Entomopathogenic Bombs – Sweet Potato Weevils Be Gone

In demonstrating EPN efficacy, we experienced extremes in sweetpotato weevil pest pressure. In two demonstrations, the sweetpotato weevil pressure was so extremely high, EPN applications were ineffective. The EPN were overwhelmed by the weevil population and any reduction in sweetpotato weevil populations were not sufficient to produce a marketable crop. In other demonstrations, the untreated controls yielded similarly to the EPN treatments because there were not sweetpotato weevils to control.

EPN Efficacy

We compared water applications of EPN to an EPN infected-cadaver application. EPN application rates were 2×10⁹ Infective Juveniles (IJs)/ha for all EPN treatments. Three days after application of EPN-infected cadavers, 100% of cadavers were located demonstrating that the cadavers were not eaten by birds, ants, geckos or other scavengers. The application method (water vs cadaver) did not have a statistically significant impact on sweetpotato yield. A numerical difference in yield was noted between the EPN treatments compared to the untreated control. The aqueous application of Steinernema feltiae and the cadaver application of Heterorhabditis indica increased sweetpotato yield the most (P<0.05). We did not detect the sweetpotato weevil in the first or second repeat of the experiment. All sweetpotato yield was marketable due to the lack of sweet potato weevil damage in the first repeat. In the second repeat, the plots suffered from substantial stem borer damage. Of the little sweetpotato yield obtained, none suffered from weevil damage. The EPN treatments had no effect against the stem borer.

In a separate experiment, the efficacy of 2, 3, and 6 infected cadavers/m of row were compared. This was equivalent to  IJ applications of 0.5X, X and 2X or 500 million, 1 billion and 2 billion IJ/ha. We found that increasing the number of cadavers/row did not increase EPN efficacy against sweetpotato weevil. Unexpectedly, the low cadaver application, ie. 2 cadavers/row, resulted in the least amount of sweetpotato weevil damage. We hypothesize that the increased number of cadavers made it easier for scavangers (like birds, geckos, slugs, or rodents) to locate and consume the EPN-infected cadaver, thus reducing the overall application rate of EPN and subsequent efficacy against the sweetpotato weevils.

The results from our field demonstrations prompted us the undertake two additional investigations. A more accurate measurement of sweetpotato weevil populations might allow adjustments to EPN applications to provide better management of the pest. Since EPN-infected cadavers were likely consumed by scavengers, alternatives that protect the EPN like the cadavers but are even less palatable to scavengers might also enhance EPN efficacy.

Sweetpotato Weevil Populations

Currently sweetpotato growers base their assessment of sweetpotato weevil populations on their experiences from the previous cropping cycle and plant their IPM strategies on that information. A few growers use pheromone traps to monitor sweetpotato weevil populations. Neither approach is particularly accurate or timely. Consequently, the growers employ a calendar-based pesticide application approach in most instances. Sweetpotato weevils are cryptic in nature. The insects are not particularly good fliers and females deposit their eggs near the soil line or follow cracks in the soil peds to lay eggs in the swollen roots of the sweetpotato. We evaluated pheromone traps, pit fall traps, Berlese funnels, sweep nets, and elutriation as methods for early detection and monitoring of sweetpotato weevils. In one field, we monitored using pitfall and pheromone traps. Over the 17 week crop growing period, nearly 1000 weevils were collected in the pheromone trap but only 2 weevils were recovered from 16 pitfall traps over the same time span. In a separate field, we placed pitfall and pheromone traps, collected soil samples, and used a sweep net to monitor sweetpotato weevil populations. The pheromone traps collected 5 weevils over the cropping period. The pitfall traps, the Berlese funnel, soil elutriation and the sweep net of the same field recovered no sweetpotato weevils. However at harvest, all of the sweetpotato roots collected were infested with sweetpotato weevils at a moderate level of 3-4 weevils/swollen root. We have not identified a timely method to determine sweetpotato weevil presence and population in the field. We have been targeting EPN application commencing at root swelling but have had the greatest management of sweetpotato weevil when monthly applications started 1 month after planting rather than at commencement of root swelling.

IMG_9300

Better Applications

Infected-host cadavers, however, are fragile thus developing an EPN infected-cadaver that resists breakage would be beneficial. Laboratory experiments were conducted with Galleria mellonella and Tenebrio molitor infected with Heterorhabditis indica OM-160 or Steinernema feltiae MG-14 respectively. The cadavers were desiccated to 85% relative humidity with KCl. Cadaver weight of G. mellonella infected with H. indica was reduced by 40% with cadavers becoming leather-like and more resistant to rupture. Desiccated cadavers of G. mellonella infected with S. feltiae remained pliable and ruptured when handled. IJs of S. feltiae emerged prematurely from cadavers of T. molitor during the desiccation period and H. indica OM-160 reproduced poorly in T. molitor.  Infective Juveniles (IJs) of H. indica collected from desiccated and non-desiccated cadavers of G. mellonella were evaluated for infectivity and viability on larvae of G. mellonella, T. molitor, and Oryctes rhinoceros. Desiccated cadavers of G. mellonella infected with H. indica  yielded equal numbers of IJs compared to non-desiccated infected cadavers (P>0.05) with both averaging over 200,000 IJs of H. indica/cadaver. IJs of H. indica emerging from desiccated and non-desiccated cadavers killed 100% of G. mellonella larvae, 40% of T. molitor larvae, and 94% of O. rhinoceros larvae. The delivery of H. indica through infected cadavers of G. mellonella can be improved by desiccating the infected cadavers. The number, infectivity, and viability of IJs were not adversely impacted by desiccation.

While desiccated proved easy to handle, the may be prone to scavaging. Consequently, the survival, recovery, and infectivity of IJs of S. feltiae were formulated on cellulose and microfiber sponges and stored for 14, 30, and 60 days. Nematodes formulated onto 1 cm sponge cubes and stored at 15°C. IJ infectivity was assessed on larvae of G. mellonella. Both cellulose and microfiber sponges maintained IJs infectivity significantly better than the filter paper control. However, no significant difference was observed between the two sponge types, suggesting that either could be used as a substrate for further formulation. Storage duration significantly affected infectivity, with IJs stored for 14 and 30 days causing higher larval mortality within 24 hours compared to IJs stored for 60 days. No significant interaction was found between sponge type and storage duration. Cellulose and microfiber sponges preserve EPN viability for up to 30 days, while longer storage (60 days) reduced infectivity. Further optimization of formulation methods is necessary to address desiccation, ultraviolet radiation, and temperature protection to enhance the long-term EPN storage for biocontrol applications are required and these treatments can be compared to the infected-cadaver delivery method for efficacy.