Fear No Weevil

Fear No Weevil


Solo-type cups or deli cups (top) are good examples of pitfall trap containers. The rim of the container should be even with the soil surface, otherwise insects will bump into the rim and walk away. The slightly more complex linear pitfall trap (bottom) uses a PVC pipe with a slot running the length of the pipe and a capturing cup at one end. This trap design covers more area and is useful for larger turf areas like golf courses.

Spring 2013 is off and running and warm temperatures are quickly increasing insect activity. The billbug, a complex of weevil species in turf, has spent the winter primarily as an adult. Overwintering typically occurs in sheltered sites around the edges of turf where there might be leaf litter and woody debris, and also in thatch where they can be buffered from the winter conditions. Insects are poikilotherms (or “cold-blooded”), meaning their body temperature fluctuates with ambient temperatures. Once it warms up, insect metabolic and enzymatic activity begins to speed up. As temperatures start to exceed 65°F, adult billbugs become active and make their way to turfgrass where they will feed and deposit eggs. We can use adult activity to our advantage and monitor weevils to better predict when or if to manage.

Visual observation of adult billbugs making their way into turfgrass is doable in a home setting, but it is not all that practical on a larger scale or in recreational areas. Instead, pitfall traps can be used to monitor activity. Billbug adults rarely fly, and play possum when disturbed, so they easily get caught in pitfall traps. Traps can be as simple as digging a hole so that a chosen container fits snug in the soil with no side gaps. Billbugs crawling into an area will drop into the cup without being able to escape.

Pitfall traps are a useful monitoring tool but it is important to note that this is not an effective control method. In addition, pitfall traps collect ground-dwelling organisms (wolf spiders, millipedes, worms, and many insects) so identification at a basic level will be needed to distinguish pest and non-pest. In turf, predatory ground beetles and spiders are common and beneficial. Checking these traps regularly early in the spring will help to evaluate the start of billbug activity and increasing activity of adults as more are collected from week to week.

We can pair pitfall trapping with degree day models that predict insect activity based on maximum and minimum temperatures and the temperature at which an insect begins activity in the spring (lower developmental threshold). As a certain number of degree days (or heat units) are accumulated, we can predict the occurrence and activity of an insect for which a model has been developed.

A model for bluegrass billbug was developed in Ohio and has been used elsewhere but it has not been specifically validated in Utah. The model is calculated from a base temperature of 50°F and relies on a March 1 start date. If you have not visited the Utah TRAPs website, this is a good way to follow degree days for many locations in Utah. On the site, select the closest weather station on the map, then the growing degree day (GDD) model (base 50), and select a March 1 start date. According to the Ohio model, first activity of adults was recorded between 280 and 350 degree days with 30% of first adult activity occurring between 560 and 624 degree days.

Pitfall trap modifications

  1. Add a sleeve insert in the trap for quick monitoring (works well with solo-type cups where the rim is removed and slipped into the soil-bound rimmed cup)
  2. Poke small holes through the bottom of the container for water drainage
  3. Add a protective cover that sits just above the trap to shelter from water
  4. Add a wire mesh cover so small animals don’t eat the insect sample

Combining these monitoring tools can be useful in making management decisions. Some have used these methods to target active and incoming billbugs with border treatments of pyrethroid or organophosphate products. Preventive products (e.g., Acelepryn, Arena, Merit, and Meridian) are often used for turf insect management and target newly emerging larvae from eggs deposited in turf stems. Pairing monitoring strategies with preventive applications is important for improving efficacy.

Given the drastic differences in temperature and degree day accumulation from year to year in Utah, as in 2011 (cooler) versus 2012 (warmer), applications too early or late will not be as effective. Too early, and there will be less residual product left, and too late will not allow enough time for these products to enter the plant so that emerging billbug larvae can ingest it as they feed. As larvae become larger, they are less susceptible to these applications and how quickly they develop will be, in part, dependent on temperature.

-Ricardo Ramirez, Extension Entomologist


Murray, M.S. 2008. Using degree days to time treatments for insect pests. Utah State University Extension Fact Sheet. IPM-05-08.

Shetlar, D.J. and J.E. Andon. 2012. Billbugs in turfgrass. Ohio State University Extension Fact Sheet. HYG-2502-12.