums (Pelargonium spp.) and castor beans (Ricinus communis). The four o’clock was not mentioned as a candidate.

Later, geraniums and castor beans were tested, and only geranium had evidence of being directly toxic to Japanese beetles. Adding geraniums to protect the garden may seem like a good idea, but a University of Kentucky study found that planting garden geranium (Pelargonium × hortorum) actually increased Japanese beetle damage to nearby roses. The reasons why are not clear, but the scientists suggested that geraniums may have attracted more beetles in the area due to their flowers’ scent or color.

A recent Colgate University study in 2013 seems to put another hole in the “four o’clock is toxic to beetles” theory. In this experiment, a four o’clock leaf was put in a cage with a starving Japanese beetle. While the scientists did find that some varieties of four o’clock were tastier to the beetles than others, no dead or sick insects were observed in any cage.

This plant does affect humans, however. The sap from four o’clocks is a mild irritant, causing itching on the skin. Also, eating the plant may cause stomach upset or other intestinal issues as well.

Can four o’clocks be used as a trap crop?

Another suggestion is to use four o’clocks as a “trap crop.” Trap cropping is when a plant very attractive to pests is used to lure them in a confined area, away from other vegetables or flowers. Usually, the trapping area is then treated with insecticides, minimizing the total amount of chemical used.

This technique is really hit or miss, and there are few studies that directly look at Japanese beetle trap cropping. Still, if you are running out of options, it may be good to experiment with planting some of these trap crops around the garden edge.

Two plants that have been touted as a Japanese beetle trap crop include evening primrose (Oenothera biennis) and borage (Borago officinalis). In my experience, borage might be the easier of the two to purchase and sow in the garden. Be aware though, both species can be weedy if left to their own devices.

I can also attest a bit to the effectiveness of borage as a trap crop. This year, we had some self-seeded ones pop up in our pollinator garden. Sure enough, the borage plants were visited by hungry Japanese beetles. However, it was not nearly as tasty to them as a nearby crabapple (Malus sp.) about 20 feet away. Remember, the most important thing is for the trap crop to be more attractive than the plant you are trying to save!

If you find that your variety of four o’clock (or another plant) is exceptionally delicious to Japanese beetles, feel free to try trap cropping using it. But be aware that using insecticides on flowers can also impact non-pests such as bees.

What is the verdict?

This summer has been particularly difficult to control Japanese beetles. As a result, many solutions pop up that are too good to be true. Four o’clock seems to be one of these. To learn about Japanese beetle control, I urge you to contact your local extension office or visit our Japanese beetle page.