Among the bevies of beasties that take a bite out of the delight of growing dahlias are beetles.
Which of them are the most unwanted dahlia pests? It depends on where you are. But nationwide, it might just be Japanese beetles. University of Minnesota's Jeff Gillman, an associate professor in the department of horticultural science, wrote recently that he was invited to speak on garden remedies to an audience of gardeners from all over the country at Epcot Center. When he got to the question portion of his talk, he said, the most frequent issue that came up was Japanese beetles, a major problem from the east coast to the midwest.
Japanese beetles, Popillia japonica, are half-inch beetles, metallic blue-green with coppery wing-covers. The 3/4-inch larvae are white, C-shaped grubs that do serious damage to lawns, turf grass and nursery stock, and collateral damage occurs when heavy infestations attract raccoons and Canada geese that dig up the turf to get at them. Adult beetles aren't picky; they feed on over 275 plant species. On dahlias, they may concentrate on the flowers, or skeletonize leaves, sometimes defoliating entire plants. They also hide in the convenient florets of dahlia blossoms.
Unfortunately, Dr. Gillman writes, "The first rule of Japanese beetle control is that you can't control Japanese beetles. Nobody has found a sure-fire cure yet and, if you try too hard, you're going to poison yourself and everyone in your neighborhood." Most recommendations you'll find come under the heading of "damage control," like covering valuable plants with row covers and destroying beetle eggs in lawns by allowing turf to dry out completely between waterings.
But the beetles are tough. Gillman advises. And some strategies, he says, "make the problem worse than it already is. Using a trap to lure Japanese beetles to their demise will kill a few—and may make you feel like you're doing something—but you will be attracting more beetles to your yard than you kill." And killing grubs, he adds grimly, "won't prevent adults from flying into your yard after they've hatched from someone else's."
Biological controls for Japanese beetles "just don't work that well," he says. Dish-soap spray and citrus insecticides aren't very effective and can damage dahlia foliage. Pyrethrum, an organic insecticide, will kill Japanese beetles, but it doesn't last long. Milky Spore Disease, a biological control, "usually kills less than 50 percent in good conditions," he adds. "If you're willing to go to a little bit of trouble, lose a few leaves, and use a little bit of synthetic insecticide, there is a way to protect your plants to some degree." His strategy, called "trap cropping," is to spray the beetles favorite plant—he recommends roses, but the beetle favorites are often dahlias, to judge from the blogs we see—with permethrin, and if you can get 7-14 days of clean plants, you'll kill Japanese beetles before they move on to other plants.
However, he warns, Japanese beetles "will seek revenge for their dead relatives."
Corn rootworm beetles include the western (Diabrotica virgifera virgifera) and northern (D. barberi) species. The western beetle is stripes and the northern is solid green. They are, as their name implies, signficant pests in the corn-growing Midwest. According to Iowa State University horticulturalists, the larvae feed on the roots of corn plants and emerge in July to feed on the tassels and silks of the corn. By August, when the corn is drying and the dahlias are beginning to bloom, the beetles migrate to other feeding locations and display an appetite for dahlia petals. Control of corn rootworm beetles, according to university agricultural bulletins, is difficult at best and cost-effective only when multiple pests are involved.
Cucumber beetles are pests that afflict cucumbers and melons, and incidentally other vegetation, across the country. The western spotted cucumber beetle, Diabrotica undecimpunctata, targets ornamental plants, according to Oregon State University's nursery pest bulletin. When large populations do appear they can cause leaf damage. In addition to a number of broadleaf trees, the beetles have been found guilty of pillaging among dahlias, peonies and hisbiscus. However, counsels the OSU bulletin, the western cucumber beetle rarely shows up in such large numbers that it must be controlled.
Bad enough that beetles are such greedy dahlia pests, chewing on our champions; the other issue associated with their presence is that they can serve as vectors for plant diseases, bacterial and viral. So it's just as well to do what you can, in as environmentally friendly a way as you can, to reduce or eliminate their presence.
Coming next: the vicious flea beetle, Lynch Creek Farm's nomination for the nastiest dahlia pest.