Vol. 5 No. 4 April 25, 1997

Weekly Crop Update
University of Delaware Cooperative Extension

Volume 5, Issue 4

April 24, 1997

Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.

Field Corn. Economic levels of white grub larvae can easily be found in the top six inches of soil, especially in fields where corn will be planted into double crop soybean stubble. If the weather remains cool and wet during corn germination and emergence, significant stand loss can occur if economic levels are present. Although the greatest stand losses have occurred in no-till fields, economic losses can still occur in conventionally tilled fields. In general, earlier planted fields are more susceptible to economic losses. Since the beetles will generally pupate by late May, later planted fields are less susceptible to damage. If you find one or more white grubs per square foot, a soil insecticide is recommended. Counter, Force Lorsban, and Fortress will provide will provide white grub control, These soil insecticides should be placed in-furrow to achieve the best white grub control.

Small Grain. Along with cereal leaf beetles, be sure to begin checking fields for the presence of grass sawflies. Since sawflies should be detected when they are small and before head clipping begins, a sweep net should be used to sample for larvae in rank areas of the field and along field edges especially near woods. Once larvae are detected, examine 5 linear foot of row in 5- 10 locations and treat if you find 0.4 larvae per linear foot of row. If cereal leaf beetles are also present, the threshold for each insect should be reduced by 1/3. If both insects are present, Lannate or Warrior will provide control.

Beware of Last Years= Poor Grass Control.

Mark VanGessel, Extension Weed Specialist.

Last year=s heavy rainfall caused many of our soil-applied grass herbicides to break sooner than anticipated. As a result, many fields had heavy grass pressure and heavy seed production. If this describes any of your fields be sure to use a good soil-applied program and use rates at the upper end of the rate range for your soil type (refer to the label).

Bermudagrass Control in Corn

Mark VanGessel, Extension Weed Specialist.

The best thing for wiregrass in corn, is planting SR corn and using Poast Plus. A rate of 36 fl oz/A of Poast Plus after full emergence of the wiregrass and often a second application at 24 fl oz/A of Poast Plus is needed.

Because Poast Plus is good on all of our annual grasses, and because of the high rate needed for wiregrass control, I suggest not using a preemergence grass herbicide (i.e., Lasso, Dual) and save the money for the postemergence program.

One year of good wiregrass control is not going to be enough to make the problem go away, so follow-up with a sound program next year. That would be a program with a postemergence grass herbicide (in soys or veggies you should consider Fusilade, Select, or Prism). Fusilade and Select are a little better on wiregrass than Poast Plus, but the SR corn is not resistant to these herbicides. You must use Poast Plus with SR corn. That also means that volunteer corn can be controlled with Select or Prism.

Cold Weather Damage and Diseases in Small Grains -

Derby Walker, Sussex County Ag Agent

Small grains are showing signs of cold weather damage. Those yellow tips or yellow bands across the leaf are signs of cold weather. Sunshine and warm temperatures will correct the problem.

Continue to check wheat for powdery mildew. If you think spraying is necessary, select the fungicide for control based on the growth stage of your wheat. Tilt must be applied before the flag leaf emerges. Bayleton can be applied up to 21 days before harvest. Some early planted fields have already passed the stage where Tilt can be applied. Bayleton can be mixed with Dithane M-45 or Benlate to expand the disease control spectrum to include leaf rust, septoria leaf spot, glume blotch, and tan spot.

Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.

Cabbage. Low levels of diamondback larvae can be found feeding on cole crops in Sussex County. Since larvae can move quickly to the heart of the plant and cause significant damage, treatments should be applied when 5% or more of the plants are infested with larvae. If temperatures remain cool, Monitor should be used at 1 quart per acre. Once the weather turns warmer, the Bts have provided effective control.

Potatoes. As plants emerge, fields should be scouted twice a week for overwintered Colorado potato beetle adults. Although an occasional adult can be found on the earliest planted potatoes, no egg laying has been observed. A foliar insecticides will be needed for adult control if you find 25 beetles per 50 plants and defoliation is greater than 10%, Provado at 3.75 oz/acre is still providing excellent control.

Weed Control in Peas -

Derby Walker, Sussex County AG Agent

Scout pea fields for grass and weed breaks. Basagran is the only broadleaf material we have to use in peas, and you will have more success with this material,when using it on small weeds. Assure II or Poast can be used to control grasses. See the label for instructions on use and read caution statements concerning tank mixing and the use of crop oil.

Weather Summary

University of Delaware,


Week of April 19 to April 24.


0.01 inches: April 19

0.10 inches: April 22

0.55 inches: April 24

Readings taken for the previous 24 hours at 8 a.m.



Ranged from 62EF on April 21 & 22 to 49EF on April 19.


Ranged from 44EF on April 22 to 39EF on April 20.

Soil Temperature:

44EF. Average for the week.


Compiled & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops

Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State College and the United States Department of Agriculture cooperating, John C. Nye, Dean and Director. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age or national origin.