Feasibility of Reducing Slug Damage in Cabbage: Part II

FINAL REPORT Feasibility of Reducing Slug Damage in Cabbage: Part II Christy Hoepting and Rebekah Edgell, Cornell Cooperative Extension Regional Vegetable Program COOPERATORS Chris Martin, Martin Farms, Brockport, NY Don Sweet, Crop Advantage crop Consulting, Henrietta, NY BACKGROUND The board of the New York Cabbage Research and Development Program made slug control one of their highest research priorities for the first time in 2009. Slugs are considered a sporadic pest in cabbage and are favored by cool and moist conditions, especially where crop residues are left on the soil surface. In conventional production of cabbage, slugs tend to be a problem later in the growing season along tree lines and hedgerows and in weedy patches within the field. Slugs leave large holes in the leaves with the veins intact, and can be a contaminant in the heads when they squeeze between the leaves. During the cool wet growing season of 2009, slug contaminants were the cause of several rejected loads of cabbage in New York. It is predicted that the frequency of slug problems in cabbage will increase, because more cabbage is being grown in rotation following field corn. The newer varieties of field corn are Bt tolerant and have tougher stalks that take longer to break down, thus, these fields have more crop residue and are more favorable for slugs. In 2010, Hoepting evaluated the following treatments for control of slugs in cabbage: 1) Deadline MP @ 20 lbs per acre ($50/acre) 2) 30% urea based nitrogen (30 0 0) @ 0.55:1 and 1:1 N:water in 20 gal per acre ($15 $27 per acre) 3) Lannate LV @ 3 pts per acre ($26 per acre) 4) Browseban @ 1.28 and 15 fl oz per acre ($1 to $15 per acre) 5) Asana XL @ 9.6 fl oz per acre 6) Sluggo @ 20 lbs per acre A small plot research trial was conducted in a grower s field that had a very high infestation of slugs. In the untreated controls, 95% of the plants and 80% of the cabbage heads were infested with slugs with an average of 3.2 slugs per plant. Rescue treatments were applied only once using a CO 2 backpack sprayer at 30 psi and 40 gpa. Since treatments No. 2, 3, 4 and 5 required direct contact with the slugs in order to be effective, applications were made at night, because slugs are nocturnal. Best control of slugs was achieved with Deadline MP, which provided 63% control, followed by Browseban (46 to 63% control), Lannate LV (36 to 50% control) and 30% urea based liquid nitrogen (32 to 44% control). Dead slugs were found in the Deadline MP and Lannate LV treatments. The 15x rate of Browseban that was used caused the cabbage leaves to turn lighter green in color. To complement the field trial, a bioassay was also conducted. Deadline MP again provided the best control and had the highest slug mortality at 71% 60 hours after treatment, which was significantly 1

higher than all other treatments. The 30% urea based nitrogen (1:1 N: water) and Sluggo had significantly higher slug mortality than the untreated with 35% and 31% slug mortality, respectively. We observed that the slugs in the Browseban treatment promptly relocated themselves from contact with the Browseban and sought shelter on the under sides of the leaves. This observation in combination with reduced incidence of infested heads in the field, fewer slugs per head, and relatively low mortality in the bioassay suggest that Browseban may be functioning as a repellent to slugs. The results of this first year of study are encouraging and it continues to be worthwhile to investigate the feasibility of managing slugs in cabbage. Since Deadline MP is a bait, and requires slugs to be attracted to it and then ingest it in order for it to be effective, evaluating the highest label rate of 40 lb per acre rate as well as two applications of the 20 lb rate is warranted to determine if a higher level of control can be achieved. Lannate LV is not currently labeled for slug control, but it is labeled in cabbage for worm control and DuPont is interested in pursuing a label for slugs and funded a portion of this trial. Further research is needed to optimize the efficacy of Lannate including as a preventative application. Browseban is not labeled specifically on slugs, but it is labeled in cabbage as a deer and raccoon repellent. In 2010, Browseban was accidently applied in the field trial at 15 times the labeled rate, and it needs to be evaluated at the labeled rate and as a preventative application. We also need to make sure that the lower rate does not change the color of the cabbage leaves. The active ingredient, copper sulfate is labeled on cabbage for control of black rot, but not on cabbage. Copper has been shown to work as a repellant to slugs and copper sulfate has long been used with great success in tropical countries to clear disease carrying snails from ditches and lakes. It has been found to be especially effective against the field slug when sprayed on the soil. It is a contact poison and would be most effective when slugs are active. In New York, there are certain formulations of copper sulfate, such as Cuprofix 40 Disperss that are labeled on cabbage for black rot and should be evaluated for their ability to control slugs. It should be relatively easy to add slugs to the labels of materials that already have cabbage on the label, as a 2ee, and growers could get control of multiple pests, which may also make them economically feasible to use preventatively. OBJECTIVES 1. To optimize management of slugs in cabbage using Deadline MP, Lannate LV and Browseban. 2. To evaluate copper sulfate for slug control in cabbage. PROCEDURES Field Study: A small plot field trial was set up in a grower s cabbage (c.v. Constellation) field that had a spotty slug infestation in a weedy patch along one of the hedgerows. The trial was set up as a randomized complete block design with 15 treatments and 5 replications. Each treatment replicate consisted of a single row of cabbage by 15 feet long. Plant spacing was 18 inches and row spacing was 32 inches. An untreated guard row was left on each side of the treated row. Slugs are nocturnal, so to ensure optimal contact with slugs, spray applications were made at night (with exception of 2 Lannate LV apps) using a CO 2 pressurized backpack sprayer with three XR8005 nozzles spaced 19 inches apart, with 28 psi delivering 40 gpa and other specifications according to Table 1. Deadline MP (mini pellets), active ingredient metaldehyde 4% was applied at 20 lbs, 40 lbs and as 2 applications of 20 lbs per acre. It was applied by hand between the cabbage rows, taking care not to contaminate the edible portions of the cabbage plants. Lannate LV, active ingredient methomyl 29% was applied at 1.5 pt and 3 pt per acre as a single and a double application with the double application also being applied during the day. Lannate LV at 1.5 pt per acre was also applied with as a double 2

application with and without Induce at 0.25% v/v at night and during the day. Browseban, active ingredient, capsaicin and related capsaicinoids 2%, was applied at 1.28 fl oz per acre as a single and a double application. Cuprofix Ultra 40 Disperss, active ingredient, basic copper sulfate 71.1% was applied at 0.75 lb per acre as a single and a double application. Table 1. Spray conditions for application of treatments to cabbage for control of slugs: field trial, 2011. Time Wind speed (mph) Tempe Relative Spray Date rature humidity Field conditions Start Finish Average Maximum ( F) (%) A day Oct 6 6:30 PM 6:45 PM 0 0 54 75.5 Dry foliage Heavy dew on cabbage A night Oct 7 6:40 AM 7:15 AM 0 0 45 77 leaves, lots of slugs out Cabbage leaves dry, B day Oct 13 5:00 PM 5:15 PM 4.6 8.6 68 81 ground wet Heavy dew, light B night Oct 13 9:30 PM 10:30 PM 3.4 6.3 64 86 rainfall, lots of slugs out Four and 11 days after the first and second spray, respectively, the trial was harvested on October 17, 2011. All except the heads on the ends were harvested (5 6 heads per plot). Per plant, the total number of slugs was counted on both the head, which included two wrapper leaves, and on all of the remaining lower frame leaves. Any dead slugs found on cabbage plants were recorded. Bioassay: Slugs were collected from the field where we conducted our trial. Treatments included baits, Deadline MP and Sluggo, active ingredient, iron sulfate 1%, both at 20 lb per acre; Browseban at 1.28 fl oz per acre, Cuprofix Ultra 40 Disperss at 0.75 lb per acre; and Lannate LV at 3 pt per acre and 1.5 pt per acre with and without Induce at 0.25% v/v. Each treatment was replicated 5 times. Each replicate consisted of a 12 quart plastic Sterilite bin, which were lined with a moist paper towel unto which a cabbage leaf that was free of chemicals was placed. Six slugs ranging in size from small (< 1 cm), medium (1 2 cm) and large (> 2 cm) were placed on the cabbage leaf, which was then spritzed with water and covered with another moist paper towel. The bait treatments were measured out according to the 12.5 x 8 area of the containers and sprinkled into each container by hand under the cabbage leaf. The liquid treatments were applied to the exposed slugs within the containers using the backpack sprayer as described for the field trial. After the treatments were applied, each container was closed with tulle fabric allowing the slugs to breath. The bioassay was set up on October 14, 2011 and evaluated after 18, 42 and 77 hours. The number and size of live, dead and missing slugs were recorded. Missing slugs were not presumed dead or alive. RESULTS Field Trial (Table 2): Slug pressure was very high at our field location with the untreated control having 70.8% (up to 93.3% in one treatment) infested plants and 3.6 slugs per plant (up to 4.1 in one treatment). However, incidence of infested cabbage heads was low with the untreated control having 14% infested heads and 0.23 slugs per cabbage head. It is unknown why the slugs were not found more inside the cabbage heads, like they were in the 2010 field trial. 3

No significant differences occurred among treatments in the incidence of infested whole plants (head + lower frame leaves), and the only treatment that had less than 50% infestation was the single application of the low rate of Deadline MP (43%), representing 39% control. The next best treatments were the double application of Cuprofix 40 Disperss (58.7%) and Browseban (single app 62.2%; double app 66.1%). Although no significant differences occurred among treatments in the incidence of infested cabbage heads or the number of slugs per cabbage head, all treatments had fewer contaminated heads than the untreated control with the single applications of the high rate of Lannate LV, Browseban and the high rate of Deadline MP, and the double application of the low rate of Deadline MP having 100% clean heads. Table 2. Evaluation of selected treatments for controlling slugs in cabbage: field trial, Martin Farms, Brockport, NY, 2011. % infested No. slugs % dead % Lower Head slugs unmarke Treatment Whole Whole (whole table plant 2 Head plant 2 frame (dead + leaves alive) plant 2 ) heads 3 Untreated Control 70.8 14.0 3.4 a e 5 3.4 a e 0.23 1.3 7.8 a Deadline MP 20 lbs AB 9 85.3 0.0 4.1 ab 4.1 abc 0.0 9.0 0.0 e Deadline MP 20 lbs B 43.0 3.3 1.9 ef 1.9 ef 0.04 7.5 4.3 abc Deadline 40 lbs B 72.7 0.0 2.2 c f 2.2 c f 0.0 1.4 3.6 a d Lannate LV 3 pts AB 74.7 4.0 3.5 a d 3.5 a d 0.05 9.9 4.2 a d Lannate LV 1.5 pts AB 75.3 7.3 3.9 abc 3.9 abc 0.07 1.2 3.7 abc Lannate LV 1.5 pts AB day 1 82.0 0.0 3.1 a d 3.1 a d 0.0 6.5 3.9 abc Lannate LV 1.5 pt AB + Induce 0.25% v/v AB 93.3 3.9 3.6 ab 3.6 ab 0.04 17.6 4.4 ab Lannate LV 1.5 pt AB day + Induce 0.25% v/v AB day 86.7 5.0 4.3 a 4.3 a 0.03 9.0 5.7 a Lannate LV 3 pts B 84.0 0.0 2.9 a d 2.9 a e 0.0 0.3 0.0 de Lannate LV 1.5 pts B 84.0 5.0 3.1 a d 3.1 a d 0.05 9.0 1.5 b e Browseban 1.28 fl oz AB 66.1 5.0 2.6 b f 2.6 b f 0.10 7.1 1.3 cde Browseban 1.28 fl oz B 62.2 0.0 1.7 f 1.7 f 0.0 9.1 0.7 cde Cuprofix 40 Disperss 0.75 lb AB 58.7 10.0 1.9 ef 1.9 ef 0.13 0.03 4.3 abc Cuprofix 40 Disperss 0.75 lb B 70.7 6.7 3.1 a d 3.1 a d 0.12 2.4 5.6 a P Value NS 4 NS 0.0012 6 0.0012 6 NS NS 7 0.0006 7 1 sprayed during the afternoon, other treatments sprayed at night. 2 whole plant includes lower frame leaves and head. 3 includes heads contaminated with slugs and slug frass. 4 NS: not significant, Fisher s Protected LSD test, p>0.05. 5 Numbers in a column followed by the same letter are not significantly different, Fisher s Protected LSD test, p<0.05. 6 statistics performed on transformed data, y=log(x+1); non transformed data presented. 7 statistics performed on transformed data, y = arcsin(x/100)^0.5; non transformed data presented. 9 spray dates: A Oct 6 (day) and Oct 7 (night); B Oct 13. The majority of the slugs occurred in the lower frame leaves where significant differences among treatments occurred. The only treatment that had significantly fewer slugs per plant (lower frame leaves only and leaves + head) than the untreated was the single application of Browseban (1.7 slugs per plant). Slug mortality within the plants was observed in all treatments, but no significant differences 4

occurred. Numerically, the double application of the low rate of Lannate LV + Induce had the highest slug mortality (17.6%). Significant differences occurred among treatments for incidence of unmarketable heads, which included contamination with live and dead slugs and slug frass. The double application of the low rate of Deadline MP and the single application of the high rate of Lannate LV had 100% marketable heads. Both the single and double applications of Browseban and the single application of the low rate of Lannate LV also had significantly fewer unmarketable heads than the untreated control. Deadline MP: Two applications of the low rate of Deadline MP resulted in significantly fewer unmarketable heads and numerically fewer heads with slug contamination than the single application, but the plants had significantly twice as many slugs in the lower leaves. No significant differences occurred among single applications of the low and high rate of Deadline MP, although numerically, the high rate had fewer infested and unmarketable heads. Overall, the single applications of the low and high rates of Deadline MP were the second and third best treatments in the trial, representing 35 to 100% control. These results suggest that using the higher rate of Deadline MP and making two applications (1 week apart) are not necessary, especially for the added cost at $50 per acre for 20 lb. Lannate LV: Rate No significant differences occurred among the low and high rate of Lannate LV when applied in single or double applications, although numerically, the high rate had fewer infested heads and number of slugs per head. In the double applications, more dead slugs were found in the high rate, while more dead slugs were found in the low rate in the single applications. There were slightly more unmarketable heads at the high rate in the single applications, but slightly more unmarketable heads at the low rate in the single applications. These data suggest that the high rate of Lannate LV provides improved control (71 to 100% control of infested heads) over the low rate (49 to 64% control), especially when only a single application (i.e. a rescue) is made. An additional 1.5 pts per acre of Lannate costs about $13 per acre. Lannate Single vs. Double Apps There were no significant differences between the single and the double applications of Lannate LV at either rate. Numerically, there was a general trend that the single application had lower incidence of infected heads, number of slugs per plant, per lower leaves and per head, and unmarketable heads, and more dead slugs per plant, except the double application had more dead slugs per plant at the high rate, and the double applications had lower incidence of infested plants at both rates. These data suggest that once a slug infestation has occurred, only a single application is necessary. An additional application of Lannate costs $13 (1.5 pt) to $26 (3 pt) per acre. Lannate Night vs. Day No significant differences occurred between the treatments that were sprayed at night and during the day. Numerically, there were no notable differences or consistent trends, except that Lannate LV had 100% clean heads and 5.4 times more dead slugs per plant when it was applied during the day compared to at night. In the Lannate LV + Induce treatments, slug mortality was 2 times higher when it was sprayed at night and number of slugs per plant and lower frame leaves were also lower. Induce Although not significant, when applied during the day, Lannate LV consistently performed better without Induce. When applied at night, there were no significant differences and numerical differences were slight, except that Lannate LV 1.5 pt + Induce had 1.9 times fewer infested heads and 14.7 times higher slug mortality. Overall, these results suggest that Lannate LV works slightly better during the day than it does at night, which is counterintuitive, because Lannate LV is most likely to come in direct contact with slugs when it is sprayed at night. Our results also show that the addition of Induce 5

to improve the efficacy of Lannate LV to control slugs is only important when it is sprayed at night, a concept that requires further investigation. Browseban: No significant differences occurred between single and double applications of Browseban, but numerically, the single application performed slightly better. Despite our bioassay results showing that Browseban does not kill slugs, our field data shows that it is comparable to Deadline MP for reducing the number of slugs in the plants and in the cabbage heads. We did not observe any phytotoxicity at the 1x rate. Cuprofix Ultra 40 Disperss: There was a general trend that two applications of Cuprofix 40 Disperss was slightly more effective than one with significantly fewer slugs per plant and lower frame leaves. Unfortunately, this treatment had some of the highest incidences of infested and unmarketable heads in the trial. Perhaps, Cuprofix 40 Disperss acts as a repellant and deters slugs from crawling onto the treated leaves. Unfortunately, it does not have any effect on slugs that are protected from the spray inside the heads. Our data suggests that preventative applications of Cuprofix 40 Disperss prior to slug entry into the plant may be more effective. Bioassay (Table 3): Significant differences occurred among treatments after 42 and 77 hours of exposure to the treatments. After 42 and 77 hours, the untreated control had 15% mortality. After 18 hours of exposure, Sluggo had the highest mortality (16.7%) followed by Lannate LV 1.5 pts + Induce (13.3%), and Lannate LV 3 pts (10.1%) Deadline MP (10%). After 42 hours of exposure, Lannate LV 1.5 pt + Induce (76.7%), Lannate LV 3 pt (72.8%), Deadline MP (66.7%) and Sluggo (43.3%) had significantly higher mortality than the untreated. After 77 hours of exposure, all treatments except Browseban and Cuprofix 40 Disperss had significantly higher mortality than the untreated with Lannate LV 3 pt having the highest mortality (87.1%) followed by Lannate LV 1.5 pt + Induce (80%) and Deadline MP (73.3%). This data in combination from the field study suggest that Browseban and Cuprofix Ultra 40 Disperss have slug repellent properties. Table 3. Efficacy of selected treatments for control of slugs: bioassay, 2010. Mortality (%) after Size of dead slugs (%) Treatment and rate (per acre) 18 h 42 h 77 h Small Medium Large Untreated 0.0 15.0 bc 2 15.0 d 20 0 0 Deadline MP 20 lbs 10.0 66.7 a 73.3 ab 59 21 6 Sluggo 20 lbs 16.7 43.3 ab 63.3 ab 53 18 3 Browseban 1.28 fl oz 0.0 3.3 c 3.3 d 20 0 0 Cuprofix 40 Disperss 0.75 lb 0.0 23.3 bc 27.3 cd 53 2.5 0 Lannate LV 3 pt 10.1 72.8 a 87.1 a 38 24 30 Lannate LV 1.5 pt 0.0 30.0 bc 53.3 bc 58 16 2 Lannate LV 1.5 pt + Induce 0.25% v/v 13.3 76.7 a 80.0 ab 66 5 23 P Value (alpha = 0.05) NS 1,3 0.0009 0.0000 NS NS 0.0005 3 1 NS: not significant, Fisher s Protected LSD test, p>0.05. 2 Numbers in a column followed by the same letter are not significant, Fisher s Protected LSD test, p < 0.05. 3 statistics performed on transformed data, y = arcsin(x/100)^0.5; non transformed data presented. 6

No significant difference occurred among the two bait treatments, although numerically Deadline MP had 10% higher mortality after 77 hours. There was a significant difference between the high (3 pt) and low (1.5 pt) rate of Lannate LV with the high rate providing 42.8% and 33.8% higher mortality than the low rate after 42 and 77 hours, respectively. However, when the adjuvant Induce was added to the low rate of Lannate LV, it performed as good as the high rate. These data show that doubling the rate of Lannate LV increased efficacy by 39% and the addition of the adjuvant Induce improved the efficacy of Lannate LV by 33%. It would be interesting to see whether the addition of Induce to the high rate of Lannate LV would further improve control. CONCLUSIONS Deadline MP applied at 20 lb per acre may be used as a rescue treatment after a slug infestation is discovered and provide 50% or greater control of contaminated heads. Our study did not demonstrate that two applications of 20 lb or an application at 40 lb were worth the additional expense. We did not demonstrate whether an application of Deadline MP prior to slug infestation would have made a difference. Based on our bioassay results, Sluggo, which has an organic and greenhouse label, also provided significant control of slugs. Lannate LV provided 49 to 100% control of contaminated heads. Our study showed that a single application of the high rate (3 pt) once a slug infestation occurred was sufficient. The addition of the adjuvant, Induce also appeared to be beneficial. DuPont should pursue adding Lannate to their cabbage label. Browseban and Cuprofix Ultra 40 Disperss appear to have repellency properties against slugs. If they are being applied in cabbage for control of diseases (especially black rot) and to repel deer, even though they are not labeled to control slugs, growers may get some control of slugs. It appears that Cuprofix Ultra 40 Disperss functions best as a preventative treatment before slugs enter into the heads. 7