MANAGEMENT AND CONTROL OF CABBAGE MAGGOT, DELIA RADICUM, IN CAULIFLOWER, III. Oregon Processed Vegetable Commission

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1 MANAGEMENT AND CONTROL OF CABBAGE MAGGOT, DELIA RADICUM, IN CAULIFLOWER, III. Research Report to Oregon Processed Vegetable Commission by James G. Todd, Entomologist Willamette Agricultural Consulting, Inc 7555 Conifer Street, NE Salem, OR November 21, 2000 INTRODUCTION: The cabbage maggot, Delia radicum, is a chronic pest problem in Willamette Valley cauliflower. Current management of this pest relies exclusively on prophylactic chemical applications; usually with Lorshan. Lorsban. Treatments are made after transplanting or seeding to kill larva hatching from eggs laid by flies attracted to to plant odors. Essentially all cauliflower plantings are treated, but control is inconsistent. Some cabbage maggot larva survive treatment in all plantings and occasionally there is severe stand loss and yield reduction in spite of treatment. This report summarizes the third year of research to improve management of cabbage maggot. This study has included investigation of cabbage maggot life cycles, evaluation of treatment practices and evaluation of sampling techniques for making management decisions. During 1998 and 1999, it was demonstrated that registered chemical treatments, Lorsban and Guthion, are often ineffective because of improper application technique. These products only suppressed cabbage maggot if they were applied in narrow bands over plant rows and followed immediately with irrigation to leach chemical down to the root zone. Single applications of Lorsban and Guthion were only effective for a short period, even when properly applied. Two applications, eleven days apart, were required to prevent loss to cabbage maggot in a heavily infested planting. Field studies indicated that cabbage maggot densities are quite low, if plantings are isolated from alternate hosts and are highest if located adjacent to overwintering hosts. Eighty percent of the study sites had less than 2 cabbage maggot/plant in

2 2 untreated plots and had no measurable yield loss. One study site exceeded twelve maggots/plant and had a small but significant yield loss. These fmdings findings provide the basic elements of an integrated pest management strategy for cabbage maggot in cauliflower. We know that cabbage maggot damage may be avoided in some cases by isolating plantings from alternate hosts. We know that maggot densities are variable and often do not reach damaging levels. Thus there are instances when chemical treatment for cabbage maggot might be reduced or eliminated entirely. When damaging populations of cabbage maggot do occur, we know that proper application technique is essential and that multiple treatments may be needed. To make further improvement in cabbage maggot management we need to improve the longevity and efficacy of our chemical treatments, determine more precise treatment thresholds and develop a reliable, economical monitoring technique to make treatment decisions. We sought to achieve these goals in 2000, by studying treatment effects and yield impact at high densities of cabbage maggot and by testing traps for cabbage maggot flies as monitoring tools. METHODS: Two emergence cages (3' x 3') were used to determine when first generation cabbage maggot flies emerged from the pupal stage. In the spring cages were located near Gervais and were placed over decayed cauliflower stems that remained in the spring from a heavily infested 1999 planting. A preliminary test of traps for adult flies was also conducted at this site. Several types of "sticky traps", often painted yellow to attract flying insects have been used in cabbage maggot research (Finch, 1990; Bomford, et. al., 2000). We tested a "Sticky Card", a 9"x11" waxed yellow card coated with Stickem Special (Seabright Enterprises); and a "Sticky Cup", a 16 oz. yellow plastic cup (Solo), coated with Stickem Special. The traps were mounted on stakes approximately 12-18" above the ground and placed near the emergence cages.. Four cauliflower fields (Gervais, Monitor, Mt. Angel and Pratum) were selected for intensive study. These sites were located as close to host plants as possible to increase the chance of a high density of cabbage maggot. Gervais was transplanted to early cauliflower in late June. The remaining sites were transplanted in mid-july. Details of crop history, transplanting and treatment are shown in Tables 1 & 3. Cauliflower was transplanted in 40" rows and plots were 40' (12 rows) x 80' at Gervais, Mt. Angel and Monitor; and 100' (30 rows) x 40' at Pratum. At each of these sites an untreated check was compared to chemical treatment in a arandomized design with 4 replicates for each treatment. Two different traps for cabbage maggot flies were tested at each study site. One wasthe "Sticky Cup" described above; the other was a "Water Pan" trap (Bracken, et. al., 1988; Finch, 1992). Water Pan traps were six quart yellow plastic bowls (Arrow Plastic) or 20 oz. styrofoam dishes spray painted yellow. The 20 oz. dishes were used because of a shortage of the six quart bowls. Water Pan traps were placed on the ground between cauliflower plants and filled nearly to the brim with water and a small amount of liquid detergent. Each type of trap was replicated four times at each site. Traps were placed in the untreated plots at transplanting and examined twice a week. Fresh Sticky Cups were placed at the trap sites on each sample date; and Water Pans were rinsed and refilled on each sample date. Both Sticky Cup and Water Pan traps collect a diverse array of flying insects including aphids, beetles and many species of fly. Most of these can be classified quite easily as "non-

3 3 cabbage maggot". However, there were still many different species of calypterate flies, mostly of the families Anthomyiidae (cabbage maggot family) and Muscidae (house fly family) that are similar to cabbage maggot, Delia radicum. All calypterate flies were counted. It was hypothesized that Delia radicum would be so abundant in cauliflower that other fly species would be an insignificant percentage of the total number of flies recorded. The calypterate flies were counted and discarded from the Sticky Cups because the glue generally damaged flies beyond the point of positive identification. The calypterate flies from the Water Pan traps were counted and preserved in 70% alcohol for possible later positive identification to species. The replicated plots at each site were sampled to estimate cabbage maggot density five to six weeks after transplanting. Soil was excavated in a four inch diameter circle around each plant, the stem and roots were examined and all larva and pupa were counted. Twenty plants were randomly sampled from each plot for a total of eighty plants per treatment. This sampling data was analyzed using analysis of variance and Duncan's Multiple Range Test. The cabbage maggot counts were transformed to square root (x+.5) before analysis. A root damage index (Todd, 1998) was used to evaluate cabbage maggot injury at all sites and visual assessments of stand density and plant growth were made prior to harvest. RESULTS & DISCUSSION: Cabbage maggot flies emerged from overwintering pupae in April, as indicated by emergence cages (Table 2). The time of emergence and the number of flies trapped was similar to trapping results the previous spring (Todd, 1999). The number of flies trapped was quite low (ten flies caught in three weeks), even though traps were placed in a site that had a high density of cabbage maggot (12/plant) the previous summer. Emergence cages can be useful for studies of the cabbage maggot life cycle, but they do not trap enough flies to be used for predicting infestation levels and making treatment decisions. Both Sticky Cards and Sticky Cups caught substantial numbers of calypterate flies during the cabbage maggot fly's spring emergence period (Table 2). The number of flies caught by the two different traps were not significantly different (P<.05). In subsequent tests only the Sticky Cup trap was used because it was easier to handle in the field. Flies caught on both sticky traps were heavily coated with glue. It was difficult and time consuming to remove flies from traps without damaging them; and it would have been difficult or impossible to identify these glue coated flies to species. Therefore the flies on Sticky traps were counted and discarded. Data from these traps is a measure of total calypterate fly activity. It was hypothesized that cabbage maggot would be the primary species caught and that cabbage maggot density would be strongly correlated with calypterate fly numbers. Treatments for cabbage maggot at the four study sites are summarized in Table 3. A single band spray of Lorsban was applied at Monitor and Mt. Angel, two Lorsban band sprays were applied at Gervais and a broadcast spray of Lorsban was applied at Pratum. The check plots at Pratum were inadvertently sprayed along with the treated plots, so it was impossible to test treatment efficacy. However, since broadcast sprays have been ineffective in all previous tests, the check plots at at Pratum Prat= are analyzed as "untreated" along with the check plots from the other three sites. Cauliflower plants in the test plots were sampled for cabbage maggot and evaluated for root damage five to six weeks after transplanting. Sampling data are summarized in Table 4a&b.

4 4 None of the study sites had high densities of cabbage maggot in untreated plots, even though all were relatively close to alternate hosts. There was a very low density of cabbage maggot (0.1/plant) at Mt. Angel and low densities (1-2/plant) at Gervais and Monitor. The Pratum site had a moderate density of cabbage maggot (4.7/plant), but the level of infestation was well below the density that affected productivity in previous trials (Todd, 1999). The Monitor site was treated at transplanting with a band application of Lorsban and followed immediately with irrigation. This early treatment did not produce a significant reduction in cabbage maggot density in our previous study (Todd, 1998); and the treatment at Monitor was not significantly different from the untreated check this season. Gervais and Mt. Angel also had no significant difference (P<.05) between treated and untreated plots, even though they were treated in the manner that has been effective in previous studies (application delayed a week after transplanting, banded over plant rows and irrigated immediately). The cabbage maggot damage index was low to very low at all sites (Table 4b). There was no significant difference between treated and untreated plots and no visible stand loss or stunting in untreated or treated plots. Both Sticky Cup and Water Pan traps caught flies throughout the cauliflower growing season (Table 5). Calypterate flies counts on Sticky Cups ranged from /trap/day; and counts from Water Pans ranged from /trap/day. There was no consistent trend in fly numbers over time with either trap. Counts seemed to vary at random with no sign of a flight activity peak. The number of flies caught during the first three weeks after transplanting was used for more detailed comparisons between trap types and among study sites. Trap catches during this period should be more closely related to damaging maggot infestations than the numbers caught later in the growing season. This early flight period had distinct differences between the two trap types and among the four study sites. The greatest number of calypterate flies caught was 19.3/trap/day in Water Pans at Gervais and the lowest was 0.8/trap/day on Sticky Cups at Pratum. Water Pan traps caught more calypterate flies than Sticky Cups at all sites. The difference was significant (P<.05) at all sites except Mt. Angel (Table 6a). There were also significant differences in calypterate fly catch among the four study sites (Table 6b). Both traps at Gervais caught significantly more flies than at all other sites. Sticky Cups caught the lowest number of flies (0.8/trap/day) at Pratum, while Water Pans caught the lowest number of flies (7.0/trap/day) at Monitor. If these differences are consistently correlated with maggot infestation, calypterate fly traps might be a useful monitoring tool for cabbage maggot treatment decisions. The potential of calypterate fly traps to predict cabbage maggot densities was evaluated with regression analysis. The trapping and cabbage maggot density data are summarized in Table 7, and portrayed graphically in Figures 1 & 2. The regression of cabbage maggot density on calypterate fly catch during the first three weeks after transplanting was not significant (P<.05) with either trap. The R-square value was for Sticky Cups and for Water Pans. This indicates no significant correlation between calypterate fly catch and cabbage maggot density at the densities encountered this season. Some of the flies collected from Water Pans were examined microscopically, but it was not possible to identify Delia radicum reliably. With greater taxonomic expertise flies could be identified to species, and the cabbage maggot counted separately from other calypterate flies. This may be useful in cabbage maggot research; but even if Delia radicum fly catches are closely correlated with maggot density, field identification to species is probably too difficult for Water

5 5 Pans to be a practical monitoring tool. If Water Pans are used in future experiments, improvements in trapping techniques should be made. Water Pans should be checked, cleaned and refilled every two to three days. At longer intervals there is serious damage to specimens from microbial growth. Soap solutions (Finch, 1990) did not prevent this damage; and alcohol, copper sulfate or other products should be used to minimize decay. Trap placement at the edges of fields (vs. within rows) should also be investigated because tillage operations can fill pans with soil. The egg sampling technique used in 1998 and 1999, was reevaluated as a monitoring technique to predict cabbage maggot density because of the poor performance of the adult fly traps. Sampling data for eggs during the first two weeks after transplanting and for larva and pupa five weeks after transplanting are summarized in Table 8, and portrayed graphically in Figure 3. The regression of cabbage maggot density on number of eggs was highly significant (P<.05). The R-square value was 0.973, which indicates that cabbage maggot density is highly dependent on the egg counts just after transplanting. This strong correlation may be partly due to the nature of the data, with a cluster of sites at relatively low densities and only a single site with a high density. Additional data points at moderate and high densities might lower the R-square value; but it seems likely that cabbage maggot density would still be more closely correlated with egg counts than with calypterate fly catch in traps. CONCLUSIONS: Densities of cabbage maggot were relatively low at all study sites this season; and no visible damage to cauliflower occurred. This is similar to results from the previous two years. Twelve trials have now been conducted; only four trials have had cabbage maggot densities exceeding 2/plant; and only one (12 maggots/plant) had measurable losses to cabbage maggot. It seems unlikely that area wide cabbage maggot densities have been abnormally low for three successive years. Instead, it is probable that severe losses to cabbage maggot in cauliflower are a sporadic occurrence that requires a season of unusually high densities or a very close proximity to overwintering sites and alternate hosts. Previous trials have shown that improper application practices and the short residual effectiveness of Lorsban and Guthion cause many control failures. This season's Lorsban trials at Gervais and Mt. Angel were applied with the optimal technique (application delayed about one week after transplanting, spray banded over plant rows and irrigated immediately), but still did not produce significant reductions in cabbage maggot density. This documents once again the marginal effectiveness of available chemical treatments for cabbage maggot. However, 75% of the treatments applied in this manner have been effective and this is the best practice available at present. Multiple sprays of Lorsban or Guthion must be applied for greater insurance against cabbage maggot damage. Reliable prediction of cabbage maggot densities would permit growers to eliminate unnecessary sprays when numbers are low; and it would help minimize damage by warning of the need for multiple treatments when densities are high. Development of a sampling method that would achieve these goals is difficult. Sticky traps and Water Pan traps have been used in research projects, but specialists are needed to identify flies accurately. Soil sampling for eggs has also been used (Mukerji & Harcourt, 1970). This technique does not require taxonomic expertise, but it is quite time consuming. A technique is needed that is fast, reliable and readily

6 6 accomplished by growers and field professionals. The trapping for calypterate flies tested this season is reasonably fast; and it is fairly easy to learn to identify this group of flies (which includes the cabbage maggot). Unfortunately, neither the Sticky Cup nor the Water Pan count of calypterate flies was correlated with cabbage maggot infestation levels at the low densities encountered this year. There might be a better correlation of trap catches with cabbage maggot infestations at high densities, but that remains to be demonstrated. The reevaluation of egg sampling data indicates that sampling for eggs may be a reliable predictor of cabbage maggot infestations. It is relatively easy to learn to identify cabbage maggot eggs; but this technique requires large numbers of samples and a great deal of time for accuracy. The time required for egg sampling could be minimized by sequential sampling, a statistical model that reduces the number of samples required at low and high densities (Onsager, 1976). Data collected in 1998 and 1999, provides some of the information needed for a sequential sampling plan; but it would take additional sampling at a range of cabbage maggot densities to develop and validate a model. It is uncertain whether sequential sampling would be fast enough to be economical, but it may be the best hope we have for a reliable cabbage maggot monitoring technique. RESEARCH PRIORITIES: An attempt should be made to develop a sequential sampling plan based on egg counts to predict damaging cabbage maggot infestations. Additional field trials would still be beneficial to refme refine an economic threshold and clarify our understanding of effective treatment practices. The essential requirement for any such trial is a location with a high density of cabbage maggot so that treatments and economic injury can be assessed under extreme conditions. If sites with a strong probability of high cabbage maggot densities are found, it might also be beneficial to evaluate fly traps again. ACKNOWLEDGEMENTS: Tom Buchholz, Inc., Dickman Farms, Five Oak Farms and Scott Miller, Inc. offered use of their cauliflower fields for this study. They took the risk of leaving untreated check plots and made the extra effort needed to apply different treatments to to small plots. This study would have been impossible without their generous assistance and cooperation. Jonnetti Bernard, Curtis Cheney, Angela Colton and Kale Haggard spent many hours digging, dissecting and counting. This study would not have been completed without their perseverance.

7 7 LITERATURE CITED: Bomford, M. K., R. S. Vernon & P. Pats Importance of collection overhangs on the efficacy of exclusion fences for managing cabbage flies (Diptera: Anthomyiidae). Envi. Entomol. 29: Bracken, G.K Seasonal occurrence and infestation potential of cabbage maggot, Delia radicum (L.) (Diptera: Anthomyiidae), attacking rutabaga in Manitoba as determined by captures of females in water traps. Can. Entomol. 120: Finch, S The effectiveness of traps used currently for monitoring populations of the cabbage root fly (Delia radicum). Ann. appl. Biol. 116: Finch, S Improving the selectivity of water traps for monitoring populations of the cabbage root fly. Ann. appl. Biol. 120: 1-7. Mukerji, M. K., & D. G. Harcourt Design of a sampling plan for studies of the population dynamics of the cabbage maggot, Hvlemya brassicae. Can. Entomol. 102: Onsager, J. A The rationale of sequential sampling with emphasis on its use in pest management. USDA Tech. Bull pp. Todd, J. G Management and control of cabbage maggot, Delia brassicae, in cauliflower. Report to Oregon Processed Vegetable Commission. 11pp. 'pp. Todd, J. G Management and control of cabbage maggot, Delia brassicae, in cauliflower, II. Report to Oregon Processed Vegetable Commission. 16pp.

8 8 Table 1. Study sites for cabbage maggot management and control, Proximity to Predicted Sum '99 Spr '00 Cabbage Maggot Cabbage Maggot Site Crop Crop host plants Risk Rating Gervais Grass Peas <1/4 mi. to overwintering cabbage seed field Monitor Strawberry Strawberry <1/4 mi. to '99 cauliflower, weed hosts in area Mt Angel Grass Peas 1/2 mi. to '99 cauliflower, weed hosts in area High High Moderate Pratum Grass Fallow <300' to '99 cauliflower High Table 2. Spring emergence trapping for cabbage maggot; Gervais, No. Flies / Trap Cabbage Maggot Fly Calypterate Flies Date Emergence Cage Sticky Cup Sticky Card 4/4/00 start start start 4/14/00 1.5(2.1) 34.4 (3.3)* 35.6(10.5) 4/25/00 3.5(3.5) 117.0(79.7) 82.6(47.3) *mean(st dev), n=5.

9 Table 3. Transplant and treatment information for cabbage maggot study sites, Site Transplant Date Rate Application Chemical (ai/ac) Method Spray Volume 1st Time to 2nd Time to (gal/ac) Spray Irrigation Spray Irrigation Gervais 6/30/00 Lorsban 4E 1 lb 12" Band 80 7/3 Monitor 7/17/00 Lorsban 50W 1 lb 10" Band Jul Mt Angel 7/18/00 Lorsban 50W 1 lb 10" Band 40 7/27 +Guthion 50W.75 lb <2 hr 7/17 <2 hr <1 hr - <2 hr - - Pratum 7/20/00 Lorsban 4E 1 lb Broadcast 40 7/24 >24 hr - - 9

10 10 Table 4a. Control of cabbage maggot with post-transplant applications of Lorsban. No. Larva & Pupa / I Plant* Treatment*** Untreated Treated w/lorsban Site: Gervais Monitor Mt Angel Pratum 1.6 a** 1.3 a 0.1 a a 2.0 a 0.1 a Table 4b. Cabbage maggot damage to cauliflower, Cabbage Maggot Damage Index Treatment*** Untreated Treated w/lorsban Site: Gervais Monitor Mt Angel Pratum 0.74 a** 0.79 a 0.16 a a 0.91 a 0.09 a *n=80. **Densities in columns followed by same letter are not significantly different (P<0.05). ***See Table 3 for treatment detail. ****0=no ****Ono tunnels; 1=surface tunnels, no root loss; 2=deep tunnels, no root loss; 3=deep tunnels, minor root loss; 4=deep tunnels, severe root loss.

11 11 Table 5. Summary of trapping for calypterate flies in cauliflower, Mean No. Flies/Trap/Day (n=4) A. Sticky Cup Traps B. Water Pan Traps SITE: Gervais Monitor Mt Angel Pratum Gervais Monitor Mt Angel Pratum DATE 7-Jul Jul Jul Jul Jul Jul Jul Jul Jul Aug Aug Aug Aug Aug Aug Aug Aug Aug

12 12 Table 6a. Comparison of calypterate fly catch in cauliflower with two trap types. No. Calypterate Flies/Trap/Day Trap Site: Gervais Monitor Mt Angel Pratum Sticky Cup 12.5 a* 1.3 a 7.6 a 0.8 a Water Pan 19.3 b 7.0 b 8.4 a 11.2 b *Numbers within columns followed by same letter not significantly different (P<0.05). Table 6b. Comparison of calypterate fly catch among four cauliflower fields. No. Calypterate Flies/Trap/Day Site Sticky Cup Water Pan Gervais 12.5 c* 19.3 c* Monitor 1.3 a 7.0 a Mt Angel 7.6 b 8.4 a Pratum 0.8 a 11.2 b *Numbers in columns followed by the same letter not significantly different (P<0.05). Table 7. Comparison of cabbage maggot density to trap catches of calypterate flies, No. Calypterate Flies/Trap/Day No. Larva & Pupa Site Sticky Cup Water Pan /Plant Gervais Monitor Mt Angel Pratum

13 13 Table 8. Relationship of cabbage maggot infestation to egg density, 1998 & No. Egg & Shell No. Larva & Pupa Site Year /5 Plants /5 Plants Monitor Mt Angel S. Falls N. Howell Brooks Brooks Gervais Mt Angel

14 Figure 1. Relation of Cabbage Maggot Density to Calypterate Fly Catch in Sticky Cup Traps CD CD i ci z ra2= I No. Calypterate Flies/Trap/Day 14

15 Figure 2. Relation of Cabbage Maggot Denisty to Calypterate Fly Catch in Water Pan Traps ,, c 3.5 as 02=0.028 ra2): cc _I No. Calypterate Flies/Trap/Day 15

16 Figure 3. Relationship of Cabbage Maggot Density to Egg Density Estimates I! 4 (t1 c co ii: 13: in -... U) a. = c o_ ea ca 2 ( J dci Zz C No. Egg & Shell / 5 Plants 16