JoAnne Skelly: What is milky spore and how is it used? |

JoAnne Skelly: What is milky spore and how is it used?

JoAnne Skelly

My friend Paul recently asked me to tell him about milky spore. I knew nothing about it, so decided to look it up.

Milky spore is a bacterial disease used as an alternative to chemical insecticides to reduce Japanese beetle populations. This microbial or biological insecticide was one of the first registered in the United States in 1948. I asked Paul what he was trying to control, since we don’t have Japanese beetles in Nevada according to US Department of Agriculture. Paul said he was hoping it would control Jeffrey pine beetle.

He brings up an interesting concept. If an insecticide, whether a traditional chemical one or a nontraditional biological insecticide, works on one beetle, won’t it work on all beetles, or for that matter, all insects?

While both types of beetles are in the Order Coleoptera, they are in different families, Japanese beetle in the Scarabaeidae and Jeffrey pine beetle in the Scolytidae. Why this matters to a non-entomologist is that members of different families develop differently and feed differently. Because of this, a management strategy that works for one may not work for the other.

For example, Japanese beetle grubs eat the roots of grass. Adult Japanese beetles eat flowers and leaves, but only in their first two weeks as adults. The Jeffrey pine beetle grubs and adults live under the bark of the Jeffrey pine tree and feed on the vascular tissue and inner bark.

The milky spore disease spreads via Japanese beetle grubs in the soil finding and eating the bacterial spores. If they eat enough and if the soil temperature is high enough, they die. The spores reproduce within the grub and spread through the soil. Milky spore only affects Japanese beetles, not Jeffrey pine beetles, other beetles or other insects even though the others’ larvae (grubs) might live in the soil rather than under the bark of trees as the Jeffrey pine beetle larvae do.

This illustrates the importance of identifying an insect pest before deciding on or implementing a management or control tactic. All insecticides don’t control all insects. As with milky spore, they can be very specific, not only to one insect, but also to a stage of growth of that particular insect, such as the larva versus the adult. Without proper identification, not only might you waste money on an ineffective control, you may also harm the environment and beneficial insects that may help you manage your pest problem.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at