Kentucky Hemp: Corn Earworm Outbreaks

Industrial hemp. Photo: Clemson University

Helicoverpa zea (formerly Heliothis zea) is an insect that affects many crops and is known with different common names, depending on the crop it affects: cotton bollworm, soybean podworm, tomato fruitworm, or corn earworm. The larva of this noctuid moth is polyphagous and feeds on many other crops, including industrial hemp plants. In this article, we are going to use the name utilized in corn production.

Biology, and Description of Larva

Female moths lay a single egg on suitable host plants. Once the egg hatches, first instar larva feed search and start feeding on beans in soybeans, kernels in corn, fruit in tomato or peppers, or buds and developing seeds in hemp.

The larval instar is completed in 15 to 20 days depending on temperatures. Late instar larvae move into the soil to pupate.

In Kentucky, one or two generations of corn earwoms can be completed every year. Larvae can reach up to 1.5 inch in length. The coloration of the larvae varies greatly ranging from green, pink, dark brown to almost black. In many hemp areas scouted for this report, the green form is the most common (Figure 1).

Figure 1. Green form of corn earworm collected in a hemp fields in Caldwell and Clinton counties in September 2019 (Photos by R.T. Villanueva and C. Guffey)

Occurrence in Kentucky,

Dr. Ric Bessin wrote that the abundance of the corn earworm in Kentucky’s sweet corn increases as the season progresses; adult corn earworms (moths) migrating from southern areas especially affect sweet corn planted late. Thus, this coincides with current observations in industrial hemp where this pest has been found feeding in hemp plants since late-August across several areas of Kentucky.

Unfortunately, this pest may become the key pest for the production of industrial hemp in Kentucky. Its management in late planted sweet corn includes the application of insecticides every 3 to 5 days during the silking period.

However, the insecticides commonly used in corn (i.e. Bacillus thuringiensis, spinoteram, spinosad, or synthetic pyrethroids) cannot be used in industrial hemp as they are not registered to be used in this crop.

Areas Scouted and Damage

During late August, only small size larvae were found in Caldwell and Lyon (Susan Fox) counties, whereas during the first two week of September greater than 0.5-inch larval instars were found in the Caldwell, Graves (Figure 2) (Samantha Anderson), and Clinton (Colby Guffey) counties. Larval instars ranging from 0.5 to 1.5 inches in length were collected at Graves, and Caldwell counties.

Figure 2. Several instars of corn earworm collected in hemp fields in Graves County in September 2019 (scale in yellow is in millimeters) (Photo by Raul T. Villanueva)

Corn earworms are well camouflaged in the hemp shoots (Figure 3a) and sometimes well-hidden deep within the hemp canopy and tunnels it makes (Figure 3b), but with a meticulous search, the worms can be easily found on plants. Damage of corn earworms can be easily observed in hemp grown for CBD oil due to the frass (excrement from larva) (Figure 4a), tunneling on the buds, and wilting and dead of leaf and tissue (Figure 4b) surrounding the area of attack.

Figure 3. Corn earworm can be (a) camouflaged within the hemp foliage or (b) found deep hidden in tunnels. In hemp for CBD oil buds are being severely damaged (Photos by R. T. Villanueva and C. Guffey)

Management

For the management of pests affecting hemp, the Kentucky Department of Agriculture is the entity responsible for regulating pesticide use in hemp. Currently there are few insecticide options to conduct foliar applications for corn earworms. The list of insecticides registered for hemp is in the KDA, and a previous publication in Kentucky Pest News summarize these pesticides. For all pesticides registered in Kentucky it is necessary to follow all label guidelines.

Currently there are no guidelines to scout or tally for insects in hemp. However, during a collection of corn earworm in Caldwell County for approximately 45 minutes, 15 worms were captured; and from this number, three larvae showed the presence of a tachinid fly egg near the head (Figure 5), thus in this particular case 20% of larvae might been potentially affected by this natural enemy.

Figure 4. (a) Damage of corn earworms can be easily observed in hemp grown for CBD oil by the presence of frass (excrement from larva) or (b) wilting or dead of foliage. (Photos by Raul T. Villanueva)


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