Jeff A. Davis and Nathan Arey

Research efforts across all aspects of hemp production are currently needed to aid in the successful reestablishment and long-term success of this crop in the U.S. A 2020 survey of licensed U.S. hemp farmers determined one of the most challenging aspects of production was the lack of information on the efficacy of currently labeled hemp insecticides.

Of particular concern for hemp producers are insects that consume floral buds and defoliate fan leaves. Recent surveys have identified an insect pest complex associated with hemp in the order Lepidoptera. These include corn earworm (Helicoverpa zea), fall armyworm (Spodoptera frugiperda), yellowstriped armyworm (Spodoptera ornithogalli), beet armyworm (Spodoptera exigua), and cabbage looper (Trichoplusia ni). All these caterpillar pests are present in our Louisiana agroecosystem and have been found in hemp within the state.

These pests are polyphagous, which means they can feed on a variety of plant species from different families. They are also found in corn, cotton, rice, sorghum and soybean and are typically controlled using insecticides and plant-incorporated protectants. However, the recent removal of hemp from the Controlled Substances Act in 2018 has only allowed for a limited number of pesticides to be used on this crop. Currently, the EPA has added hemp to the label of 99 pesticide products, and 49 of those are insecticides. These products contain active ingredients classified as minimum risk pesticides under Title 40 of the U.S. Code of Federal Regulations (40 CFR 152.25). They are exempt from registration under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) and can be applied to hemp if state registration permits. Louisiana has approved their use.

The approved products are not the typical synthetic chemistries our growers are used to, and there is almost no information on their efficacy on hemp insects as previously stated. Thus, to better inform our state’s hemp producers, we conducted insecticide efficacy tests on three hemp floral bud pests: fall armyworm, beet armyworm and soybean looper. The list of products tested can be found in Table 1. These products were purchased online or at a local cooperative. We tested five different types of minimum risk insecticides: potassium salts, azadirachtin, Bacillus thuringiensis (Bt), GS-omega/kappa-Hxtx-Hv1a, and essential oils. Potassium salts and potassium silicate are abrasive, creating pores within the insect cuticle and leading to desiccation. Azadirachtin, a secondary plant chemical from the neem plant, is found in pure neem oil and acts as an insect growth regulator, preventing the insect from molting. Bacillus thuringiensis is a microbial disrupter of insect gut membranes. The spider venom peptide GS-omega/kappa-Hxtx-Hv1a works only on insects and essential oils work as deterrents, repelling insects with some insecticidal activity.

To test these insecticides, hemp variety Southern Luck from The Hemp Mine company in Fair Play, South Carolina, was grown in a hoop house at the Plant Materials Center at the LSU AgCenter Doyle Chambers Research Station. Leaves were then collected, dipped in one of the test products at the maximum labeled rate, dried and then placed in Petri plates with a single neonate of either fall armyworm, beet armyworm or soybean looper. Mortality was assessed at five days after treatment. Results are found in Figure 1.

AzaMax, Leprotec and Leprotec with Spear-Lep provided the best control of all products tested. These products provide three distinct modes of action for insect control and should be rotated in a hemp integrated pest management program to reduce the chance of insecticide resistance. The other products tested provided some benefit with soybean looper being the hardest to control. The least control was provided by the potassium salts of fatty acids and potassium silicate. This is not surprising as these products work best against soft-bodied insects, such as aphids and whiteflies. Thus, when applying potassium salts to aphids and whiteflies, do not expect additional control of caterpillars. These results begin to address the lack of information on insecticide efficacy in hemp and provide a basis for growers to choose between products to test on their own.

Jeff A. Davis is a professor of entomology at the LSU AgCenter, and Nathan Arey graduated with a Master of Science degree in entomology in 2022.

Figure 1: Efficacy of selected insecticides against soybean looper (SBL), fall armyworm (FAW) and beet armyworm (BAW).