Trap cropping for flea beetle control in long-season brassicas

View the project final report

Commodities

• Vegetables: broccoli, cabbages, cauliflower, greens (leafy), brussel sprouts

Practices

• Crop Production: application rate management
• Education and Training: demonstration, on-farm/ranch research, participatory research
• Pest Management: biorational pesticides, chemical control, economic threshold, field monitoring/scouting, integrated pest management, row covers (for pests), trap crops, traps

Crucifer flea beetles are a consistent pest of Brassica crops. They emerge every year in the early spring and, unless controlled adequately, causing economic losses every year. Their feeding damage may defoliate small plants, severely stunting their growth, and feeding damage causes leafy crops to be unmarketable. The control methods currently utilized by organic farmers are not very conducive to use in long-season Brassicas.

Trap cropping is a management strategy that takes advantage of insects feeding preferences for different plants. By planting the pests’ food of choice around a less desirable crop, it is possible to attract the pest in high numbers to the “trap” crop while few pests move on to the main crop.

It has been observed that certain Brassica species are particularly attractive to crucifer flea beetles, while others are attacked at lower levels if the beetles have a choice. Brassica oleracea, which includes most of the traditional European crops such as cabbage, broccoli, cauliflower, kale and collards, is significantly less attractive than Brassica rapa crops, which include many of the greens (bok choi, tatsoi, mizuna, etc). This trial seeks to test the effectiveness of using Komatsuna, a Brassica rapa trap crop, to control flea beetles on long-season Brassica oleracea crops such as kale and Brussels sprouts.

Komatsuna will be planted in a double row around perimeter of the main brassica crops, flea beetle populations in the trap crop will be controlled by Entrust sprays, an organically approved form of spinosad. Flea beetle damage to the trap crop and the main crops will be monitored throughout the season, to determine if trap cropping is an effective management tool for flea beetles in long-season brassica crops.

This trial will be carried out on three different organic farms in three different states, so each farm will have a slightly different crop mix, timing, soil type, etc. The strength of this approach is that we will be able to see how the trap crop performs in three different situations. The challenge is that specific set-up details for each farm are not yet known. Farmers will look over their rotations and plan for planting during the winter, so by the spring we will have more precise plans for acreage and timing of plantings.

The methods in general terms: We will plant a double row of Komatsuna around a field of long-season Brassicas. In-row and between-row spacing of the borders will be the same as the main crop or closer if that is practical. The fields used will not have had Brassicas in them last year, so we do not expect overwintering beetles to emerge from within the field. On the NY farm, the Brassicas planted will be three different types of kale (red Russian, green, and dinosaur), collards, and Brussels sprouts. In VT the crops will be Red Russian Kale, Lacinata Kale and collards. In MA the crops will be Brussels Sprouts, kale, broccoli, and some early sauerkraut cabbage. Red Russian kale is a Brassica napus, and more attractive to flea beetles than Brassica oleracea, but probably still less attractive than Brassica rapa. In the trial we will find out if trap crops can still pull beetles from B. napus to the same degree as they attract them away from B. oleracea.

The NY farmer puts in successive plantings, one planting each month for the four months of April-July. We plant to reserve one large (~2 acre) field or two half fields (separated by distance) for the succession plantings. In NY we will direct seed the komatsuna around the early field perimeter 10-14 days before the first crop is set out as transplants (April), and this field will be used for the May crop planting as well. We will sow the komatsuna around the second field perimeter 10-14 days before the June crop planting, and transplant the July-planted crop into this same field. In VT and MA they will transplant komatsuna and have it in place by the time the crop is planted.

Yellow sticky cards will be placed in the trap crop perimeter, and when the first flea beetles are detected the trap crop will be sprayed with Entrust at the highest labeled rate (3oz./A) for controlling caterpillars. Beetle populations within the trap crop and within the main crop will be evaluated biweekly at first colonization of beetles, then weekly. Borders will be sprayed at the first observation of flea beetle presence, and weekly thereafter as long as pressure is significant and the main crop is in a susceptible stage. We will use direct observation of crop damage (feeding holes) as well as yellow sticky cards to measure beetle populations and damage levels. Direct beetle counts are difficult and time consuming. From previous studies, we know that sticky card counts correlate with beetle counts and with damage at harvest, and are faster and easier to do (Andersen et al, in press). Yellow sticky cards will used in the trap crop and in the field to estimate beetle population levels. Crop and trap crop damage levels will be evaluated by sampling two leaves per plant of similar age in the respective plantings and rating them visually for beetle damage (leaf holes) on a simple scale of 1-5. We will sample five plants at a randomly chosen location in the border, then five crop plants half way between the center of the field and the sampled border.

We have decided not to set up a control plot because to do that effectively, we would need another field of the same crops far enough away from the first planting to be unaffected by the trap crop or flea beetles migrating from the first field. This situation does not exist on most organic farms.

The farmers consider the critical time for control of beetle damage to be the first 3 weeks after transplanting. If Brussels sprouts look good at this point, no further control is applied. The leaves of kale and collards are themselves sold, so if the trap crop does not control the flea beetles adequately these crops may be sprayed after their first 3 weeks in the field to control feeding damage on the leaves. Since specific economic thresholds for flea beetle do not exist for many of these crops, we expect to establish them for the purpose of the project. Growers and consultants will determine in advance their threshold for acceptable damage on the main crop, which we expect will vary with the crop. If the threshold is exceeded within the first three weeks after transplanting (or while the plants are under 6 inches high), beetles will be knocked back with an organically approved spray. For crops where greens must be protected for a longer period (eg collards, kale), action thresholds will be established for the later growth stages as well.