Tuesday, June 6, 2017
Last week I noticed an unusually large number of daylily (Hemerocallis sp.) leaves turning completely yellow and dying. Upon closer inspection, I found that they have aphid feeding on the lower stems.
Aphids are a soft-bodied insect with piercing mouth parts that they use to suck out the plant's sap. They multiply quickly, resulting in large populations.
I have aphids on other plants in my yard, including climbing honeysuckle and hibiscus. However, the infestations on those plants have not been at a high enough level to warrant control. I don't bring out the insect control toolbox unless they begin severely damaging the plant. Usually, the natural predators such as ladybugs and lacewings keep the aphids at a manageable level.
In this case, the aphids are completely defoliating some of the daylilies. Evidently, the ladybugs and other predators cannot eat the little morsels fast enough. Not only is the yellow and brown foliage ugly, but the plants are also severely weakened by the injury. I had to mow back one large patch of daylily to remove the yellow and dead leaves. I'm hoping they will regrow, but those will not bloom this year.
When natural enemies are not enough, we sometimes have to resort to a chemical control option. There are some organic, less-toxic options available such as garlic extract, insecticidal soap, and neem oil. However, on daylily, the aphids are feeding very deep in the folded leaves near the ground. The organic options are all contact insecticides because they will only kill the aphids that it touches. There are many other contact insecticides available to treat aphids. However, in this case, it is nearly impossible to reach all the aphids feeding deep in the daylily leaves. Also, the organic and contact insecticides do not last very long, and thus need to be reapplied every few days.
In this case, I may go to my last resort in the insect control toolbox, which is a systemic insecticide. Systemic insecticides are sprayed on the leaves or applied around the base of the plant on the soil. The plant takes up the chemical and moves it throughout its leaves, stems, and roots. Insects feeding on those plant parts are killed by the chemical within.
This is the reason it works so well, but it is also the reason it is the last tool in the box. Because the chemical stays in the plant killing insects for several weeks, it will also kill the predators and pollinators that might visit the plant during that time. I am concerned about inadvertently killing predators and pollinators. Large pollinators like honeybees don't typically visit daylily flowers, but some small bees and flower flies do feed on the pollen there.
Therefore, I will not treat all the daylilies in my yard. Some cultivars are not infected, such as the 'Stella de Oro'. I only plan to treat the ones that are in a high profile location or are greater value. Of the hundreds of daylilies in my yard, I will likely only treat four plants.
Read more about aphids at http://hyg.ipm.illinois.edu/pastpest/200715e.html
Update on June 7, 2017 - I still haven't treated any daylilies yet. Will wait a few more days and see how they are doing with just general cleanup.