THIS WEEK IN PLANT PROBLEMS:
Q: My cannas have been decimated by Japanese beetles! Yet there are so many songbirds in my garden. Are there no birds that eat Japanese beetles?
A: There is a long list of birds that eat Japanese beetles, both the adults and the grubs, as part of their insectivore diet.
One of the most voracious feeders of Japanese beetles is the European starling, which is often under-appreciated in the urban gardens they frequent. The reason it seems that nothing eats Japanese beetles is because there are so many of these insects at one time! Excessive numbers of Japanese beetles create an imbalance in the ecosystem— there are not enough predators to keep the beetles’ population in check—which is why these insects are such significant horticultural pests. (But we don’t want too many predators, either; the right balance is important.)
Birds are not the only predators of Japanese beetles: raccoons, skunks, moles, shrews, spiders, assassin bugs, ants, ground beetles, and parasitoid wasps are also on the hunt for these annoying insects, but like birds, they can only consume so much.
Controlling Japanese beetles on canna can be tricky, as these flowers are a favorite of this pest and there is a short list of products that can be applied to blooming plants because of risks to pollinating insects.
Consider visiting your canna often to hand-pick Japanese beetles from the plant into a bucket of soapy water to drown. Some people use pyrethrin, which kills Japanese beetles on contact but without residual chemical that would affect beneficial insects.
Resist the temptation to try a trap; traps work by releasing pheromones that attract beetles. The problem is that Japanese beetles can fly long distances (on average 250 feet to 2 miles per flight, but up to 5 miles at a shot and up to 15 miles total from where it lived as a grub) to find the source of the pheromone but then they don’t necessarily get into the trap—meaning that traps can attract way more Japanese beetles to a garden than there would’ve been without the trap.
Without traps, the ecosystem in your garden will be able to adapt easier to the presence of Japanese beetles. This doesn’t mean that Japanese beetles will go away, but there is potential for a better predator balance.
The hard news is, ecosystem adaptation is not a fast process—taking 5 to 10 years after Japanese beetles have reached a damaging threshold. The good news is, eventually it should be easier to grow canna in Nebraska again.
Q: I wanted to get more potatoes to harvest this fall, so for the first time, I fertilized my potatoes this spring with something I bought at the garden center in addition to mixing in chicken manure last fall like I usually do. My potato plants had the most lush, beautiful top-growth I’ve ever seen in all my years of gardening! But then that top-growth died by early July, leaving behind some potatoes to harvest but fewer and smaller than usual. What happened to my potatoes?
A: There are several possible causes of early death in potato plants, including environmental stress and some diseases, so it’s important to ask your local UNL Extension horticulturist to investigate.
When you slice into your potato tuber, do you notice any brown staining toward the stem end? If not, something you mentioned that throws a red flag is that you fertilized your garden soil last fall with chicken manure and then again, for the first time, this spring with a commercial fertilizer mix.
It’s possible that your potatoes received too much nitrogen, if you did not do this according to recommendations from a soil analysis. While nitrogen is an essential macronutrient for plant growth— especially of the stems and leaves—root, flower, and fruit development use more of the other macronutrients, phosphorus and potassium.
Excess nitrogen can lead to potato plants expending more energy on stem and leaf development at the expense of root development. Nitrogen imbalance can also lead to early death of the top-growth and fewer, smaller tubers.
This is not a surefire reason for your potato plants’ early demise, but it’s a possibility to talk about with your UNL Extension horticulturist.
Rita Brhel lives in Fairfield, has a Bachelor of Science in Agricultural Sciences and Natural Resources from the University of Nebraska- Lincoln, is a certified Master Gardener and Master Naturalist trainee, and coordinates the Horticulture Program at the Adams County UNL Extension office in Hastings.
Have a horticulture question? Contact your County Extension office to get connected with your local UNL Extension horticulturist.
