The Carrot Weevils (Listronotus oregonsis and Listronotus texanus) are major pests of carrots and celery. Carrot weevils overwinter as adults in the soil in the field or or other secluded areas near the field and emerge in spring, (or in late winter in warmer climates.) Soon after the adults emerge, females begin to lay eggs on celery or carrot leaves. Larvae emerge from the eggs and bore down to the roots. Adults typically emerge in summer and in most cases, there is only generation per year.

Researchers have developed a growing degree day model for each species to help predict when life stages will occur based on that year’s weather.

Calculating the Growing Degree Day Units

The Carrot Weevil (L. Oregonsis) growing degree day model uses a lower threshold of 7C, and an upper temperature threshold of 49C. The Carrot Weevil (L. texanus) uses a lower threshold of 13.3C, with an upper threshold of 30C.

These units can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

The first larvae typically emerge at 131 GDD(C) for L. Oregonsis and 62 GDD (C) for L. texanus after the first adults have emerged and have been consistently observed in the field, which is the “biofix” or date at which growing degree day unit accumulation should begin. Pupation will occur approximately at  501 GDD(C) for L. Oregonsis and 341 GDD (C) for L. texanus

Setting the Biofix Date

For growing degree day models, there needs to be a “biofix”, a set date after which growing degree day units will accumulate. For Carrot Weevils, this is the date at which adults are first consistently observed in the field in spring, or late winter.  In the Pest Prophet app, this will correspond to the growth stage, ” Adult / Eggs.” The bio-fix can also be re-adjusted after later growth stage are observed, in order to better predict later life stage events.

Sources:

Woodson, W. D., and J. V. Eldelson. 1988. Developmental rate as a function of temperature in a carrot weevil, Listronotus texanus (Coleoptera: Curculionidae). Ann. Entomol. Soc. Am. 81: 252-254. ]