Eric Sideman's Pest Report
May 28,2002

Crop Condition

The 2002 season started out very warm and looked as if it were going to be very early. Then it got cold and stayed cold and we are at about an average season at this time. The same is true for water. At first it looked as if we were going to catch up on the drought, but lately there has been little rain and I suggest you check your soils. Sandy soils may already be too dry for good germination.

Weeds-

The big weed this season that I see around is shepard's purse. This is a winter annual that has laid dormant over the winter and is now producing seeds. Get these seeds out of your fields. Even the seeds that do not look fully mature can mature after you mow this plant and those seeds will mostly germinate in the late summer/early fall. If shepards purse is a problem for you, plan a tillage in the early fall to kill new seedlings before you plant your cover crop.

Quack grass is growing very well this spring. Pig weed is coming along. I have not seen much growth of crab grass yet.

Crops-

CRUCIFERS:

FLEA BEETLE
Flea beetles are active. Crop rotation gives a significant benefit. Putting spring crucifers close to last fall's later-season crucifers provides a ready food source for overwintered beetles. Populations are much lower in rotated fields. The all-black crucifer flea beetle, Phyllotreta cruciferae, and the striped flea beetle, Phyllotreta striolata, which is black with two light tan stripes on its back (you need a hand lens and a quiet beetle to see these), are the primary flea beetles that attack brassicas in the Northeast. The adult beetles overwinter primarily in field edges, in leaf litter or in soil and move out of these sites after temperatures exceed 55 oF in the spring. Beetles rapidly colonize new plantings of brassicas and feed on cotyledons and leaves, causing shot-hole damage which can delay growth, reduce yield, or kill whole plants. Eggs are laid in the soil and larvae develop underground, feeding on roots. Beetles are long-lived, and may be present until the summer generation of beetles has completed its development in the soil and emerges as new adults.

Flea beetles show differences in preference among the brassica species, tending to prefer the other species of Brassica (B. juncea, B. rapa, B. napus) to Brassica oleracea. Generally, the standard European vegetable crops (cabbage, broccoli, cauliflower, Brussels sprouts, collards, kale, kohlrabi) belong to B. oleracea, while the leafy greens and Asian crops belong to other species of Brassica. The Brassica oleracae can generally tolerate and outgrow more feeding damage than Chinese cabbage, bok choi, mustards, mizuna, or other leafy greens.

However, feeding can kill seedlings, delay growth or reduce yield on any brassicas. Scout crops to assess beetle numbers, damage and plant stress. Beetles can be counted if leaves are not disturbed. Threshold studies conducted by M. A. Al-Doghairi at Colorado State tested thresholds of 0.2, 5, and 10 beetles per plant in broccoli and cabbage, where sprays were applied when the average number of flea beetles per plant reached the desired threshold. This study concluded that a threshold of 2 to 5 beetles per plant would provide control without yield loss. When controls were applied at 10 beetles per plant or no controls were applied, significant yield reductions and reduced head diameter occurred. The leafy greens quickly become unmarketable and can thus no flea beetle damage can be tolerated.

For non-chemical control, floating row covers provide a barrier that prevents beetles from reaching the crop. Covers must be put in place at seeding and sealed around the edges with soil. For weeding or harvesting, remove and replace the cover as quickly as possible Pyrethrum products are an effective option as a last resort. They can be sprayed right through the row cover if you suspect that you have trapped beetles inside. The problem is that the control is short lived and new beetles will be back and I personally cannot justify that much spraying, especially on greens.

CABBAGE ROOT MAGGOT

Watch for cabbage root maggot eggs, especially on transplants. Flies spend the winter as small brown pupae in the soil. Adults emerge in spring (around the time forsythia flowers) and seek out crucifer plants to lay eggs at the base of the stem. Adults can travel considerable distance in search of host plants (1/2 to 1 mile). (Onion growers should also watch for root maggot activity at this time.)

Hot dry soils are not a favorable environment for maggot egg survival. Eggs are killed when temperatures exceed 100 oF, and are also subject to dessication under dry conditions -- both of which have occurred in many fields at some time this season. But much of the season has not been hot and dry.

Recent research by Tony Shelton at Cornell University produced a degree day model for cabbage maggot emergence in New York State. According to the model, the first emergence of cabbage maggot occurs at 160 degree days +/- 8 degree days (DD). 25% emergence occurs at 203 DD +/- 3 DD, 50% emergence occurs at 251 DD +/- 8 DD, 75% emergence occurs at 304 DD +/- 37 DD and 95 % emergence occurs at 387 DD +/- 8 DD. We do not know if this model fits for New England maggot flies, as there can be regional variations, but it gives an estimate. As of May 6, GDD accumulation in Massachusetts (base 50 oF) ranged from a low of 118 GDD on Cape Cod to a high of 203 GDD in Boston and Westover (NASS New England Report). Growing Degree Days are calculated by taking the average of the maximum and minimum temperatures for the day (Max + Min /2) and subtracting the base temperature (either 50 or 55 oF for most plants and insects). Below the base temperature, growth and development are minimal.

Cabbage root maggot flies are smaller and more delicate than a housefly, and can often be seen moving from plant to plant in early crucifers. To monitor for adult flies, use a flat water-pan trap painted bright yellow (Federal Safety Yellow), placed on the ground in the field. These can be made from any sturdy plastic dish, or purchased. Add a drop of soap to break the surface tension. Flies are attracted to the yellow color and to the moisture. Yellow sticky traps can also be used - these are placed vertically on stakes, near the soil. Check traps twice weekly. This will tell you when the flight peaks, and when it declines. This report is late, sorry, for typically, flights decline after mid-May and many late May and later plantings do not need protection.

Floating row covers provide an effective barrier against this pest. Use in a rotated field only, as flies overwinter in fields of late season crucifers. Remove and replace after weeding operations. If covers are not a practical option, and eggs are found by scouting, cultivation that brings soil up around the stem may help encourage formation of adventitious roots from the stem., which can help compensate for root loss. This assumes some soil moisture is present in the upper layers of soil.

--Ruth Hazzard

SWEET CORN:

FROST DAMAGE

Last week we frosted 4 days in a row and my tranplanted corn took a beating, as did my strawberries and asparagus. Here are some notes on frosted sweet corn from Liz Maynard of Purdue University: The growing point of sweet corn remains below the ground until plants reach the 6-leaf stage (6 leaf collars visible). While underground, the growing point is protected from light frosts. A frost on small sweet corn will injure or kill leaves that are above ground, but the growing point will typically survive and continue to produce new leaves. What affect will frost have on maturity and yield of sweet corn? Sweet corn maturity depends on the cultivar and growing temperatures. While the growing point is below the ground, the soil temperature plays a bigger role in determining the developmental rate than air temperature. Cool soil temperatures will slow development whether or not a frost occurs. The damage that frost causes to leaves above ground robs those leaves of the capacity to make sugars, the primary source of energy and building materials for plant growth. Frost that occurs before the 3- leaf stage generally does not reduce yield.

--Vegetable Crops Hotline, Purdue, May 3, 2001.

POTATOES:

Late Blight and Cull Piles

Cull potatoes are a well-known source of late blight inoculum. Because late blight is more difficult to control now than in recent memory, eliminating late blight inoculum sources is critically important. Cull tubers can be buried. This method is effective if they are placed deep enough so that the sprouts will not each the surface. Generally this requires at least two feet of soil on top of the tubers. There are safety issues to consider. Deep piles can create sinkholes after the tubers degrade. Another technique is to completely cover the cull piles with black plastic or an opaque tarp. It is important that no light reaches the tubers, as this will allow the volunteers to survive and encourage sprouts to push through or around the covering.

--Rosemary Loria, Cornell & D.Moyer- Cornell Cooperative Extension, Suffolk Co.

DRY TRANSPLANTS

Here is a warning for transplanting into dry soil:

All this dry weather has led to a few farm visits to look at recently transplanted vegetables that appear to be drying out, although the trickle irrigation is on. We have come up with a couple of explanations about what we may be seeing in these situations. First, transplants are grown in trays in the greenhouse in a soil-less mix specifically designed to maximize water availability and minimize drying out of the seedling. Placing the moist transplant root-ball into dry soil will tend to wick water away from the root-ball. Thorough watering while transplanting is a must, but be aware that in unusually dry years, more water may be required than in past years. A second area of concern in water supply for young plants is the placement of the trickle irrigation tape. In loose sandy soil there is little movement of water laterally away from the drip tape emitters. Water availability decreases dramatically as you move away from the drip tape. Depending upon where the drip tape is placed under the plastic mulch at the time it was laid down, it could be 6-12 inches away from the site of the new transplant. This may mean that, while drip tape irrigation would be fine for a mature plant with a large established root system, the transplant roots are receiving very little water even though the trickle irrigation is on. Close monitoring of the water status of the soil around the transplant will help to eliminate drying out of the new plants. Supplemental irrigation may be necessary if the plants are experiencing water stress.

--Chris Gunter, from Vegetable Crops Hotline, No. 390, May 16, 2001, Purdue University Cooperative Extension Service.

STRAWBERRIES:

CLIPPER BEETLE

There is a fair amout of clipper damage out there. If you are very small scale, then get a bag and go collect the cliped bud and destroy them. Each hanging bud is carrying an egg. Each female clipper beetle can lay about 300 eggs. A population can build up very quickly. Large scale growers depend on crop rotation to stay ahead of the beetle. Plow fields under right after picking.

ROOT WEEVIL

Strwaberry root weevil is active. The adult is a brown to black, quarter inch long beetle with an elogated snout. The adults emerge from pupae around now. They cannot fly so they walk through fields and feed on strawberry leaves leaving a hook-shaped notch along the leaf margin. After about two weeks the weevils lay eggs in the soil. The eggs hatch over the summer and the larvae go down and feed on the plant roots. There is feeding in the fall and the spring before pupating. Most of the feeding is in the spring and areas of damaged plants are often in circular shape.

Plow under infested fields quickly after picking. Repeatedly disk the field to expose the grubs to the elements and predators. Non-susceptible crops include corn and pumpkins. Clean equipment before using in another field.

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)