Evaluation of persistent versus commercial nematode strains for management of curculio caryae (horn) and other weevils in pecan

Authors: Slusher, Eddie; SHIELDS, ELSON - Cornell University; HARGES, WILL - Strawberry Creek Ranch; Shapiro Ilan, David

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2025
Publication Date: 3/1/2025
Citation: Slusher, E.K., Shields, E., Harges, W., Shapiro Ilan, D.I. 2025. Evaluation of persistent versus commercial nematode strains for management of curculio caryae (horn) and other weevils in pecan. Biological Control. 202. https://doi.org/10.1016/j.biocontrol.2025.105705.
DOI: https://doi.org/10.1016/j.biocontrol.2025.105705

Interpretive Summary: Pecan weevil is a major pest of pecan. Pecan weevil is usually controlled using chemical insecticides. However, due to environmental and regulatory concerns, alternative methods of control are needed. Beneficial nematodes, also called entomopathogenic nematodes are small round worms that are used as natural bio-insecticides and are applied commercially to control a wide array of economically important pests. In previous research beneficial nematodes were found to also control pecan weevil. However, the costs of application can be high. The goal of the current research was to determine if the nematodes, applied at low (less expensive) rates, could control pecan weevil, and if pest control would last more than one year. Additionally, nematode strains that were thought to be especially persistent in the environment were compared with strains that are already commercially available. The study was conducted in pecan orchards in Georgia and Oklahoma. Results indicated that the beneficial nematodes reduced pecan weevil populations in the year they were applied as well as the the next year (when no additional nematodes were applied). In Georgia, other kinds of weevils such the two-banded Japanese weevil were also controlled with nematode applications. The commercially available strains performed as well or better than the suspected persistent strains. The findings indicate that beneficial nematodes may be able to control pecan weevil even when using very low application rates, and the effects of the applications can last into a second year (when no nematodes are applied). Thus, low-cost biological control of pecan weevil appears feasible.

Technical Abstract: Pecan weevil is a key pest of pecans. In addition, other weevils, such as Fuller rose beetle and two-banded Japanese weevil also feed on pecan foliage and roots. Entomopathogenic nematodes (EPNs) have previously been shown to be an effective tool for pecan weevil management. However, EPNs can be costly to apply and need frequent reapplication. Thus, there is a need to develop persistent strains of EPNs that can be applied less frequently and at lower rates. In this study, we compared two putatively persistent strains of EPNs, NY01’ (Steinernema carpocapsae) and NY04’ (Steinernema feltiae), against two commercially available EPN strains, ScAll (S. carpocapsae) and SfSN (S. feltiae), in the lab and field. For the field study, the suppressive ability of each pair of EPNs on pecan weevil, Fuller rose beetle, and two-banded Japanese weevil was compared. EPNs were applied in a 50/50 mix (ScAll + SfSN and NY01’ + NY04’) at substantially lower-than-normal rates for EPN application (~125 million IJs per ha in the current study whereas 2.5 billion per ha is commonly considered a standard minimum). A water-only control was also included. EPNs were only applied in the first year of the study (2022) and subsequently insect populations were monitored in 2022 and 2023. For the field study in Georgia, we caught significantly less two-banded Japanese weevil in trees treated with either nematode type in both years of the study. More pecan weevils were caught in trees treated with persistent nematodes in 2023 than the control or commercial EPN application. For the field study in Oklahoma, significantly less pecan weevils were caught in trees treated with commercial nematodes compared to the persistent nematodes and the control in both years of the study. In lab trials, comparison of the two S. carpocapsae strains and the two S. feltiae strains, found no significant differences across the 56-day sampling period. The results of this study indicate that commercial nematodes can have substantial carryover across two field seasons. In addition, commercial EPNs can be applied at a significantly lower rate and still provide some pest suppression. This can be beneficial as it means growers can apply nematodes less frequently and at a lower rate while still managing pest populations. Additional research is needed to optimize EPN rates to maximize efficacy and economic feasibility.