Managing Bt Resistant European Corn Borer

European corn borer larvae

European corn borer (ECB) has long been a pest of Manitoba corn crops, but it is not only a nuisance to corn – crops like potatoes and hemp are affected as well. The larval stages of the insect are most economically significant due to their tunneling (boring) capabilities which disrupt the flow of nutrients and water, and the integrity of the stalk. Yields can certainly be affected by ECB presence, around 3-5% yield decrease being possible in standard incidences (5-9 bu/acre in a 175 bushel crop) and increasing in more significant infestations.

Until Bt (Bacillus thuringiensis) traits were introduced to corn in Canada and the United States in 1996, corn farmers would rely on cultural practices and insecticidal control when economic thresholds were met. Cultural practices include crop rotation, residue management – destroying stalks where larvae overwinter successfully, and tillage that buries residue deep enough that larvae cannot survive. Insecticidal control is difficult due to timing between egg hatch and the boring phase. Diligent scouting to monitor egg hatch progress is extremely important to time when most eggs have hatched and larvae have not begun entering the stalk tissue yet. Once larvae reach the 3rd instar stage (7-10 days following hatch), they begin to bore into the stalk, and rarely resurface, rendering insecticide applications ineffective.

The introduction of Bt hybrids allowed farmers to not rely so heavily on residue management and insecticide application. Farmers were able to choose a fitting Bt-traited hybrid for their farm and had to match that hybrid with a refuge, or non-Bt, hybrid in 20% (or more) of the field in a block, strip or perimeter method. In more recent years, seed companies have come out with a 5% refuge system, called refuge-in-a-bag, making the system a lot easier for farmers to adhere to.

Unfortunately, non-compliance with pesticide requirements weakens the system and creates an opening for resistance. While the Bt trait is very strong, there is a small portion of the ECB population that are naturally resistant to the trait that controls the rest of the population. If farmers were to plant 100% Bt hybrids, those resistant populations would thrive and reproduce, eventually being the only population remaining.

BT CORN 101 – Manage Resistance Now
HOW RESISTANCE DEVELOPS – Manage Resistance Now
HOW TO MANAGE INSECT RESISTANCE IN YOUR BT CORN – Manage Resistance Now
MANAGING INSECT RESISTANCE VIA REFUGE PLANTING – Manage Resistance Now

Managing resistance on-farm can feel daunting, but it is very straightforward. Best management practices to avoid European corn borer resistance to Bt traits include:

  • Scout for pests and damage
  • Rotate crops and traits
  • Plant a refuge
  • Manage with insecticides
  • Keep accurate records

If there is a suspected case of ECB resistance in a Bt corn field, the following should be monitored to identify the issue:

  • Scout – both Bt and non-Bt corn for damage
  • Field Investigation – verify trait(s) present, evaluate presence and damage caused by ECB, rule out external reasons for damage
  • Contact Seed Company – seed company representative must be informed if ECB damage is found in Bt-traited crop, where it is determined the pest is resistant
  • Best Management Practices
  • Collect Insects – the seed company will likely arrange for live ECB samples to be taken from affected field(s)
  • Resistance Mitigation – if resistance is confirmed, farmer will be notified of next steps (see Managing Resistance in your Bt Corn)

Resources have been developed to help farmers, agronomists and seed companies identify issues in Bt corn fields as resistance incidents have occurred in Canada. Canadian Corn Pest Coalition is a group of industry members that work to develop extension and support to Canadian farmers and industry as insect issues arise. The CCPC has extensive resources available on their website on this specific topic, as well as other insect pests in corn. It is important for members of the corn industry to be educated on pest pressures that could turn into serious resistance incidents. Together we can improve the longevity of Bt traits so farmers can continue to use them safely and effectively.

Act Now ECB Campaign – Canadian Corn Pest Coalition

Contact your provincial Extension Entomologist (John Gavloski, Manitoba Agriculture) or MCA’s Agronomy Extension Specialist – Special Crops (Morgan Cott) for further information on European corn borer resistance, what to do to avoid it, and how to determine if you see possible resistance.

Phantom Yield Loss

Phantom Yield Loss – A phenomenon related to yield loss with little to no explanation why, aside from letting the grain dry naturally, prior to harvest.

Farmers who have to pause harvest, after opening a field, do record decreased yields when they return to continue combining. A few things come to mind when considering what the losses are a result of. Could it be ear drop? Or lodging is a common occurrence, the longer the crop stands in the field and is exposed to wildlife, snow, or wind. Another consideration is that as the grain dries in the field, it does loosen from the cob and can fall to the ground when disturbed. Low moisture grain is also susceptible to cracking or breakage at harvest, resulting in losses.

Speaking to Manitoba farmers, it appears that phantom yield loss is caused by none of the above. All the obvious culprits (ear drop, lodging, kernel shattering) can be accounted for and there are yield penalties beyond these factors, still.

Purdue University performed research on this topic in the early 90’s – before the phenomenon even had a name. The project looked at three hybrids over the course of four years and measured kernel dry weight until physiological maturity and again after maturity until they were ready for harvest. The study found that kernel dry weight increased until reaching physiological maturity, which occurred at about 25% moisture for all three hybrids. Following maturity, during the dry down period, kernel dry weight decreased by an average of 1.1% for every one per cent decrease in grain moisture content. This is an average across three hybrids in the four years of study. There was one year where none of the hybrids experienced any significant changes in kernel dry weight. The “bottom line” of the project is that there is a potential average 1% yield loss per point decrease in grain moisture content. That is to say that if a field is left to naturally dry down 5 moisture points following physiological maturity, there is a potential +/- 5% yield loss.

So, how does this occur and why? We now know that this is part of the drying process, but why is so much dry matter being lost as the grain dries a small amount?

Quite simply, physiological maturity occurs and each kernel develops a “black layer” where it connects to the cob and had gained access to nutrients and water throughout the season. Once black layer is achieved, grain continues to use up the starch and sugar reserves, which decreases kernel dry weight and quality. Grain is typically alive following black layer until it has dried down to around 15% moisture, so it is not surprising that this process results in loss of dry matter.

Unfortunately, it is impossible to predict losses as a result of this phenomenon. Factors affecting losses include harvest timing, soil type, hybrid/genetics, and of course, environmental factors. That being said, it is difficult to predict what genetics are most susceptible to respiration losses following black layer, so that isn’t something that would normally factor into hybrid choices. Earlier harvest timing is the best way to avoid significant losses, in this case. As mentioned, losses are impossible to predict, but measuring drying costs against the alternate potential yield loss is key in finding the best management practice for your farm.

Holcus Spot

What is holcus spot?

Holcus spot on corn leaf

Holcus spot on corn leaf

A bacterial leaf disease affecting mainly corn crops, though it can overwinter in both monocot and dicot species. Holcus spot begins as a water-soaked spot on lower leaves and develops into small (1/4 to 1/8 “ in diameter), circular to elliptical, white to tan lesions. Lesions commonly develop a brown margin and sometimes a light halo is visible around the lesions. In severe infections, holcus spot can cause significant lesions on plant leaves, though it is more common to have minor spotting, covering less than 20% of a single leaf’s surface.

Conditions for Development

Holcus infections follow typical Manitoba spring conditions. This includes high winds and heavy rains, followed by extended moisture and warm summer temperatures (24C – 30C). The bacteria is interesting because it infects the leaf via wounding, but it doesn’t need a wound for development. The pathogen also does not spread from an infected leaf to a healthy leaf, as in many other leaf diseases. 

Disease Management

The holcus spot pathogen lives and overwinters on crop residues. Best management practices to gain control of the pathogen are crop rotation and tillage. As a bacterial pathogen, fungicides will have no effect on the disease.

Fortunately, holcus spot affects a very small area of each infected leaf and photosynthesis of the green leaf material is still very effective. This is a concern in more disruptive leaf diseases or killing frosts that affect large areas of each leaf and photosynthesis is allocated to a small area or none at all. As a result of the small area affected, yield is not penalized and holcus spot is more of an aesthetic disease than a concern for farmers. 

Don’t get confused…

Holcus spot infections are relatively uncommon. It is easy to see them and be unsure of what it means because lesions are most often minute and don’t draw attention. 

In the rare occasion that the disease does grab attention, lesions can be confused with drift of a contact herbicide, like diquat (image below), or fertilizer burn. Key tips to determine if it could be fertilizer injury would be to ask the farmer or applicator if anything was applied recently or in the sprayer tank. If there is a possibility of herbicide drift, there will be a clear pattern in the area that would have gotten “hit”. The lesions would likely be worst along the outer rows and lessen the further into the field you look.  Early in the season, injury would not grow with the plant and new leaves would be injury-free. 

Diquat drift on corn leaf

Diquat drift on corn leaf

The Impact of Mid-Season Excess Moisture

It is well-known that spring weather in Manitoba is unpredictable. Farmers endure drought conditions one season and excess moisture the next, never knowing for sure what is ahead. These dubious conditions make crop planning particularly difficult because no one knows what extremes of moisture crops may or may not have to grow through that season.

Generally, crops should endure excess moisture fairly well in early summer, when they are actively growing vegetatively, and environmental conditions are usually conducive to evaporation. The growth curve is quite steep during this time, especially in the large-sized crops like corn and sunflower and their water uptake is generous if conditions are good. Flax is not going to be a crop that tolerates “wet feet,” and it will be evident if it is in standing water for extended periods.

Corn

Corn that is past V6 staging has the growing point above ground, so flooding at this stage isn’t quite as detrimental as it would be at earlier stages. Remember that where there is standing water, there is no oxygen exchange and living cells cannot survive without it for very long. Ideally, conditions do not get too hot (crop stress) and evaporation and/or water drainage can happen quickly. Depending on how many times the flooded areas have been flooded this season, this influences the ability of the crop to “bounce back.” Root death is possible in this scenario and warm, dry soils will be required to generate new root growth. New root growth is possible in corn in these situations, but the new growth will extend horizontally, which leads to a few implications with nutrient uptake and plant stability.

In younger plants, V5 or smaller, being waterlogged for four days would be a maximum time span to survive and recover. It is harder to determine what that is for larger plants that are growing much more quickly, especially if there have been multiple heavy precipitation events that have left fields saturated and/or puddled. It is also exceedingly difficult to determine what nitrogen losses may be, and even more so when top-dress applications have occurred recently. At this stage and in the days ahead, it would be very important to keep an eye out for nitrogen deficiency symptoms. Corn nitrogen uptake is about 60 per cent of total uptake from the V8 to silking stages, so losing access to nitrogen via leaching or denitrification could seriously impact yield.

Sunflower

Sunflowers are growing rapidly in July and moving quickly into the reproductive stages. At this time, the crop can be using up to 1/3 inch of water each day. It is hard to believe that with this excessive water use that the crop wouldn’t manage saturated soils very well, but the roots do still need to breathe. Photosynthesis also slows down while stomata remain open in wet conditions, which slows plant development. In flooded conditions, sunflowers may have a tolerance for about three-plus days in an anaerobic environment. During those conditions and following, crop recovery is better with cloudy and cool-warm weather rather than hot and sunny weather.

Sunflowers are also very susceptible to stalk diseases during this vegetative growth, including sclerotinia basal rot. Sclerotinia infections can occur anytime between early vegetative stages through to seed fill and generally need precipitation to spread their spores. It is an important consideration for farmers and agronomists and recommended to know the high risk of disease that the crop carries in wet environments.

Flax

Flax has the lowest tolerance to flooding of the three specified crops. It is a small, shallow-rooted crop that does not adapt well to extreme conditions, nor does it have a need for high amounts of water to grow. If it remains in standing water for longer than three days, flax will become stunted, yellow and there will be a high risk of yield loss.

Flax requires the bulk of its water during flowering and seed fill, at roughly 0.28 inches/day. It is also known that dirty (weedy) flax fields use water much less efficiently than clean flax fields. The one benefit to flax in wet fields is that it is not as susceptible to stem diseases as most other Manitoba oilseeds, therefore wet conditions are not a matter of concern with regards to yield or quality loss due to disease.

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