Certified Organic Produce
Determination of an Economically Optimal Organic Method to Control Onion Flies
L. A. HAYNES
Abstract Several organic methods to control onion fly, Delia antiqua, damage were tested. The three methods that showed the most promise were spraying of parasitic nematodes (S. feltiae), application of organic cinnamon, and application of kaolin clay (surround). Trap crops of mature leeks showed an apparent effect, however none of the trap crops became infested so this was an anomaly. This year had an abnormally low incidence of early season damage, so these results are not conclusive. It was seen, at the small scale done on this farm, in this year, that these methods were not particularly economically feasible, mainly due to sprayer difficulties and associated labor costs. It should be emphasized that on a larger scale they may be extrapolated to be practical to organic farmers.
Severe onion fly damage at our farm ranges from 10-50% of our onion crops. Leeks are typically much higher. Severe damage is defined as damage that impacts marketing. Smaller blemishes are considered mild damage. The flies seem to prefer leeks first, softneck garlics second, and onions third.
Onion flies locate their host both through visual and olfactory cues. Eggs are deposited on the ground near the host plant. It was hoped to deter oviposition through the methods of row cover, cinnamon, mulch and kaolin clay. Trap crops were tried in one bed in an attempt to concentrate oviposition to one area. Parasitic nematodes were applied to one bed to control the larvae. The expectation was that the control patch would sustain much higher than normal damage due to the oviposition deterrence in other beds.
While important to plant health, crop rotation on a small scale is not practical since the flies will travel over 1 km to find allium crops. (Martinson 1988) The previous year’s crop was located to the west of this study by only a hundred feet.
Allium crops, including onions and leeks were planted in 8 separate beds between April 9th and 15th. Each bed included 9 varieties of onions and a mix of two leeks. The varieties, amount planted and the order they were placed was as follows (from west to east):
Ailsa Craig- 125 (softer, mild yellow onion)
Red Long of Tropea-125
Candy-125 (hard, sweet, white/yellow onion)
Greek salad-91 (red)
Copra-91 (yellow storage)
Varsity-91 (yellow storage)
Lincoln & King Sieg leek- 312
Scallions- Evergreen hardy white- succession plantings of 14 bunches per week for 6 weeks.
West ß----- East---à
Attempts to use heritage turkeys in pens adjacent to the beds as a method for fly control were abandoned in early June when a particularly nasty predator attacked nightly. Heritage turkeys were chosen since they are much more active than either commercial turkeys or chickens. The remaining birds were moved closer to the house.
Similarly, the row cover was abandoned in mid-June (post first generation) after it kept getting ripped and flies were found (of some sort, they were never caught and identified) inside. Additionally, the tops of onions and leeks were getting damaged.
Parasitic nematodes (S. feltiae) were applied twice during the year; once during the first generation of flies, and once during the second cycle.
Kaolin clay was applied as a spray on a weekly basis. Ground organic cinnamon was applied as a spray on a biweekly basis. The sprayer was cleaned out well between each spraying- a task which took longer than the spraying itself. The cinnamon itself was difficult to get into suspension and clogged the sprayer. Sprinkling it might be a better option.
The mulch used was a layer of partially composted stable shavings and was applied after the first several weedings. (Compost was heated fully to destroy pathogens, but the wood shavings were not fully decomposed).
Trap crops of large over wintered leeks were planted in early June. These were removed prior to leek harvests in August and were not infested.
The first three onions (Ailsa Craig, Red Long, and Candy) as well as scallions were mainly harvested weekly as fresh onions beginning in late June and marketed with their tops on. Other types of onions were harvested just as the tops were falling over and drying. The dirt was brushed off and they were checked over for damage and allowed to cure in the greenhouse (our only dry covered structure suitable for messy crop storage). Approximately 60% of the leeks were harvested before mid-September, and the others were harvested between mid September and early October.
There was no real difference in the size or quantity of onions harvested from each bed, with the exception of the ‘turkey’ bed. This was due (entirely) to the bed being situated on newly reclaimed pasture so was quite a bit trashier and weedier. Onions in this bed were half the size of others. The mulched bed had very slightly larger onions (5%) overall.
There was difference in appearance. The kaolin clay was extremely difficult to wash off the fresh onion greens completely without damaging the tops. They looked clean when wet, but had a film when dry! Onions from the row cover bed had damage to their tops.
Onion maggot damage in onions was predominantly superficial and no live maggots were observed pre-storage. While the first actual maggot seen was in a leek on July 22nd, the number of leeks damaged prior to mid-September was quite small (Table 1).
Table 1: Overall percent of crop damaged PRE-STORAGE:
In the above table, M refers to mild damage. Often, this was just a small hole/nick that may or may not have been caused by an onion maggot. This could be easily ignored, or stripped away with one layer of skin. S refers to severe damage. This was definitely due to onion maggots and would affect several layers of the onions or leeks. This type of damage is also a problem since the maggots migrate to different locations between layers so a hole that looked relatively small could not be ignored. Mild damage also is more apparent in fresh onions than dried.
Post storage damage, and late season leek damage were fairly extensive. Post harvest onion damage was not quantified since the onions were inspected before going in to storage. Much of the damage was sustained several weeks after harvest so it was assumed that it was new egg laying activity.
Costs incurred in treating the area were:
These were sprayed twice at a cost of $45 for each treatment plus two hours of labor. ($16)
This was on-hand at farm. The cost was two hours of labor ($16) since it was applied post planting and needed to be placed carefully. Weed pressure in that area of the field was high early on, so the mulch was unable to help much and was disturbed when weeding was done. In a more controlled weed environment, the mulch could go on sooner.
No additional cost.
The cost for 6 heritage turkey poults was $48.60. Portable coops were on hand and are part of the general farm operation. However, due to predation in the field, most of these were lost.
Two pounds of organic cinnamon was used, plus eight hours of labor to spray. ($72) This amount of cinnamon could probably have been reduced.
Abandoned. The labor for this farm would amount to about one extra hour per week since onion harvest occur twice per week, but expectation would be zero damage with some deficiencies in fresh appearance of leaves and possibly size.
No extra cost.
Kaolin clay (Surround):
This was sprayed weekly. The costs were one bag of clay ($30) plus 16 hours of work ($128). Spraying frequency could be reduced if a spreader sticker was used, or drip irrigation instead of overhead in a dry season. Stickers would make cleaning of onion greens more difficult but would be suitable for leeks and dried onions.
Table 2 summarizes these results. The extra crop harvested relative to control is shown. Because early season damage was so low, and the late season leek damage is more typical here in a given year, an extrapolation is shown that assumes early season damage to everything would occur that is equal to late season damage in these beds. The figure used is 312 leeks and 739 total onions.
Table 2: Income and costs for various onion fly control methods
Typically, organic onions marketed here sell for $1.25 per pound and leeks for between $1 and $1.50 apiece. Average onion size this year was around ½ pound.
This year’s early season allium crop sustained much lower than normal damage in both the control and experimental test plots. This may due to the weather, especially the dryness and fluctuating temperatures in spring/early summer. Early season rain totals were below normal, with rains frequently totaling less than .2 inches at time. This was interspersed with deluges (over 7” of rain was received over the third week in July). Applications of control methods may have been negated or delayed due to these heavy rains.
The applications of Surround, cinnamon, and to a lesser degree, parasitic nematodes, seem to protect leeks. However, in absence of choice, onion flies will lay eggs despite deterrence measures, and there were not many leeks left in the late season (Cowles, 1989). It is also apparent that the highest damage areas were adjacent to the cinnamon and surround beds. Therefore, an important part of organic control would likely involve a carefully managed trap crop with another control methods.
A standard lightweight row cover was used. A heavier row cover would be beneficial since onions and leeks are relatively long season so the row cover is removed more often. These crops also get quite tall and need a higher system than was used.
A third nematode spraying should be added into the season for the leeks as well as more aggressive control tactics during the latter part of the season.
Hot peppers have shown promise with trap crops (Cowles 1989)- this would be a useful tactic if the problems of skin burning could be addressed. (Around 90% of the time, the author manages to get hot pepper in her eye when working with powders, or the peppers themselves.) They would have the advantage of being free if hot peppers are already produced on the farm.
Repeating the study in a more normal year would be beneficial as well. There was not enough fly activity early in the season to determine effectiveness in onions. Also, having a larger stand of leeks would allow more accurate data and give a better idea of the economics of control methods late season.
It was also apparent that due to post harvest infestation, a better system of curing and storing onions is needed. A pungent spice might be useful here as an oviposition deterrent.
It is hoped to repeat this study with above changes in the 2006 season.
Partial funding for the work reported here was provided by a grant from the USDA Sustainable Agriculture Research and Education (SARE) Program. I would like to also thank Joe Spencer (Illinois Natural History Survey) for generously supplying me with advice and a large body of onion fly literature.
Cowles, R.S, Keller, J.E., Miller, J.R, Pungent Spices, Ground Red Pepper, and Synthetic Capsaicin as Onion Fly Ovipositional Deterrents, J.Chem. Ecol, Vol 15 No 2, 1989. pp719-730
Martinson, T.E., Nyrop, J.P., Eckenrode, C.J., Dispersal of the Onion Fly and Larval Damage in Rotated Onion Fields, J. Econ. Ent. 81:508-514