June 27, 1997 Vol. 97.7

IN THIS ISSUE:

Calendar

• Weed Field Days - Landisville, July 10 and Rock Springs, July 11, 1997
• Northeast Branch American Society of Agronomy - College Park, MD - July 13 to 16, 1997
• Penn State Crops Clinic - Rock Springs Agronomy Farm - July 22 and 24, 1997
• CCA Exam - PDA Building in Harrisburg - August 1, 1997
• NCRS Program on Soil Conservation postponed until September 23 & 24, 1997

Production

• Reduced corn populations - how much will they effect silage yields

Pest Management

• Insect Alert
• Scout now for weeds in soybean
• Preharvest weed control in wheat
• Roundup-Ready soybean and perennial weed control

Agent's Corner

• Dauphin
• Lebanon

Degree Day Accumulations

Production:

Reduced corn populations - how much will they effect silage yields?

Many fields this spring had less than optimum emergence and now have less than desired plant populations. A good question now is: How much will silage yields be reduced due to these lower populations? Recent studies at the University of Wisconsin and Cornell University compared silage yield and quality at plant populations of 18,000 compared to 30,000 plants per acre. The Wisconsin study showed that the 18,000 population yielded 19.1 tons/A compared to 22.2 tons/A for the 30,000 population or a 14% yield reduction. The Cornell study indicated a similar 12.5% yield reduction with 24.0 tons/acre at 18,000 ppa and 27.4 tons/A at 30,000 ppa. Consequently, reasonable yields can still be achieved with less than ideal stands due to the crops ability to compensate for missing plants.

On the positive side, both studies indicated an improvement in silage quality with the lower populations. Across both studies, protein levels were increased by about 0.5 percentage units and NDF was decreased by 1.0 to 1.7 percentage units with the lower plant populations compared to the normal. These responses would be expected because of the increased soil nitrogen availability per plant and higher grain to stover ratio in the lower plant populations.

Greg W. Roth

Pest Management:

Insect Alert

Bean leaf beetle - Feeding by this pest in first trifoliate leaf stage soybeans has been reported along a ridge starting at Kempton, Pa. and running to New Tripoli, Pa. by Gary Snyder of Reading Bone Fertilizer. This insect has typically not been a pest of soybeans in Pennsylvania, but has caused significant problems in Ohio and states further south. Economic damage by this pest is caused by the adult stage, although the larvae feed on bean roots. Threshold Guidelines are: Seedlings - 20 percent or more of plants cut and the stand has gaps of 1 foot or more; or if at least 1 seedling per foot of row is destroyed - Before Bloom - treat if defoliation reaches 30 percent and there are 5 or more beetles per foot of row - Bloom to Pod Fill - treat when defoliation reaches 20 percent and there are 16 or more beetles per foot of row - Seed Maturation - treat when 5 to 10 percent of the pods are damaged, the leaves are green, and there are 10 or more beetles per foot of row.

This pest has two generations in Pennsylvania. Early in the summer the overwintering adults may be found on the newly emerging soybean leaves, chewing holes in the leaves. The second generation injury is primarily to the pod, because foliage levels are high on the plant. The beetle is about 1/4 inch long and is green, yellow, tan, or red, often with a distinct black band around the outer edge of the wing covers. A black triangle is always found behind the head on the wing covers. Usually there are two black spots on each wing cover, but they are not always present. This information is from Illinois's "Field Crop Scouting Manual".

Mexican bean beetle - One report of possible Mexican bean beetle injury to soybeans was reported in Montour Co. by Jennifer McGrail. Again, as with bean leaf beetle, this pest has not been prevalent in Pennsylvania soybean fields. It typically is more commonly a problem in southern states and southern Ohio and Indiana. As with most crops, defoliation by insects is more significant when plants are small. Larvae and adults feed on the undersides of leaves, between the veins, and leave a lacelike network of the tougher leaf tissue. The adult is a round orange-brown beetle with 16 black spots on its back that resembles a lady-bird beetle (ladybug). The larvae is a bright yellow bristly grub which is found on the underside of the leaves. Mexican bean beetles survive the winter as adults in woodland areas. There are two generation per year in Pennsylvania.

Treatment Threshold Guidelines for the pest are if defoliation reaches 30% before bloom and 20 percent between bloom and pod fill. Products available to control the pest include, Sniper, Sevin, Furadan, Lorsban, Dimilin, dimethoate, Asana, Warrior, malathion, Lannate, methoxychlor, Penncap-M Ambush, Pounce, Larvin, and Scout. See the Agronomy Guide for specific rates and restrictions.

Slugs - Significant stand reductions to soybean fields have been reported in Centre County. In one field, stands were reduced from 230,000 plants per acre to 100,000 plants per acre. These fields were no-till planted into heavy corn residue with a history for slug activity. The plants were attacked from germination to the first trifoliate leaf stage. Managing slugs under these conditions can be difficult. The only rescue treatment is to apply Deadline M-P mini-pellets to the field at 10 lb per acre. The label does not allow lower than the 10 lb rate, but research from several states indicate that a rate of 7.5 lb per acre will provide adequate levels of control. This is not an inexpensive treatment, but under heavy pressure it can pay for itself and prevent or minimize problems for next year. If the soybean plants are growing rapidly, then the field will soon outgrow the problem as the stems harden off and leaf area is added. If you attempt control with Deadline M-P, make sure that the slugs are actively feeding at night. If conditions are dry and there is little dew formation, the slugs will move down into worm holes and soil cracks and become inactive until moist conditions return. If it is too late to manage the pest this year, several culture methods can help minimize or eliminate the problem next year. If the field can be plowed, then plowing will eliminate the residue required for shelter and break the reproductive cycle of the pest. If plowing is not an option, cultural practices that encourage more rapid growth of the crop can help minimize the problem (like early planting and trash wheels that move residue out of the row).

Stalk Borer - As of June 21, degree day accumulation indicate that stalk borer larvae should be moving from grass hosts to young corn seedlings. Some activities has been reported in the state. See the last newsletter for economic thresholds and treatment guidelines . Lorsban, Asana, Pounce, and Ambush are labeled to control the pest. The period that larvae move from grasses into young corn plants can be significantly longer than the effective period of an insecticide. Therefore, high levels of control are seldom realized. In fields where orchardgrass is common following a hay field in rotation, Pounce or Ambush applied with a pre-emergence or postemergence herbicide will provide protection of the young seedling when larvae are forced to leave the dying orchardgrass.

European corn borer - The first adult corn borers captured in blacklight traps occurred on June 2, 1997 at Rock Springs, Pa. As of June 25, significant flights were continuing. Flights in warmer areas of the state would have begun approximately one week earlier than in Central Pa. As of June 23, 1997, significant pheromone trap catches were reported in Bucks (Doylestown), Montgomery, Berks (Reading), Schuylkill (Zion Grove), Lehigh (Heidelberg and S. Whitehall Twps.) Lancaster (Brooklawn), York (York), Cumberland (Boiling Springs), Franklin (Chambersburg), Monroe (Stroudsburg, Broadheadsville), Columbia, and Centre Counties. Extremely high number of moths are being captured in pheromone traps in Lancaster and Cumberland counties. This may or may not relate to insect population levels in the area. Early planted corn fields should be scouted for evidence of insect feeding when 450 to 650 degree days have accumulated in the area. The southeastern and southwestern regions of the state are rapidly approaching this period. In these areas, leaf feeding should be evident by the first to second week of July. In low areas of Central Pa., watch for evidence of feeding during mid-July. In northern and higher elevations of the state, injury symptoms should not appear until late-July to early August. In these areas, expect only a single generation of the pest. An insecticide treatment for first generation control should be applied if 50% or more of the plants show evidence of feeding and live larvae are found in the whorl. Granular insecticides are more effective for first generation in whorl stage corn plants. If a granular cannot be applied make sure a liquid formulation is directed into the whorl. The most effective products for first generation are Pounce, Ambush, Lorsban, Furadan, and Warrior.

Corn rootworm - Degree day accumulations, as of June 21, are approaching or surpassing the minimum of 575 required for the beginning of larval hatch from overwintering eggs. A more sophisticated computer model had predicted the beginning of hatch at June 14 at the Russel E. Larsen Research Center at Rock Springs, Pa. See the last newsletter for state-wide predictions of hatch. Evidence of injury should begin to appear in early July in the Central region of the state, during late June in the warmer areas of the state (southeast and southwest), and mid to late July in higher elevations and more northern counties where temperatures are cooler. The time required from egg hatch to observable injury symptoms (stalk lodging and stunting) is about two weeks under average temperature conditions. In cooler areas this time period may be up to a week longer. In warmer areas it may be a few days shorter.

Potato leafhopper - Numbers of the pest are increasing in Centre Co. and across the state. Recent thunderstorms have carried new adults into the area that will be laying eggs in alfalfa during the second growth period. Keep an eye open for the development of economic infestations.

Scout now for weeds in soybean

Now is the time to scout those soybean fields for potential problem weeds that often slip by. In particular, eastern black nightshade and common ragweed can go unnoticed until it is often too late. Both these weed species can be effectively controlled with several postemergence herbicides when applied at the correct time. This generally means weeds with less than 6 or 8 leaves. Do not wait until the nightshade has berries on it or the ragweed is towering over the beans. Scout those bean fields now.

Preharvest Weed Control in Wheat

Here's a quick reminder on what's available for preharvest control of weeds in Wheat. One other plus with this type of application, is the potential to control some problem perennials prior to double-crop soybeans or a new forage seeding. Late summer is still the best time for perennial weed control, but this is certainly better than early summer applications and may help fit into certain crop rotations.

Roundup Ultra may be applied after the hard dough stage of grain (30% or less grain moisture) and at least 7 days before harvest. Do not apply more than 1 quart per acre, and do not treat wheat being grown for seed as a reduction in germination or vigor may occur. Additionally, do not apply to wheat with a legume underseeding. There are no double-crop soybean planting restrictions if Roundup Ultra is used as a preharvest treatment.

Some formulations of 2,4-D ester are labeled for preharvest applications in wheat. Use 1 to 2 pints per acre depending on the product . Applications should be made after the wheat is in the hard-dough stage. Do not apply to wheat with a legume underseeding. If a preharvest application of 2,4-D ester is made to wheat, double-crop soybeans can be planted 7 days later for 1 pint or 30 days for 2 pints.

Roundup-Ready soybean and perennial weed control

The following article was written by former Penn State Weed Specialist Bob Hartzler and colleague Mike Owen both at Iowa State University. It does a nice job summarizing some research from Wisconsin.

A new option for controlling perennials is the use of Roundup Ultra in combination with Roundup-Ready soybeans. This technology will allow Roundup to be applied later in the season than was feasible in the past. The availability of Roundup-Ready soybeans has some weed managers thinking that problems with perennial weeds are solved.

Jerry Doll at the University of Wisconsin evaluated hemp dogbane control in this system during the 1995 and 1996 growing seasons. Hemp dogbane control increased as Roundup applications were delayed (see table). The effect of hemp dogbane maturity on control could be partially overcome by increasing the rate of Roundup. For example, 1.5 pint of Roundup Ultra per acre at full flower provided 88 percent control, compared to 73 percent control at the bud stage. Increasing the rate of Roundup at the bud stage to 3 pints per acre resulted in dogbane control that was equivalent to the 1.5 pint per acre rate at full flower.

The researchers also evaluated the impact of the Roundup Ultra treatments on hemp dogbane the year following application. While all treatments reduced hemp dogbane populations the year following application, applications made at flowering provided more effective control than Roundup applications made earlier in the season. For example, the 2 pint per acre rate at early or full flowering reduced hemp dogbane populations by 95 percent the year following application, whereas, the bud stage treatment reduced populations by 80 percent.

Can we expect similar results with other perennials as was observed on hemp dogbane? Keep in mind that hemp dogbane is one of the more susceptible perennial broadleaf weeds to Roundup (this is one reason the rope-wick applicator works well for hemp dogbane but not for many other perennials). Other perennial weeds may require higher rates than is necessary for hemp dogbane. Also, the optimum time for controlling perennials usually will not coincide with the best time for controlling annual weeds. Since annuals have a greater potential to impact yields than perennials, these weeds should take priority when determining the optimum time for application. While Roundup-Ready beans provide an excellent opportunity to manage perennials, it will take efficient management of the product to achieve maximum performance.

Source: Jerry Doll, 1996, University of Wisconsin.
¹ Control ratings made on August 1.
² LSD 0.05 = 11%.

Bill Curran

AGENT'S CORNER

Dauphin County

Heat has finally arrived and corn and soybeans have responded with dramatic increases in growth and development. In past week the corn has grown sufficiently to "hide" all of the skippers in the stands due to poor establishment from soil insects. I investigated a corn field in Lebanon County with severe wireworm damage, many plants had 4 to 7 worms at each stalk. I have not seen any fields of corn that are outstanding, many good stands of corn, but no really super looking stands. Most problems are related to poor stand establishment. In general, no-till stands are worse than conventional tillage.

Second cutting hay harvest is progressing with yields of second cutting exceeding first cutting in many stands. There is very limited evidence of potato leafhoppers, but populations appear to be increasing. Some interest by producers in treating first cut hay stands with Roundup prior to harvest and then planting no-till corn. I will be watching these stands very closely.

Weed control is quickly decreasing in most fields with grasses and triazine resistant weeds breaking through. Incorporation prior to planting has been a definite advantage this year. Perennial weeds continue to increase in populations in corn and bean fields.

Soil conditions are becoming dry. With increasing air temperatures, moisture availability becomes a concern. Hopefully, thunderstorms will arrive and provide some relief.

Paul H. Craig
Dauphin County

Lebanon County

Currently Lebanon is dry. Corn is showing stress symptoms such as cupping of the leaves and that off color green. In addition much of the corn looks yellow and needs some nitrogen. Early corn is at v-8 and late corn is at v-4 after first cutting of hay. Some fields had heavy cut worm damage, but the cutworms I found are in the 7th instar stage, so no treatment is recommended.

Wheat is in the dough stage and so far is showing very little rust or other disease damage. I have found some aphids but not enough to warrant control.

Oats are heading and filling rapidly. Leafhoppers have arrived and most of the second cutting growth exceeds any treatment. I have been advising producers to check third cutting immediately after the harvest of the second.

Del Voight
Lebanon County

1997 DEGREE DAY ACCUMULATIONS:

To track corn and insect development, compare the 1997 cumulative degree-days shown in Table 1 to the required values in Tables 2 and 3. Choose a location that is closest to your site when making comparison.

The cumulative degree day value given is for the number of degree days needed after tillage of manure or green surface vegetation before it is recommended to plant corn in a field to avoid injury from seed corn maggot.

Dennis Calvin

William S. Curran
Associate Professor Weed Science
email: wsc2@psu.edu

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