Spatial variability in slug emergence patterns - third year results

Authors: Mueller Warrant, George; ANDERSON, NICOLE - Oregon State University; SULLIVAN, CLARE - Oregon State University; Trippe, Kristin

Submitted to: Seed Production Research at Oregon State University
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2017
Publication Date: 4/30/2017
Citation: Mueller Warrant, G.W., Anderson, N.P., Sullivan, C.S., Trippe, K.M. 2017. Spatial variability in slug emergence patterns - Third year results. Seed Production Research at Oregon State University. 153:41-45.

Interpretive Summary: Slug control treatments used in grass seed production are baits formulated using metaldehyde or iron phosphate as active ingredients along with grain products as the major physical component. Despite recent improvements in pellet design (higher density to better withstand moisture and smaller size to minimize loss to earthworm feeding), slug baits still disappear from the soil surface within two weeks after application, forcing growers to make multiple applications to protect their crops. This multi-year field study focused on three main objectives of interest to grass seed growers. First, could the timing of greatest crop damage by slugs be identified in real-world conditions of continuously changing weather, crop development, and slug emergence, and intermittently applied slug baits? Second, could damage within this critical period be quantified relative to numbers of slugs? Third, could spatial patterns in slug counts be detected and shown to be stable over time or are they only ephemeral? Field plots were located in situations in which high numbers of slugs were anticipated based on the prior crop history. Slug counts in no-till planting into white clover have been amongst the highest of any crop rotation and tillage method studied, and growers have applied slug bait more times per planting in this situation than in any other fields monitored. Satisfactory crop stands have been achieved using no-till planting into white clover in part because the firmer soil structure makes it easier to drive across fields applying slug bait in a timely manner. Results in 2016 across all four crop rotations were distorted compared to previous years by unusually heavy rainfall in October that interrupted field work and delayed planting using conventional tillage by as much as six weeks. Hot-spots of slug activity existed at all four sites in 2017, with many of their locations suggestive of possible influence of sub-surface water flow on slug survival over the summer and early emergence in the fall.

Technical Abstract: Gray field slugs damage new plantings of crops such as perennial ryegrass grown for seed, and growers routinely make multiple applications of metaldehyde and iron posphate based slug baits. Two major challenges for growers are: (1) choosing the best timing for the first heavy application of slug bait, and (2) deciding whether to treat entire fields uniformly or to focus baiting on particular areas with highest populations of slugs. In this third year of study, weekly counts of slugs, earthworms, predatory beetles, and cutworms along with gravimetric measurement of moisture in the surface 2-inches of soil were conducted at four sites whose cropping history was similar to those previously monitored. Field conditions in 2016 were conventional tillage into red clover (site 1), wheat (site 3) and canola (site 4), and no-till into white clover (site 2). Unlike previous years, unusually heavy rainfall during October spread out perennial ryegrass seedling emergence from mid-September at site 1 (where irrigation was available) to mid-November at site 4. Only one out of the four fields (site 2) saw crop emergence centered in the normal time of October. Early emergence of perennial ryegrass seedlings at site 1 due to irrigation allowed them to escape the threat of slugs by becoming too large to be injured by the time slugs were present for prolonged periods in high numbers. Although slugs were present at sites 3 and 4 while perennial ryegrass seedlings were emerging, stand loss that occurred was primarily a result of flooding, debris deposition, and freezing temperatures during a series of strong winter storms from mid-December through early February. Although this unusual weather prevented us from linking crop damage to slug numbers at most sites in 2016, we did measure strong spatial autocorrelation in slug counts, with statistically significant “hot-spots” at all four sites. In many cases, the “hot-spots” appeared to occur in lower elevation positions within fields, possibly related to areas likely to possess shallow sub-surface moisture flow during late summer.