Project Details:

Can Slug Egg Hatch be Predicted?

Parent Project: This is the first year of this project.
Checkoff Organization:Delaware Soybean Board
Categories:Insects and pests, Crop management systems
Organization Project Code:AGEXDA2008
Project Year:2020
Lead Principal Investigator:David Owens (University of Delaware)
Co-Principal Investigators:

Contributing Organizations

Funding Institutions

Information and Results

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Project Summary

Slug damage to germinating field crops is a significant problem of no-till or conservation-till fields in the mid-Atlantic. Cover crops also provide a favorable microclimate; all of these management practices are being widely adopted and encouraged by farmers and various agencies for their agronomic, nutrient management, and soil management benefits. Slug damage is difficult to predict. Sampling can be difficult because they hide during the day in soil cracks. Rescue treatments are expensive ($20 – 30/acre) and sometimes not satisfactory, and, in soybean, the field will often require replanting by the time a problem is noticed.

It has been suggested in the literature that recently hatched juvenile slugs are the most injurious to the crop and if a bait were applied when eggs hatch, slug populations can be controlled satisfactorily. If weather conditions are forecast that would favor slugs at at time when eggs were expected to hatch, planting timing could be changed to lessen slug risk to the crop.

Any information that would help producers predict and avoid or take remedial action during planting windows at greatest risk for damage will help make Delaware soybean production more resilient. In 2018, an estimated 16%
of soybean acreage was infested with slugs, but damage estimates were less than 2017 in part because excessive spring rains delayed much of the planting until later in the spring at a time when soil temperatures favored rapid growth.

Project Objectives

Determine possible relationships between soil and air temperature on slug egg hatch

Project Deliverables

More knowledge of early season slug population dynamics

Progress of Work

Updated August 3, 2020:
The objective of this study is to determine possible relationships between soil and air temperature on slug egg hatch. In one of the first investigations of slug bait efficacy, OSU entomologist Ron Hammond noted that bait applications that coincided with egg hatch were the most effective at preventing stand loss, but no follow up work was reported. To determine if slug egg hatch can be predicted, 9 fields were sampled beginning March 2020 until the first week of June, or until that field had been planted and the crop had grown enough to be safe from slug damage. The selected fields all had a history of slug damage or high slug populations. Three of the nine fields were planted into corn, the remaining were planted into soybean. Three of the fields were in Sussex County, Three in Kent County, and three in New Castle County. In each field temperature and relative humidity sensors were placed on a wooden post. Two sensors were placed approximately three feet off the ground and under a solar radiation shield to record ambient air temperature. Another temperature sensor was placed underneath residue near the wooden post to record soil surface temperature where slugs would be hiding. At weekly intervals, each field was visited and the number of adult slugs, large immatures, new hatchlings, and slug eggs were recorded from four to five 1-m2 areas by carefully sifting through residue.

Current Situation: The winter was unusually mild, and fields with marsh slugs had all active life stages present once sampling had been initiated. Five of the nine fields had gray garden slugs which appeared after sampling began. Gray garden slug eggs began hatching during the first week of April, and continued for a prolonged period of time. Unfortunately, this is well before soybean planting, meaning that weather conditions immediately prior to and during planting may have a greater impact on slug bait application and planting decisions. The data has not yet been analyzed or correlated with the temperature sensors.

Slug activity was high, and many fields throughout the state experienced stand loss due to slug feeding. Although February and March were unusually warm, cool, moist weather followed in April and May, resulting in slow crop growth and ideal conditions for slug activity. Slug updates and management were discussed in 8 editions of the Delaware Weekly Crop Update.

Remaining 2020 Activities: Data entry and analysis for spring 2020 needs to be completed, and sensors will be placed into fields as corn is being harvested to determine if gray garden slugs overwinter as eggs or as adults. Sampling will resume at the end of October and run through December.

View uploaded report Word file

Updated August 1, 2021:
This is a mid-season update to the 2020 project ‘Can slug egg hatch be predicted?’. Once again, we selected 9 fields in Delaware to monitor weekly in the fall (November and December) and in the spring (March through early June). Fields were sampled with temperature and relative humidity sensors, shingle traps for slugs, and by sifting through soil residue in 4 square meter samples. Slugs were categorized as adults, immatures, and new hatchlings, as well as counting eggs.
Current Situation: The winter was colder than 2019-2020. In many of the sampled fields, grey garden slugs that were present in the Fall had much reduced populations in the spring. One exception was a field near Lewes, DE with extremely high slug counts. The farmer elected to plant soybean at the end of May after a prolonged dry spell and very warm soil; the soybean exhibited very little slug feeding. Another field site with a large number of marsh slugs was worked with a land awl, reducing slug feeding to the crop. Once again, all life stages of marsh slugs were active during most of the sampling period. Dry weather in May also likely contributed to low slug populations in most of the sampled fields. Gray garden slug activity was much lower even in fields with historically high populations. In only 1 field were hatchlings observed on March 30. Temperature data has not yet been correlated with slug activity.

Final Project Results

Benefit to Soybean Farmers

A better understanding of slug activity and life stages present in fields prior to planting may aid in management decision making.

Performance Metrics

Slug eggs were detected, hatching was detected, and temperature data collected.

Project Years

YearProject Title (each year)
2020Can Slug Egg Hatch be Predicted?