Our research sought to evaluate in field-crop production the value of discontinuing prophylactic insecticides to gain better control of invertebrate pest populations, particularly slugs, which are increasingly important and frustrating pests of no-till production. Simultaneously, we also explored the management potential of an underseeded crop for bolstering natural-enemy populations that could aid in pest control. In collaboration with Lucas Criswell, a grain farmer in Union County, PA, who helped develop the research proposal, we conducted a factorial experiment in two locations (Mr. Criswell’s Farm and Penn State’s research farm) crossing the factors of insecticide/no insecticides with presence or absence of an underseeded rye/clover mixture.
During the season, we tracked slug populations, evaluated the damage they caused to corn, and sampled natural enemies to characterize the size of their populations. Our work clearly demonstrated that the underseeded rye crop decreased slug damage to the corn crop and increased natural enemy populations. Unexpectedly, the insecticide treatment did not depress natural enemy populations and did not explain our treatment effects. Other studies that have explored the influence of insecticides on arthropod communities suggest that in fields that have regularly received insecticide applications populations may be perennially depressed. Therefore, a few years of avoiding insecticide applications may be necessary to allow natural enemy populations to grow and become evident.
Our results demonstrate that increasing in-field crop diversity may help decrease damage on focal crop species by diluting the influence of generalist herbivores and increasing the abundance of natural enemies.
Field crop growers in the Northeastern United States have been faithful supporters of no-till farming. In Pennsylvania in 2009, about 80% of the major crop acreage received some form of conservation tillage and 57% and 70% of corn and soybean acres, respectively, were not tilled (USDA-NASS 2009). Other northeastern states appear to have similar rates of no-till adoption. This conservation practice has been widely and increasingly adopted because it reduces soil erosion, conserves water, improves soil health, and reduces fuel and labor costs.
An additional conservation benefit that is often overlooked is that no-till farming can allow populations of beneficial arthropods to build up through time. Because many no-till growers also tend to use cover crops, beneficial arthropod communities can build even further and hold great potential for helping suppress pest populations (Witmer et al. 2003). Unfortunately, the potential advantage of increased natural-enemy populations is rarely realized in conventional no-till/cover-crop field-crop systems because of routine use of insecticides prior to, or at, planting. Many growers are, not surprisingly, risk averse and over the years they have become reliant on prophylactic insecticide treatments to protect against early season pests such as black cutworm and true armyworm, which have potential to cause damage, but are notoriously variable in space and time.
Prophylactic insecticide applications violate one of the main tenets of integrated pest management (IPM) because such treatments are applied without an economic need. Numerous studies have demonstrated that ‘rescue’ treatments of insecticides, not preventative applications, are the most economical tactic to control pest outbreaks (e.g., Johnson et al. 2009). Prophylactic insecticide application prior to and at planting are undoubtedly constraining the development of natural-enemy populations in Pennsylvania crop fields and leaving these fields vulnerable to outbreak of early season pests, such as slugs and aphids but also including some of the pest species the prophylactic sprays are meant to control (i.e., black cutworm, true armyworm; Clark et al. 1994). Further, most crop fields have limited ability to maintain natural enemy populations because they usually contain few other plant species besides the crop species. It is well acknowledged that increasing plant species diversity in agricultural systems can foster improved natural-enemy populations that can contribute to better pest suppression (Andow 1991), yet there is limited opportunity to incorporate plant species diversity without disrupting crop productivity.
Pursuing these two lines of evidence as ways to bolster natural-enemy populations in grain production, we experimentally manipulated two factors, the use of prophylactic insecticides and a habitat-providing underseeded crop. We hypothesized that plots with underseeded crops without insecticides would have the highest natural-enemy populations and lowest pest populations, whereas plots receiving insecticides without underseeded crops would have the fewest natural enemies and the highest incidence of pest damage to crop plants. To test these hypotheses, we focused on ground beetles and slugs. Ground beetles (Coleoptera: Carabidae) are common and voracious predators, which can strongly regulate pest populations, whereas, slugs are among the most challenging pests of no-till systems in the Mid-Atlantic region. In fact, a recent survey by our lab group found that 80% of no-till growers identified slugs as the most challenging pest they face.
The objectives of our study were to quantify in on-farm research the benefits natural-enemy populations by:
1) Avoiding prophylactic insecticide applications,
2) Improving habitat diversity by sowing in a grain system an underseeded crop.
Improved natural enemy populations from these two tactics will then
3) Decrease pest populations
4) Improve yield
In Spring 2010, we established our factorial experiment in two locations in Pennsylvania, one on Mr. Criswell’s farm, Criswell Acres, near Lewisburg (Union County), and the other at Penn State’s Russell E. Larson Agricultural Research Center (LARC) at Rock Springs (Centre County). On Mr. Criswell’s farm, we used a 12-acre, long-term, no-till field. The Rock Springs field was also a long-term no-till field, but only a one-acre research block. Both fields grew soybeans in 2009, were planted with a cereal rye cover crop in autumn 2009, which was killed with glyphosate in April about two weeks prior to planting. On 19 and 26 May 2011, we established 16 plots per site at Criswell Acres and LARC, respectively.
The two factors that we varied for these experiments were use of prophylactic insecticides and an underseeded mixture of cereal rye and subterranean clover; therefore we had four factorial treatments:
1) plots receiving no insecticide and no underseeding;
2) plots that will receive insecticide but no underseeding;
3) plots with both insecticide and underseeding; and
4) plots that will receive no insecticides and the underseeding mixture.
Therefore, in eight plots per site we established using a no-till seed drill a mixture of cereal rye (Secale cereal, 50 lbs per acre) and subterranean clover (Trifolium subterraneum, 10 lbs per acre). On the same days, we planted corn (Dekalb variety 54-16, Yieldgard VT Triple stack containing Bt toxins against caterpillar and rootworm pests and herbicide resistance to glyphosate, and Poncho 250 seed treatment) with 30-in spacing between rows, and then treated eight plots with either the organophosphate insecticide chlorpyrifos (Lorsban, 1.5 pint per acre, Dow AgroSciences; Criswell Acres) or the pyrethroid insecticide lambda-cyhalothrin (Warrior, 3.84 oz per acre, Syngenta; LARC). Plots at Mr. Criswell’s farm were 300 X 75 ft, whereas LARC plots were 100 X 20 ft. At maturity, plots were harvested for yield with a full-sized combine (Criswell Acres) or a specialized plot combine (LARC).
We hypothesized that corn grown under the regiment of treatment #4 will have the lowest incidence of pest damage and the highest yield because the plots will not receive prophylactic insecticide applications and the underseeding mixture will provide habitat to natural enemies, which will kill developing pest populations, alternative food options for slugs, and nitrogen for the corn. Moreover, we hypothesize that treatment #2, which will have insecticides and no underseeding will have the highest pest pressure and the lowest yield because the insecticides will decrease natural-enemy populations and there will be no underseeded mixture or its associated benefits. We expect that treatments 1 and 3 will intermediate to treatments 2 and 4.
During the experiment, we tracked the phenology of slug populations to detect spring egg hatch by deploying shelter traps and their damage by rating slug injury to growing corn seedlings. Shelter traps are simply 1 X 1 cut pieces of rolled roofing shingles (Owens Corning, color Shasta White, Lowes item # 10285). Two shingles per plot were placed haphazardly near the center of each plot in different rows. Traps were separated by at least 15 m and debris was cleared away from the ground prior to settling shingles on the ground.
Once per site, on 11 and 16 June for LARC and Criswell Acres, respectively, we assessed slug damage to growing corn seedlings on four (Criswell Acres) or two (LARC) 10-ft sections of the corn rows adjacent to shelter traps. Rating occurred when corn was at the V4 growth stage. Corn was rated on a visual scale of 0-4: 0 = no slug damage; 1 = leaves showing less than 25% defoliation ; 2= leaves showing 25 – 50% defoliation; 3 = leaves with 50 – 75% defoliation); 4 = seedling with 75 – 100% defoliation, including completely cut off at the ground level.
To sample the natural-enemy community, we deployed pitfall traps. Pitfall traps were made from plastic deli containers (12-cm diameter, 12-cm deep) with lids and were filled with one-quarter cup of a 50:50 mixture of propylene glycol and water to kill and preserve trapped carabid beetles. Traps were established flush to the ground and were protected from rainfall and other falling material with a cover that was suspended ~7 cm off the ground using galvanized nails as supports. At Criswell Acres we had two traps per plot whereas the smaller plots at LARC only had one trap per plot. To sample carabid populations, we removed the lid of traps for 6 days and then returned to collect traps. At Criswell Acres, we conducted our sampling four times during the season in June, July, August, and September; whereas three efforts (June, July, August) were made at LARC. Frustratingly, only three of our sampling efforts (Criswell: June and September; LARC: June) yielded a full set of traps. Trapping for the other dates was compromised in part or nearly in full by groundhogs and other varmints that dug traps out of the ground despite our best efforts to avoid this trouble. All carabid beetles were identified to genus and the great majority of specimens were keyed out to species level. All specimens are retained in the laboratory collection of the Tooker Lab at Penn State University.
Shelter traps at both sites indicated that the gray garden slug (Deroceras reticulatum) hatched from its over wintering eggs between 4 and 11 May 2010. This slug species is the most damaging slug species in the Mid-Atlantic Region that attacks field crops, including corn and soybeans; therefore, our experiments were planted when the damaging juvenile stage were active.
Our assessment of slug damage revealed that slug populations at Criswell Acres were relatively low. Our highest mean damage rating was just under 0.47 on our 0-4 scale, indicating that slug damage was low (Fig. 1A), and likely non-economic. As a result, none of our analyses revealed significant effects of the two factors or their interaction (Fig. 1A; rye effect: ANOVA F1,15 = 4.7, P = 0.27; insecticide effect: F1,15 = 0.07, P = 0.79; rye*insecticide interaction: F1,15 = 0.45, P = 0.99). In contrast, LARC had higher slug populations which were significantly influenced by the presence of rye (ANOVA F1,15 = 10.9, P = 0.009), but not the application of insecticide (F1,15 = 0.99, P = 0.35) or the rye X insecticide interaction (F1,15 = 0.19, P = 0.67; Fig. 1B). Slug-inflicted damage was significantly higher on corn plants grown in the absence of the rye/clover underseeded crop, supporting the hypothesis that other green material in the field will decrease damage to the cash crop.
In Document 1 below, see Figure. 1: Slug-damage rating on corn planted at A) Criswell Acres or B) LARC.
Our pitfall trapping of carabid beetles revealed at both locations a significant influence of the underseeded crop on beetle abundance, but no influence of insecticide use on beetle populations. At Criswell Acres, the influence of the rye/clover crop doubled the total number of beetles that we captured per trap (Fig. 2A; Rye effect: ANOVA F1,63 = 5.8, P = 0.019), whereas at LARC the influence of the underseeded crop boosted carabid populations nearly three times compared to our bare plot controls (Fig. 2B; Rye effect: ANOVA F1,15 = 8.4, P = 0.018), supporting the hypothesis that underseeded crops will foster improved natural enemy populations. For each site, these differences in the total number of beetles captured also extended to the dominant species that we captured. At Criswell Acres, the dominant beetle species were Pterostichus permundus, Pterostichus melanarius, and Harpalus pensylvanicus, accounting for 91% of the beetles we captured. Of these species, numbers of P. melanarius and H. pensylvanicus (Fig. 3A) were significantly increased by the rye/clover underseeded crop (P. melanarius: ANOVA F1,63 = 5.8, P = 0.019; H. pensylvanicus: F1,63 = 8.7, P = 0.005; Fig. 3A). At LARC, the differences in total beetles seen between underseeded plots and bare plots was reflected in the dominant species captured, Bembidion quadrimaculation oppositum, which accounted for 36% of the beetles captured and was more than three times as abundant in when the underseeded crop was present (Fig. 3B; ANOVA F1,15 = 6.6, P = 0.03).
At Criswell Acres, where we were able to generate samples for two sampling dates, we found a significant influence of date with the September sampling date generating significantly more beetles (22 Jun: 1.91 +/- 0.32 beetles per trap [standard error]; 28 Sept: 10.5 +/- 1.91; ANOVA F1,63 = 21.4, P < 0.0001), supporting the notion that ground beetle populations are greatest latter in the season, not when most slugs are damaging young crop plants. Due to groundhog activity, the comparison was not possible for other dates or among dates at LARC.
In Document 1 below, see Figure 2. Total number of beetles trapped at A) Criswell Acres and B) LARC.
In Document 1 below, see Figure 3. Number of representative beetle species trapped. A) Harpalus pensylvanicus at Criswell Acres; B) Bembidion quadrimaculation oppositum at LARC.
At both sites, yield was significantly decreased by the underseeded crop (Fig. 1; Criswell Acres: F1,15 = 74.4, P < 0.0001; LARC: F1,15 = 63.7, P < 0.0001). This result was expected because the underseeded crop would compete with the cash crop, but our goal was to see if there were any benefits to the underseeding. Interestingly, there was no yield benefit to prophylactic insecticide usage, meaning that yield was equal whether insecticides were used or not (Fig. 4; Criswell Acres: F1,15 = 0.40, P = 0.53; LARC: F1,15 = 0.25, P = 0.63). This result supported the hypothesis that insecticide use does not guarantee increased yield and would be better used in an IPM framework driven by pest pressure and economics.
In Document 1 below, see Figure 4. Influence of the underseed crop and insecticides on yield at the two sites: A) Criswell Acres, B) LARC.
Taken in total, our findings support two main conclusions.
First, when slug populations are large enough to cause significant feeding damage, underseeded crops hold potential to decrease slug damage to the cash crop by one of two potentially interacting mechanisms. 1) The underseeded crop may provide slugs with an alternative food source that may be preferred to corn, or may just dilute the feeding pressure on corn. Previous greenhouse-based work has suggested that alternative foods may decrease slug damage to focal crops (Brooks et al. 2005). 2) Underseeded crops may foster natural-enemy populations, like carabid beetles, that may decrease slug population densities. Carabid beetles can be voracious slug predators and have been responsible for suppressing slug populations under some conditions (Douglas and Tooker, in press). Our experimental design did not allow us to distinguish between these two possibilities, but it will certainly be the focus of subsequent research efforts in our lab group.
The second conclusion from our results is that prophylactic use of broadcast insecticides does not necessarily provide a yield advantage in the absence of sensitive pests.
We hypothesized that insecticide usage would suppress natural-enemy populations, but at Criswell Acres prophylactic insecticides had been used in our research field nearly every year in recent memory (perhaps exceeding 20 yr; L. Criswell, personal communication); therefore, it is likely that the natural-enemy community has been perennially suppressed and it may have been a bit too much to expect that stopping insecticide applications in eight plots for one Spring would provide enough time for the arthropod community to rebound.
Beyond slugs, which are not sensitive to very many insecticides (Douglas and Tooker, in press), we did not find many other herbivores feeding up the corn plants at either of our sites, meaning that the insecticide treatments we applied would have been unnecessary in similar commercial fields. Certainly numerous research studies have demonstrated that broadcast insecticide use is usually not economical or efficient for managing pests in field crops (e.g., Brust and King 1994). Rather, it is far more economical to use IPM, which involves among other tactics scouting for pests, using economic thresholds to evaluate pest damage, and applying rescue treatments of insecticides if economic thresholds are exceeded.
Andow, D.A. 1991. Vegetational Diversity and Arthropod Population Response. Annual Review of Entomology 36: 561-586.
Brooks, A.S., Wilcox, A., Cook, R.T., Crook, M.J. 2005. A laboratory-based comparison of a molluscicide and an alternative food source (red clover) as means of reducing slug damage to winter wheat. Pest Management Science 61: 715-20.
Brust GE, King L.D. 1994. Effect of crop rotation and reduced chemical inputs on pests and predators in maize agroecosystems. Agriculture, Ecosystems and Environment 48: 77-89.
Clark MS; Luna JM; Stone ND; Youngman RR. 1994 Generalist predator consumption of armyworm (Lepidoptera: Noctuidae) and effect of predator removal on damage in no-till corn. Environmental Entomology 23: 617-622.
Douglas, M.R, Tooker, J. F. Slug (Mollusca: Agriolimacidae, Arionidae) Ecology and Management in No-till Field Crops, with an Emphasis on the mid-Atlantic Region. Journal of Integrated Pest Management, in press.
Johnson, K.D., Neal, M.E.O., Ragsdale, D.W., D, C., Swinton, S.M., Dixon, P.M., Potter, B.D., Hodgson, E.W., Costamagna, A.C. 2009. Probability of cost-effective management of soybean aphid (Hemiptera: Aphididae) in North America. Journal of Economic Entomology 102: 2101-2108.
Witmer J.E., Hough-Goldstein, J.A., Pesek J.D. 2003. Ground-dwelling and foliar arthropods in four cropping systems. Environmental Entomology 32: 366-376.
Our research effort resulted thus far in two publications, both are aimed at extension audiences (see below for details). We also hosted two field days at two locations for approximately 90 growers, featuring our research and the theory supporting the project. Importantly, our collaborative effort with Mr. Criswell has convinced him and his professional crop scout that prophylactic use of broadcast insecticides is not necessary. Mr. Criswell has dropped these insecticide applications from his 600 acres of grain production for two years in a row (saving nearly $3500 per year in insecticide costs) and he has discussed his experiences with other grain growers in his region. Significantly, Mr. Criswell’s crop scout Gerard Troisi has also realized benefits of reduced insecticide use and has discussed Mr. Criswell’s efforts with his other customers. Support of agricultural professionals like Mr. Troisi will help spread the wisdom of lower levels of potentially ineffective insecticide use.
Beyond these impacts and outcomes, our Partnership Project with Mr. Criswell has provided valuable data that are serving as a base for further research explorations. The Tooker Lab is continuing to explore factors contributing to improved natural-enemy derived pest control based in the findings that underseed crops may foster better natural enemy population while simultaneously diluting the influence of slugs on the cash crop.
By reducing unnecessary insecticide use, associated costs, and fostering natural enemy abundance and associated ecosystem services, our project directly linked at least four of the key themes of sustainable agriculture highlighted by NE-SARE:
1. Reduce environmental and health risks in agriculture
2. Prevent agricultural pollution
3. Improve productivity, the reduction of costs, and the increase of net farm income
4. Conserve soil, improve water quality, and protect natural resources.
Education & Outreach Activities and Participation Summary
Two publications resulted in part from our project:
Douglas, M.R, Tooker, J. F. Slug (Mollusca: Agriolimacidae, Arionidae) Ecology and Management in No-till Field Crops, with an Emphasis on the mid-Atlantic Region. Journal of Integrated Pest Management, in press.
Douglas, M.R, Tooker, J. F. Slugs as Pests of Field Crops. Department of Entomology, Penn State Extension FactSheet: http://ento.psu.edu/extension/factsheets/slugs-as-pests-of-field-crops
During Summer 2010, Mr. Criswell and I hosted two field days featuring our collaborative research:
1. Weed and Insect IPM Field Day (organized in cooperation with Dr. William Curran) Russell E. Larson Agricultural Research Center, Penn State University, Rock Springs, PA 8 July 2010. 70 Attendees
2. Criswell Acres No-Till Field Day, October 2010. 20 Attendees
During winter 2010-2011, Dr. Tooker gave seven presentations to 893 farmers and associated agricultural professionals on slug management that featured data collected with support from our Partnership Grant
Areas needing additional study
Future research will explore the relative strengths for slug suppression of natural enemies and the underseeded crop itself. Research will also pursue approaches to more precisely place the underseeded crop between the rows of corn rather than using a seed drill. Precision agriculture equipment may allow precise location of the crop so that competition between the underseeded crop and cover crop is minimized. We will also explore the potential for killing the underseeded crop with herbicides to minimize competition.