Northwest Plains Pest Management News Vol XI Issue 9

Crops have progressed very well this past week, what a difference a rain makes! The area corn crop ranges from 4 leaf to blister stage and even though some fields have taken a beating from mother nature it looks good on a whole. Much of the area cotton crop is finally blooming; nodes above white flower (NAWF) have ranged from 6-9 indicating good yield potential, time permitting. The crop is about two weeks later than what we would like to see, but currently most fields are progressing at a high pace. The reduced effective bloom period will likely limit top end yield but August and September weather conditions will determine to what extent.  Cotton will need to be carefully managed to promote earliness to preserve as much yield potential and fiber quality as possible. Area sorghum is also rapidly growing but, as with cotton, later planted fields are in a race with mother nature to mature before frost.

Spider mites must not have read the text book as they did not seem to slow a bit during recent cooler and more humid conditions. Many field have exceeded treatment thresholds and many more are near it. Even small mite colonies have a tremendous number of eggs indicating the potential for continued rapid population expansion. Treatment threshold at this point can be simplified to an established mite population in the lower 1/3 of the plant with small colonies beginning to develop near the ear leaf with end goal preventing colony establishment on the ear leaf. As I mentioned last week population dynamics are extremely important in managing spider mites, regular field evaluations should be made to document population trends and natural enemy activity.  A rapidly developing mite population should be dealt with more aggressively while a slower developing population may allow some flexibility to determine if natural enemies will keep them in check. We have several trials out evaluating different products and rates, hopefully they will provide information that will help make good pest management decisions.

Banks grass mites and eggs

Weeds continue to be problematic and difficult to control. Control problems can be attributed to several reasons and in many cases combinations of factors including environmental conditions, weed size and condition, herbicide coverage, weed resistance to herbicides. As weed management decisions are made consider all management options and don’t automatically rule out “low tech” tactics such as cultivation and hand hoeing. To my knowledge no weed has developed resistance to the separation of above ground parts from below ground roots. If weed resistance is suspected deal with it aggressively, make every effort to prevent the loss of herbicide efficacy due to resistance.

Southwestern corn borer (SWCB) field level infestations have been reported even though moth captures in traps have remained relatively low with slight increases recently. The changing agriculture landscape including  planting use of Bt hybrids, as well as previous years use of Bt technology may be making trapping less effective in predicting area wide potential pest activity. Populations of SWCB appear to be much more localized and more difficult to predict than the more uniform infestations of yester year. The established action threshold for SWCB is met when 20% of plants are infested with eggs or small larvae. Most eggs will be laid on the middle 7 leaves; the ear leaf, 2 above and 4 below.

Hatched SWCB eggs

As cotton transitions into the early bloom stage we no longer consider cotton fleahopper a risk but we need to continue monitoring cotton for significant Lygus infestations. During the early bloom period the action threshold for Lygus is 15/100 sweeps (4/beat sheet sample) with unacceptable fruit shed. Current Lygus infestation levels remain low. Fields adjacent to weedy areas should be closely monitored when the weeds are destroyed as Lygus will migrate into cotton.

Lygus Adult

Heat Unit Accumulation

The area is running a bit ahead of the 30 year long term average in heat unit accumulation but crops are behind in development due to delayed planting.

DD 60 based on May 1 planting date

Crop moisture demands near .25 inch for corn, .2 inch for cotton, and .15 inch for sorghum. More detailed crop moisture demands as of July 25, 2013 can be found here.

Northwest Plains Pest Management News Vol XI Issue 8

The area has been blessed with a wide spread slow soaking rain! The moisture will provide a much needed boost to crop production and has already provided a significant boost in producer optimism. Local NOAA weather stations recorded from near 1 inch to over 2.5 inches over the last several days; individual reports in excess of 3 inches were also received. 

Recent precipitation and cooler weather could not have come at a better time for corn which is pollinating. Stress occurring a few days prior to tasseling can cause ear development to slow resulting in a lag between pollen shed and silking which can lead to poor pollination. Moisture demand has decreased from over .40 inch/ day to .13 inch/day during the recent cool humid conditions. Moisture demand will rapidly rebound to previous levels as temperature increases and humidity decreases. Hopefully current soil moisture along with irrigation will get the early corn past peak moisture demand. A good soil moisture profile will also be a good base for later planted corn as moisture demands increase. 

Spider Mite Pressure

Spider mites have continued to progress in many corn fields, cool wet conditions may slow them down but many infestations are well established and will likely require treatment shortly to avoid excessive yield loss. The action threshold for spider mites in corn is based on crop value, percent infested leaves, and leaf area damaged by mites. The following table was developed based on fast acting miticides and may need to be amended slightly for slower acting miticides. 

Population dynamics are extremely important in accessing spider mites, regular scouting should be conducted to document population trends and natural enemy activity.  A rapidly increasing mite population should be dealt with more aggressively while a slower developing population may allow some flexibility to determine if natural enemies will keep them in check. I’ve said it before but it’s worth repeating the conservation of natural enemies is the corner stone in spider mite management.  Key natural enemies include six spotted thrips, spider mite destroyers, minute pirate bugs, and predatory mites. Management of other pest such as corn borer and fall armyworm can significantly impact mite populations.  The use of broad spectrum insecticides can flair mites by removing natural enemies which are suppressing mite populations.  Miticides may not provide adequate suppression in the absence of natural enemies which could necessitate multiple applications to get a raging spider mite population under control. 

Six Spotted Trips

Southwestern corn borer (SWCB) trap captures remain low but I expect the second generation moth flight to pick up shortly.  Moths are ¾ inch long, white,  with no distinct markings.  Eggs are flattened, approximately 1/8 inch in diameter and can be laid singly or in groups of 2 to 3 or more.  When in groups, eggs are laid in an overlapping pattern resembling fish scales. Freshly laid eggs are creamy white but develop three parallel red lines in about 24 hours. Small larvae will feed on leaves, ear shoots, husks, and silk for about 5 to 10 days before tunneling into the stalk or ear shank and continuing to feed. Second generation SWCB will lay 75% of their eggs on the upper surface of the middle 7 leaves; the ear leaf, two above and four below.  Inspection should be concentrated in this zone. The established economic threshold for second generation SWCB is when 20 to 25% of plants are infested with eggs or small larvae. Bt corn hybrids are extremely effective in suppressing SWCB making insecticide treatments unnecessary. 

SWCB Egg

Fall armyworm (FAW) infestations have increased in area sorghum, some fields have as high as 30% of plants infested. Dr. Pat Porter has been monitoring FAW moth activity and has noted that this years population is tracking very similar to 2011. If this trend continues moth activity will begin to increase the latter part of July and peak the second week of August.  FAW is a non discriminatory pest which will infest many area crops including corn, cotton, sorghum, blackeyed peas, green beans just to name a few.

Insect pest pressure remains very quiet in the cotton field. square sets are outstanding as we near bloom ranging from 90-98%  with most fields 95% or better. Weeds continue to be troublesome in many fields. Remember there are several good residual herbicide products to consider when “laying by” cotton. These layby options are another tool to prevent or manage weed resistance to glyphosate. Some products can be applied over the top while some will need to be directed or applied under a hood. The benefits of a good layby program will far out weigh costs and inconvenience of application especially in light of documented pigweed resistance to glyphosate.

Northwest Plains Pest Management News

Crops have responded very well to precipitation and cooler temperatures over the last couple weeks. The area corn crop has about as wide a range of maturity as I’ve ever seen this time of year, from just planted to tassel/silking. More mature corn fields are nearing or at peak moisture demand which could exceed .45 inch/day under hot windy conditions. The following chart graphs daily ET values based on average conditions from various planting dates.

Most cotton has “turned the corner” and is squaring but I have yet to see a bloom. Square sets have been outstanding ranging from the low 90s to near 100% with most fields around 96%.  While we are not behind in heat unit accumulation the cotton crop is behind due to delayed planting and harsh environmental conditions which will likely result in a shorter than normal effective bloom period.  Considering this, managing cotton for earliness at this point looks to be even more important.  The bulk of nitrogen fertilizer should be applied by early bloom. Pest pressure at this point is very low.


Sorghum is progressing very well, the crop ranges from emerging to growing point differentiation (GPD).  It seems some sorghum has taken a bit longer to recover from a phenoxy herbicide application than expected. A wide range of crop response was observed between hybrids in a local trial treated with dicamba + atrazine. Try to avoid yield robbing moisture and nutrient deficiencies during GPD to maximize yield potential.

Spider mites have been building in some corn fields but  remain hard to find in others. Most colonies remain relatively small but have moved up the plant to the upper part of the lower third of the plant. Occasionally we have observed colonies in the middle section of the plant. Remember if spider mites are established in a field which will be likely treated for another pest with an insecticide which is harsh on beneficials then a preventative miticide application about two weeks prior should be considered. Currently labeled miticides are designed to work in concert with natural enemies of spider mites, when these beneficials are removed from the equation miticide performance may not meet expectations. Sixspotted thrips, a key predator of spider mites,  have been observed in area corn and will help stabilize mite populations if conserved. Adult sixspotted thrips can be distinguished from other thrips species by the 6 spots on their back (3 on each wing cover). Both adults and larvae are predaceous feeding primarily on mites and can be very effective in suppressing mite populations.

Fall armyworm pressure has picked up in area sorghum and non-Bt corn. Area surveys in sorghum have ranged from 2-11% infested plants. Damaged leaves unfolding from the whorl are ragged with “shot holes.” Although this may look dramatic, leaf damage usually does not reduce yields greatly, and control of larvae during the whorl stage is seldom economically justified. Also, larvae within the whorl are somewhat protected from insecticide. Insecticide application may be justified if larval feeding reduces leaf area by more than 30 percent or is damaging the developing grain head or growing point within the whorl.

Plant growth regulators (PGR) in cotton in and of themselves do not “make more cotton” but do allow producers to push a crop with irrigation and fertility while maintaining acceptable plant structure and enhancing earliness. In other words a PGR applied to cotton without adequate moisture and plant nutrients will not enhance yield. A heavy boll load will limit vegetative growth and enhance earliness but in a high input environment where moisture and fertility are not limiting factors a heavy boll load alone may not be enough to adequately control vegetative growth in stripper harvested cotton.  Mepiquat chloride (MC) is a foliar applied PGR that is absorbed into leaves and translocated throughout the plant.  Since its introduction, MC has been used extensively to manage cotton growth in an attempt to reduce risk associated with a delayed harvest.  Mepiquat chloride regulates cell elongation by inhibiting the synthesis of gibberellin.  This reduction in cell length in turn reduces overall plant height and internode length. There are numerous PGR options most of which are based on mepiquat chloride but may contain other active ingredients to further enhance effectiveness. Early low rate multiple (LRM) applications during squaring and early bloom have shown to be more effective than later single high rate applications.  For example in a local research trial early LRM applications of Stance (4 to 1 ratio of mepiquat chloride and cyclanilide) reduced the number of days to physiological cutout which in turn translated into an earlier harvest while the single high rate application did not differ from the untreated plots.

Northwest Plains Pest Management News

Recent precipitation has provided much needed moisture to the area but did come at a cost. Streaks of hail accompanied some of the storms which damaged and in some cases destroyed crops in their path. Replant decisions are having to be made as quickly as possible as time to mature a subsequent crop is limited.

The growing point of sorghum remains at or near the soil surface and likely avoided catastrophic injury in most cases and should recover. Larger corn was at more risk since the growing point and hardened stalk below it were  much more exposed. Damage evaluations should consider crop stage, stand loss, leaf area loss, growing point injury and stalk injury.

Injured growing point

Damaged stalk

Like many other crop plants, corn does not need all of the leaf surface area that it produces. Corn fields may look ugly and ragged and still be capable of producing an almost normal corn crop. Even shredded and broken leaves are capable of some photosynthesis if they are still connected to the main plant. A strong healthy root system, good soil moisture, and favorable, sunny weather are most important to a rapid recovery.  The growth stage of the corn plant and the percent defoliation are important. Hail that is received later in the growing season can be increasingly destructive. Damage tables constructed by University of Minnesota (below) indicate the percent yield reduction observed with various percents of defoliation at defined growth stages. Notice that a near-50 percent leaf loss at the 10th leaf stage results in only a 6 percent yield reduction.

What about silage production? Leaves  only compose 10-15% of total plant weight so yield losses due to actual physical leaf removal may not be has great as one might assume. Most damage will come from the inability of the plant to produce acceptable plant structure and  ears. Research conducted by Penn State Extension suggest that corn silage yield losses due to hail are comparable to grain yield losses.

After 4 to 5 days have lapsed since the hail storm, inspect the surviving plants. Some of these plants should be split open to see at what height and condition the growing point is found. If the growing tip is black or brown, the damage is severe and the plant may soon die. Undamaged growing points will be pushing new leaves, and corn will increase in height and leaf area. A “buggy whip” condition can occur when new leaf growth becomes tangled in dead or mangled leaves, normally most plants will break through given enough time. Each field will have to be carefully evaluated to get a best estimate of crop potential and risks associated with keeping the current crop vs. replanting.

Estimating corn yield reduction due to various amounts of leaf removal at several stages of plant development (taken from tables prepared by Hail Insurance Adjustment and Research Association and Crop Insurance Research Bureau as listed in Crop News Bulletin No. 23, Extension Service, University of Minnesota).