Optimizing management of a new invasive species, swede midge, on small-scale organic farms: Part II

Objective 1 i-iv. Advance understanding of swede midge (SM) population dynamics on small-scale organic brassica farms as related to management practices.

Objective 1. i. Definitive end date of emergence of the overwintering generation.

At Quest Produce and Blue Heron, we set up three small SM emergence cages per field following a 2015 fall planting of SM-infested brassicas (Table 2 & 4). At Muddy Fingers, we set up a single small cage in each of two fields where SM-infested brassicas were grown the previous fall (Table 4). Unfortunately, at Canticle, all of our small cages had to be removed, and we were unable to obtain any usable data. In the paired open-air traps, first SM were captured on May 20 at Quest (1 of 3 traps), May 24 at Blue Heron (all three fields) and on May 25 at Muddy Fingers (both fields) (Figs 14, 15 & 16) Fig. 14 Fig. 15 Fig. 16. This was about a week later than we observed in 2015. At Blue Heron and Muddy Fingers, we observed a bimodal spring emergence peak in late-May and mid- to late-June (Fig. 15 & 16). At Quest, SM emergence occurred in a single peak from late-June to mid-July (Fig. 14). Trap catches inside small cages were low compared to the open-air traps due to the small area (4 ft²) to draw from. Although first trap catches were later in the small cages than in the open air, fluctuations in trap catches were similar. Inside the cages, first SM were captured on May 31 at Blue Heron (Fig. 15), Jun 6 at Muddy Fingers (Fig. 16) and Jun 10 at Quest (Fig. 14). The last SM small cage captures were on Jul 2 at Quest (Fig. 14), Jul 21 at Muddy Fingers (Fig. 16) and Aug 5 at Blue Heron (Fig. 15). Although we know that the majority of SM emergence occurs during May and June, these results confirm that a small proportion of SM will continue to emerge from an overwintering site throughout July and into early August.

 Objective 1. ii. Intensity and duration of SM emergence in the spring at a site where an infested crop was grown the previous summer.

At Blue Heron, we set up three small cages each in two locations in Field No. 4 that followed a summer planting of SM-infested crop (Table 2). Unfortunately, our traps at Canticle had to be removed and we did not get usable data. In open-air, the first SM trap catch in the field following a 2015 SM-infested summer crop occurred at the same time as the first catch following a SM-infested fall planting (Fig. 15). Interestingly, at both locations, only a single peak of emergence occurred in late-June following a SM-infested summer planting, which was about two weeks after the first peak emergence following a SM-infested fall plantings. Although first trap catches were slightly later in the small cages than in open-air, fluctuations in trap catches were similar. Inside cages, the last SM were captured on Jul 21 and Aug 5, which was the same as the spring emergence sites following SM-infested fall crops (Fig. 15). We hypothesized that spring emergence following a SM-infested summer planting would be much lower than following a SM-infested fall planting, because we thought that in the summer SM would emerge and leave the site in search of another suitable host, whereas in a fall planting, most SM would drop to the soil to pupate and overwinter at the same site. These results indicate that SM emergence in the spring may be equally as strong following both SM-infested summer and fall plantings, which must be considered when making crop rotation decisions. Since our data was only collected from two fields, it would be worthwhile to continue to study this phenomenon on different farms under different pest pressures and different post-harvest practices.    

 Objective 1. iii. Annual SM population dynamics per farm.

Blue Heron. SM pressure generally increased at Blue Heron from 2015 to 2016, primarily due to lack of crop rotation, use of SM-infested transplants and lack of post-harvest crop destruct. In field No. 1 in 2015, SM populations were moderate-high with peak SM trap catch of 16/trap/day on Sep 15 and 25% infestation in red cabbage (data not shown). In 2016 in field No. 1, spring SM emergence was moderate (Table 6) Table 6with a bimodal peak (1^st: 5.2 SM/trap/day on May 31; 2^nd: 5 SM/trap/day on Jun 22) followed by two consecutive weeks of no trap catches until Jul 12 when the trap was taken down (Fig. 17 top) Fig. 17. SM populations built slowly and a summer planting of broccoli was harvested with only minor SM damage (data not shown). Unfortunately, the crop was not destructed post-harvest and the SM population eventually built up in late September and remained high (Table 6) until late-October (peak: 38 SM/trap/day on Sep 22) (Fig. 17 top). After main harvest and leaving plants intact, the SM population increased almost 10-fold in just three weeks from 4 SM/trap/day on Aug 26 to 38.3 SM/trap/day on Sep 22. This is an example of how SM population can build after harvest in absence of post-harvest crop destruct. It is expected that there will be a large spring emergence of overwintering SM at this site in spring 2017.

In field No. 4, in 2015, brassicas were planted from early May until late July with brassicas remaining in the ground when the last traps were taken down on Nov 11. Peak trap catches in fall included 72.2, 43.7 and 39 SM/trap/day on Aug 26 in fall Romanesco/cauliflower (73% infested, 40% unmarketable), on Aug 26 in fall cauliflower (80% infested, 37% unmarketable) and on Oct 14 in fall broccoli (97% infested, 23% unmarketable), respectively (data not shown). In 2016, spring SM emergence was moderate-high (Table 6) following 2015 fall SM-infested site with a bimodal peak (1^st: 17.7 SM/trap/day on May 31; 2^nd: 9 SM/trap/day on Jun 28). Spring SM emergence from 2015 summer planting sites were high (35.1 SM/trap/day on Jun 22) and moderate-high (12 SM/trap/day on Jun 21) and had only a single peak (Fig. 17 bottom). Despite our recommendations, the growers implemented their usual practices and planted green cabbage in early June, which was followed by a summer planting of red cabbage in mid-June. SM trap catches were high (Table 6) in green cabbage (peaks: 31 SM/trap/day on Jun 22 and Jul 27) and very high in red cabbage (peaks: 67.9 SM/trap/day on Aug 17) (Fig. 17 bottom). The red cabbage had 48% infested plants and 3% unmarketable heads (Table 7) Table 7. Fall plantings of cauliflower/Romanesco/red & green cabbage and broccoli had 55% infestation/33% unmarketable heads and 95% infestation/55% unmarketable heads in Romanesco and broccoli, respectively (Table 7). Essentially, SM population consistently doubled between the peaks of spring emergence, to peak of spring cabbage planting, to peak of summer cabbage planting and to the peak of fall broccoli planting for a total 8-fold increase across the growing season (Fig. 17 bottom). This is an example of how SM population builds in the absence of far and wide crop rotation and timely post-harvest crop destruct. SM pressures in the fall were similar between 2015 and 2016 in this field. We expected that this field would have very high emergence of SM in spring 2017.

In 2015, field No. 6 had a moderate level of SM (Table 6) in fall planting of Romanesco (10 SM/trap/day on Oct 14) (data not shown). It was expected that this field would have the lowest SM emergence in the spring of the three fields at Blue Heron. In fact, it had higher SM spring emergence in 2016 than field No. 1, but was similar to field 4 and also had a bimodal peak (1^st: 31 SM/trap/day on May 31; 2^nd: 2 SM/trap/day on Jun 21) (Fig. 18 top) Fig. 18. This field was planted to fall cabbage after the majority of SM spring emergence was complete and the fall cabbage suffered no losses from SM (data not shown). Unfortunately, the fall transplants were infested with SM and the SM population increased to moderate-high levels (peak: 17.4 SM/trap/day on Oct 6) (Fig. 18 top). This is an example of how good crop rotation may be defeated when SM is introduced on infested transplants. Spring SM emergence was expected to be lowest in field No. 6 compared to field Nos. 1 and 4 in 2017.

In 2015, the odd SM was captured in the high tunnels at Blue Heron (<1 SM/trap/day), while much higher levels (peak: 7 SM/trap/day on Jul 8) were captured in the open air transplant hardening off area outside of the high tunnels (data not shown). In 2016, SM were captured in the hardening off area with similar peak as the previous year (9 SM/trap/day on Jun 22) and generally less than 2 SM/trap per day from May 24 to Oct 13 (Fig. 18 bottom). We only monitored tunnel No. 2, which had open sides and kale and Asian brassicas planted in the ground. Here, SM pressure was moderate-high (Table 6) throughout June and July (peaks: 16.6 SM/trap/day on Jun 22; 13.8 SM/trap/day on Jul 7; 10.3 SM/trap/day on Jul 23) despite brassicas being removed and replaced with peppers in early July (Fig. 18 bottom). SM damage was observed in brassica plantings in the high tunnels and transplant hardening off area (data not shown). Apparently, at one time the grower had transplanted kale plants from the field into the high tunnel. The field-grown kale must have been infested with SM, as this could explain the SM population in the high tunnel. The constant SM catches in the open-air trap in the hardening off area also indicated a constant presence of SM in this area of the farm, despite it being about 500 ft away from the brassica production fields (Fig. 4 top). SM could also enter into the open sided high tunnels from the outside. Finding SM-infested transplants in the hardening off area implies that the newly planted transplants in field No. 6 exhibiting SM damage were most likely infested during transplant production instead of in the field.

 Canticle. Generally, it appeared that the SM population at Canticle increased slightly from 2015 to 2016, primarily due to a small land base and constant production of brassica production that make crop rotation ineffective. In 2015, in the Back field, a fall planting of broccoli suffered 92% infestation and 69% unmarketable heads in fall broccoli where SM pressure was high (peaks 20.2 SM/trap/day on Aug 28 and Sep 10). Similarly, Red Russian kale had 75% SM infestation in a summer planting in the Middle field under high pressure (40.4 SM/trap/day on Aug 28 and 30 SM/trap/day on Sep 10). In the Front field, Red and white Russian kale had 82% and 75% SM infestation, respectively under moderate-high SM pressure (peak: 13.4 SM/trap/day on Sep 17) (data not shown).

In 2016, interestingly, SM emergence was lower than expected in the Middle field following 2015 summer mixed brassicas (peak: 4.1 SM/trap/day on Jul 7) (Fig. 19) Fig. 19. Trap catches were generally slow to increase, until in the Front field SM pressure climbed to high levels (Table 6) in a spring planting of kohlrabi (peaks: 21.1 SM/trap/day on Jul 2 and 35.8 SM/trap/day on Jul 15), which had 60% SM-infested and 23% unmarketable bulbs (Table 7). The high SM population (peaks: 30 SM/trap/day on Aug 17 and 73 SM/trap/day on Sep 16) moved to the Front Corner field where it caused 97%, 63% and 46% SM-infestation in broccoli, kohlrabi and cauliflower, respectively and 53% of the broccoli was unmarketable (Fig. 19, Table 8) Table 8. In the Back field, the fall brassica plantings suffered from 40%, 18% and 13% unmarketable crop loss in broccoli, cauliflower and turnip, respectively (Table 8) despite low trap catches (3.6 SM/trap/day on Sep 2) (Fig 19). In the Middle field, under moderate pressure (peak: 5.8 SM/trap/day on Oct 12), a fall planting of mixed leafy brassicas had 15 to 60% SM-infestation (Table 8). In the absence of a break from brassica production on this small farm, SM population at least doubled from early July until mid-September (Fig. 19) and highest trap catches in September were almost doubled from the previous year. It is expected that highest levels of SM emergence in spring of 2017 will occur in the front corner field, followed by the back field, while the middle field and the field behind the barn will have lowest. Planting susceptible brassicas like broccoli, kohlrabi and Red Russian kale should be avoided in the front corner field in spring 2017.

Objective 1. iv. Differences in SM damage among crop types.

Of the five plant types in three plantings at Blue Heron and 15 crop types in four plantings at Canticle that we evaluated, the highest proportion of unmarketable produce at harvest was broccoli. In a fall planting under very high SM pressure at Blue Heron (Field No. 4) broccoli had 55% unmarketable heads, which was numerically 35% more than cauliflower (Table 7). At Canticle, in a summer planting under high SM pressure (Front Corner), broccoli had 53% unmarketable heads, which was significantly more than kohlrabi (11%), cauliflower (0%), red cabbage (3.3%) and green cabbage (0%). Also, in a fall planting under moderate SM pressure (Back Field), broccoli had 40% unmarketable heads, which was significantly more than cauliflower (17.5%) and turnips (12.5%) (Table 8). At Blue Heron, in a fall planting under very high SM pressure, Romanesco had significantly more unmarketable heads (33%) than cauliflower (0%), while both had high levels of SM infestation (Romanesco – 77.5%; cauliflower – 57.5%). In these plantings, red cabbage and green cabbage had 0-3% (n=3) and 0% (n=2) unmarketable heads (Table 7). Although we never observed significant differences between red and green cabbage, numerically red cabbage tended to have higher SM infestation and damage ratings (Table 7 & 8). At Canticle, green kohlrabi had significantly higher SM infestation and damage ratings than red kohlrabi, although they had the same proportion of unmarketable heads (24%) (Table 8). At Canticle, kale and collards did not result in unmarketable produce under moderate SM pressure. In this planting, collards (60%) and Red Russian kale (45%) had significantly higher SM infestation than purple kale (15%), green kale (17.5%) and Bok Choy (2.5%), while Napa cabbage and Lacinato kale were free of SM (Table 8).

In general, highest levels of SM damage occurred in broccoli, followed by Romanesco, and then by kohlrabi (especially green), and cauliflower. Our studies indicate that these crops are not only preferred by SM, but also cannot withstand as much SM damage, as any scarring in the head renders the produce unmarketable. Alternatively, although we tend to observe high SM infestation in Red Russian kale, because the leaves are marketed and the plants have many leaves, this crop can withstand much more SM damage before a single plant is deemed unmarketable. After broccoli, Romanesco, Kohlrabi and cauliflower, purple/Dinosaur kale and cabbage appear to be most preferred by SM. Although in 2015 we observed high levels of SM infestation and unmarketable heads in cabbage, we have observed that this crop is quite tolerant to SM when SM occurs after head formation. Turnips and green (Winterbor) kale are even less preferred and impacted by SM, while Asian brassicas (Bok Choy, Napa cabbage) are rarely attacked at all. These results are similar to our 2015 findings. Whether SM populations build to a greater degree in their more preferred brassica plant types warrants further investigation. Understanding SM plant damage is a valuable tool for growers to make crop management decisions, especially with respect to crop rotation and selection.

 Objective 2. Optimize management strategies including newly developed disruption tactics, insect exclusion netting (IEN) and garlic oil repellent, crop selection and rotation strategies.

 Objective 2. i. Evaluation of IEN compared to open air with bare ground and black plastic mulch.

Summer Broccoli Mulch/IEN trial (Canticle). In this trial, Insect Exclusion Netting (IEN) resulted in zero SM damage or 100% control compared to the broccoli that was grown in open-air, which had 97% and 100% SM-infested plants with 50% and 70% unmarketable heads on black plastic mulch and bare ground, respectively, with minor to moderate severity of SM damage (Table 9) Table 9. The open-air traps caught a total of 330 and 358 SM per 8-week trial with maximum trap catches of 97 and 102 SM per week (Table 10) Table 10. This level of SM pressure and damage is considered to be moderate-high (Table 6). In the bare ground + IEN replications, a total of 22 and 28 SM were captured inside the netting, which likely made their way into the netting when the access sleeves fell off (Table 10). Fortunately, these low levels of SM did not result in SM damage in the broccoli inside the netting. In addition to protecting the broccoli from SM, IEN also significantly reduced flea beetle damage from severe in the broccoli grown in the open air to none in the broccoli grown under the netting (Table 9, Fig. 20) Fig. 20. IEN increased marketable broccoli yield by an estimated 68 lb to 92 lb and crop value by $204 to $276 per 100-ft of bed in just SM protection alone, which is 2.3 to 4.3 times higher than broccoli grown in open-air (Table 9).

Although there were no significant differences in incidence of heat stress among treatments, heat stress ranged from 0 to 46.7% (Table 11) Table 11 Canticle mulch IEN trial maturity. The highest level of heat stress occurred in the black plastic open-air treatment, which was 14-times higher than black plastic + IEN, which suggests that IEN can greatly reduce heat stress of broccoli grown on black plastic mulch in the heat of summer. In the bare ground treatments, heat stress was numerically higher (23.3%) with IEN than without (0%). Plant maturity as indicated by plant maturity rating, was most advanced in bare ground + IEN, which was not significantly different than black plastic + IEN, which were both significantly more advanced than black plastic open-air, while bare ground + IEN was significantly the least advanced in maturity than all other treatments (Table 11, Fig. 20). With both bare ground and black plastic, but especially with black plastic, broccoli grown under IEN matured significantly faster than broccoli in open-air. Eighty percent of the plants were not ready on Sep 2 in bare ground open-air treatment, which was significantly higher than all other treatments (6.7%, 10%, 20%). Plants were also significantly 3.7 and 6.5-in taller under IEN in both bare ground (OA: 19.7-in; IEN: 23.4-in) and black plastic (OA: 20.6-in; IEN: 26.8-in), respectively (Table 11, Fig. 21). Fig. 21

The greatest differences among treatments occurred in maximum weekly temperatures, which differed by as much as 5 to 16 ⁰F, while minimum and average weekly temperatures differed by 0 to 3 ⁰F, and 1 to 6 ⁰F, respectively (Fig. 22 and 23) Fig. 22 & 23. Most notably, for the first four weeks of the trial (Jul 7 to Jul 28), black plastic + IEN had the hottest maximum temperature, while black plastic open-air was the coolest. Bare ground treatments were between the plastic extremes with open-air being hotter than bare ground + IEN (Fig. 22 top). From Aug 4 to Aug 11, bare ground open air was the hottest treatment in the trial, while bare ground + IEN was the coolest. During the last two weeks of the trial (Aug 18 to Sep 1), both bare ground treatments had cooler maximum temperatures than black plastic. Black plastic + IEN consistently had higher maximum temperatures than black plastic open-air, while bare ground open air consistently had higher maximum temperatures than bare ground + IEN (Fig. 22 top). These bare ground temperature results are similar to our observations in 2015, where bare ground open-air was hotter than bare ground + IEN from Jul 22 to Aug 26. Likely, with bare ground, the cooler temperatures inside the netting significantly advanced crop maturity compared to open-air, because broccoli does not grow well when temperatures exceed 80 ⁰F. When used with black plastic, it appeared that the netting trapped heat inside, but surprisingly the increased maximum temperatures did not delay crop maturity (Table 11). Perhaps, the added protection from flea beetles and worm pests provided by the netting reduced plant stress from pest attack, which may explain why plant height and maturity was improved when broccoli was grown with IEN (Table 11).  

 Economic analysis. In our 2015 study (NESARE Partnership ONE15-237), we estimated that IEN cost $236 per 100-ft bed on bare ground ($175.61 for IEN + $60 for setting up netting and hand weeding), and $205 per 100-ft bed with mulch (no hand weeding expense). These costs do not include initial investment in hoops, stakes and clamps, which are reusable. With no SM damage, the broccoli crop was valued at $360 per 100-ft bed (2 rows/3 ft), which would result in $124 and $155 net profit with IEN + bare ground and IEN + plastic mulch, respectively (Table 9). Interestingly, when the cost of IEN was factored in, there was no difference in net return between 0% SM crop loss with IEN and 50% SM crop loss without IEN on plastic mulch. When crop loss was 70% without IEN on bare ground, net profit was $40 (1.5-times) higher with IEN on bare ground. When we use average price of organic broccoli according to USDA ($2.32/lb) instead of $3/lb that Canticle gets, 100% SM control with IEN would result in $278 per 100-ft bed in our study, minus $205 in cost of IEN (with mulch) for a net profit of only $73 per 100-ft bed. This is less than half of the profit of absorbing 50% crop loss without netting. These results clearly demonstrate that in order for IEN to be economical, that both the potential loss due to SM and price earned for broccoli, need to be high (e.g. > 50% crop loss, at least $3/lb). However, use of IEN can also provide value via 100% control of flea beetle, a healthy beautiful crop to sell (instead of losing customers when crop is short or nonexistent due to SM damage), advanced maturity and reduced heat stress (at least with black plastic) . Also, economic feasibility of IEN will increase as it is used multiple times. For example, IEN used for broccoli may be re-used over low hoops in kohlrabi.    

 Objective 2. ii. Effect of white plastic and reflective silver mulch, and green IEN on broccoli quality.

Fall Broccoli Mulch/IEN trial (Muddy Fingers). This trial was designed to investigate whether different insect exclusion netting (IEN) and mulch combinations would alleviate heat stress when broccoli was planted during the heat of July. No SM damage occurred in any of the IEN treatments resulting in 100% control (data not shown) despite a few SM being captured inside the netting (Table 12) Table 12 and 13. This was likely a result of the unsecured sides being lifted when the weeds at the edges of the beds grew large (Fig. 11). There were no significant differences among treatments for incidence of heat stress, although numerically white plastic + white IEN had the highest (46.4%), followed by silver mulch + white IEN (22.3%), while the hay mulch treatments had the least with no difference between white (13.7%) and green IEN (13.3%) (Table 13). Significant differences occurred among treatments in maturity for the first cut and the 2-3-inch head categories (Table 13). Hay + green IEN had significantly the most delayed maturity compared to all the other treatments with only 6.6% heads in the first cut category and numerically the highest proportion of heads in the 2-3 inch head (27.6%) and ≤ 1 inch head (18.1%) categories. White and silver plastic mulches with white IEN had relatively similar maturity, which was slightly behind hay mulch + white IEN. Hay + white IEN had significantly the tallest plants (28.3 inch), which were significantly ~ 3 inch taller than all other treatments. White plastic mulch + white IEN had the shortest plants (24.2 inch), which were not significantly different than silver plastic mulch + white IEN (Table 13.)

It was our hypothesis that all treatments would have resulted in cooler temperatures and reduced heat stress compared to hay + white IEN. The greatest differences among treatments occurred in maximum weekly temperatures, which differed by as much as 6 to 10 ⁰F, while minimum and average weekly temperatures differed by 1 to 8 ⁰F, and 1 to 4 ⁰F, respectively (Fig. 24 and 25) Fig. 24 & 25. Silver plastic mulch + white IEN had the highest maximum temperature throughout the trial and the highest minimum and average temperature until Sep 2, after which white plastic + white IEN had the highest minimum and average temperatures (Fig. 24 and 25). Perhaps, these higher temperatures are the reason why white plastic mulch + white IEN and silver plastic mulch + white IEN had higher incidences of heat stress than the straw treatments. In 8 out of the 10 weeks, hay + green IEN had a lower weekly average temperature than hay + white IEN by ~1 to 2 ⁰F, while their average weekly minimum temperatures were similar and, the average weekly maximum temperatures toggled back and forth as to which was higher. Our results support the manufacturer’s claims that green netting keeps temperatures cooler. Perhaps, silver plastic mulch resulted in the highest temperatures because heat was reflected back and trapped in the netting. It is unknown why minimum temperatures in white plastic + white IEN increased compared to all other treatments after minimum temperatures dropped below 57 ⁰F (Fig. 24). The only treatment that significantly affected crop maturity was green IEN, which significantly delayed crop maturity. White and silver mulch + white IEN did not result in lower temperatures or reduced heat stress compared to hay mulch + white IEN in this study. However, determining the effect of these treatments on yield is warranted. The cooler temperatures of green IEN may result in a significant reduction in heat stress and consequent yield increase in a summer planting trial when broccoli is harvested in August, a timing that is usually prohibitive due to heat stress. Since black plastic is the most commonly used mulch type, trialing it in combination with green IEN would also be worthwhile. We observed that the green IEN was definitely more durable than white IEN, and could likely be re-used several more times than white IEN could be. Since green IEN ($187/100-ft) is similar in price to white IEN ($175/100-ft), it may actually be more economically feasible than white IEN if it can be re-used more often.    

Objective 2. iii. Evaluation of garlic oil with spreader-sticker as repellent of SM (Table 14) Table 14.

In 2015, we did not observe any efficacy with garlic oil as a repellant of SM. In 2016, we tested it with the addition of spreader sticker Nu-film P in hopes that the adjuvant would reduce plant runoff and improve the longevity of the essential oil on the plants. In the two replicated trials, there were no significant differences between untreated plots and those treated with 1% essential garlic oil + Nufilm-P 0.25% v/v for % SM incidence, % unmarketable plants, and average SM damage rating. The untreated controls averaged 90%, 46% and 45% SM incidence in the spring (Red Russian kale), summer (kohlrabi) and fall (Red Russian kale) planted trials where SM pressure was moderate, high and moderate, respectively (Table 6). SM incidence was reduced by almost half from 90% in the untreated to only 40% with garlic oil + Nu-film-P in the spring Red Russian kale trial, which was not replicated. SM was numerically lower in the garlic oil treatments than the untreated for average SM damage per plot and for average SM damage per infested plants in the summer and fall trials, and for % unmarketable plants in the summer trial. Despite promising results in laboratory trials with garlic essential oil as a repellant of SM, it unfortunately did not work in any of our field trials, even with the addition of a spreader-sticker. However, this strategy may be worth trialing again, because there were some weeks when the technicians were either sloppy making their garlic oil applications, or forgot to treat the garlic oil trials altogether. And, a fresh batch of garlic oil mixture was only prepared when the premixed supply ran out, instead of before each application.

 Objective 2. iv. Crop preference/trap crop study (Table 15) Table 15.

We set up a trial in open air to determine whether a more preferred by SM brassica crop than broccoli could be used as trap crop to protect the broccoli. We compared more and less SM-preferred crops in combination to each in monoculture. Significant differences occurred among treatments for % SM incidence, % unmarketable and mean SM damage rating per plant. Incidence of SM infestation was significantly higher in Red Russian kale than in any other plant type with 94% in monoculture, 95% when grown beside broccoli and 86% when grown beside Bok Choy. Bok Choy had zero SM infestation in monoculture and when grown beside broccoli and Red Russian kale. With shorter days to maturity than broccoli, it was possible that by the time the SM population began to build at this site that Bok Choy had already began head formation, after which time brassicas are generally no longer preferred by SM, because the meristematic tissue where they lay their eggs is protected deep within the developing head. However, Bok Choy in rep 3 & 4, which was planted on Jun 3, one month later than reps 1 & 2, was exposed to moderately-high SM pressure (Fig. 26, Table 6) Fig. 26 and still did not sustain any SM damage.

Broccoli monoculture had 60% SM infestation, which was significantly reduced to about half to 34% when grown beside Red Russian kale. When it was grown beside Bok Choy it was numerically reduced to 41%. Red Russian kale grown beside broccoli had the highest % unmarketable plants (66%), which was not significantly different than Red Russian kale grown beside Bok Choy (58%) and Red Russian kale monoculture (58%). Broccoli monoculture had 43% unmarketable heads, which was numerically reduced to 30% when grown beside Red Russian kale, and to 23% when grown beside Bok Choy. These results demonstrated that Red Russian kale was more preferred by SM than broccoli, while Bok Choy appeared to be completely tolerant to SM. Consequently, broccoli grown beside the more preferred Red Russian kale sustained less SM infestation and had more marketable heads than broccoli grown in monoculture. Interestingly, when broccoli was grown beside Red Russian kale, even though the % SM infestation and % unmarketable heads were less than monoculture broccoli, the severity of damage in the SM-infested heads was significantly higher than broccoli monoculture and broccoli beside Bok Choy. Although the concept of using a more preferred brassica as a trap crop to protect broccoli from SM holds promise, more research is needed, specifically the concept should be tested under different SM pressure and in different planting configurations. Also, management of the trap crop needs to be considered and tested to ensure that the SM population does not build and spill into marketable brassica crops, which would defeat its purpose.

Economic analysis: According to our yield estimates, growing broccoli beside Red Russian kale and Bok Choy increased marketable yield by 25% and 36%, respectively compared to monoculture broccoli, which resulted in an additional $51 and $72 per 100 feet of bed. This could be as much as $3,703 to $5,227 per acre (assuming 3-foot wide beds with 3 feet between beds). However, these figures do not take into consideration the planting configuration used in this study where half of each bed was used to grow the trap crop instead of broccoli. In this case, if half of each broccoli bed was cropped to Red Russian kale, marketable yield of broccoli would also drop in half to 41.5 lb and $125 per 100 ft bed, which would be even lower than growing monoculture broccoli with 43% unmarketable heads. Ideally, a planting configuration that utilizes proportionately less trap crop than cultivated crop will be found to be effective, because this will enhance the economic feasibility of this potential SM management strategy.

Objective 2.v. Effect of plastic mulch on SM pupation and emergence: Small Cage Study

In this study, we wanted to determine whether SM pupation and emergence was reduced when SM-infested plants were grown on black plastic mulch compared to bare ground. Our hypothesis was that plastic mulch would reduce SM trap catches, presumably because some of the SM larvae that dropped from the plant to the ground to pupate would land on the plastic, which would hinder their ability to get into the soil to pupate. Consequently, fewer SM adults would emerge if fewer SM pupated successfully. It is unknown whether or how far SM larvae can crawl over top of plastic mulch to find a plant hole in the plastic through which to enter the soil.

Although lower in number, peaks in SM trap catches inside the small cages generally resembled those of the open air traps (Fig. 27 top) Fig. 27. SM trap catches were similar between bare ground and plastic mulch in the first and third generations with bare ground having very slightly higher catches than plastic (Fig. 27 bottom). The greatest difference between treatments in trap catches occurred during the second-generation peak, where SM population in bare ground was 74-260% higher than plastic (Fig. 27 bottom). In total, SM trap catches were significantly 37% higher where Red Russian kale was grown on bare ground compared to black plastic (Fig. 28) Fig. 28. Presumably, there were no differences between treatments during the first generation because the plastic had just been removed and the SM that were captured were generated before the bare ground treatment was established. The dramatic difference between SM trap catches in the second generation suggests that in bare ground many more SM larvae dropped to the soil where they successfully pupated and subsequently emerged compared to plastic mulch. Thus, these results suggest that plastic mulch may serve as a barrier to SM pupation.

 Objective 2. vi. Effect of post-harvest practices on SM emergence: Large-Cage Study.

In this study, we investigated the effect of post-harvest practices on SM emergence. We assumed that the majority of SM from the infested Red Russian kale plants had already dropped to the soil to pupate when post-harvest treatments were applied, because we found hardly any SM feeding in the plants. Although lower in number, peaks in SM trap catches inside the large cages generally resembled SM population trends of the open-air traps (Fig. 29 top) Fig. 29. Shallow tillage to 4 inch following harvest of SM-infested kale resulted in a significant 72% reduction in total subsequent SM captures over a 4-month period compared to intact plants (= untreated) (Fig. 29 bottom). It is unknown whether the reduced trap catches in the tillage treatment occurred, because tillage destroyed SM larvae feeding in the plants, which then never pupated or emerged, or because tillage displaced SM pupae in soil from top 1 inch to deeper depths that reduced subsequent SM emergence. What happens to SM pupa when it is displaced to greater than 1 inch in soil profile is unknown. They could perish, wait until they resurface (via tillage) to emerge, or attempt to emerge from greater depths with varying success. It is believed that deep tillage is an effective management strategy to reduce SM pressure. It is important to note that in our trial there was some re-growth of the kale plants in the shallow tillage treatment, which we pulled out as soon as we discovered them. However, these plants could have served as a host for SM during this time. Perhaps the tillage treatment could have reduced SM populations even further had there not been any plant regrowth.  

Applying biodegradable black plastic mulch over top of intact SM-infested kale plants resulted in an 88% reduction in subsequent total SM captures compared to intact plants with no cover, which was not significantly different from shallow tillage. When the trial was taken down, we observed tears in some of the plastic through which SM could have emerged. Nonetheless, these results indicate that the plastic mulch served as a barrier to SM emergence. It is unknown whether the SM emerged under the plastic and then perished in the absence of an exit, or if they were alive and waiting to emerge when the barrier was removed. Further work to determine whether plastic mulch or another ground barrier could be used as a management strategy to prevent SM emergence and reduce the SM population is warranted. However, to be effective, the ground barrier would have to be thicker and more resistant to tearing than the biodegradable black plastic mulch that we used in this study.

Objective 2. vii. Effect of minimum tillage and tarping on SM emergence: Small-Cage Study.

SM trap catches in small cages were about a third of that in the open-air traps with peak emergence occurring in late June (Fig. 30) . There were no significant differences among treatments in average 11-week total SM trap catches in the small cages (Fig. 31) Fig. 30 and 31. Numerically, fall tillage and no tillage with tarp (= no tillage after tarp removed) captured 3-times as many SM as no tillage. We hypothesized that the no tillage system would result in higher SM emergence, because the cabbage re-growth in the fall could have been a favorable host for SM, unlike the tarp and fall tillage treatments in which cabbage residue was destroyed (Table 5). Presumably, there would have been no disturbance of SM pupa deeper into the soil profile in the no tillage treatment, leaving them all within the top 1 inch for easier emergence compared to shallow tillage, similar to our results from the post-harvest study at Canticle. Although our results suggest that no tillage did not increase SM population, this warrants further research, because the sample areas (3 reps of 4 ft² area) were rather small. First SM emergence occurred at the same time (May 25) in all the small cages as it did in the open air (Fig. 30 bottom), which suggests that the tillage practices and tarp did not have any effect on timing of SM emergence. Theoretically, the different treatments could have resulted in different soil temperature (such as warmer underneath tarp) and moisture levels that could have affected timing of SM emergence.

The fact that SM emerged in the cages underneath the tarp is encouraging. If SM are capable of emerging underneath a tarp, then perhaps use of tarp or another type of ground barrier such as landscape fabric or plastic mulch might be a pragmatic means of crashing a SM population at a spring emergence site. In this study, the tarp was laid over a 1.5 ft tall cage instead of being pressed against the ground. Whether SM emerge and consequently perish underneath a ground barrier laid directly on the ground surface warrants further investigation. Alternatively, they might be alive and waiting to emerge once the ground barrier is removed.  

Although overall trap catches might be too low to draw any conclusions, we did observe that the fall tillage treatment appeared to have two emergence peaks (Jun 15 & Jul 13) compared to the two no tillage treatments and open air, which had a single emergence peak on Jun 22 (Fig. 30). Perhaps the bimodal SM emergence peak in fall tillage was a result of the tillage displacing a portion of the SM pupa deeper into the soil causing them to emerge later. In general, the effect of minimum/no tillage on SM population should be investigated further.