2019 Louisiana row crop insect pest outlook

By Blake Wilson and Sebe Brown

Populations of pest insects are often influenced by factors at play well before crops are planted. Winter mortality is a key regulator of spring insect populations, particularly for pests introduced from tropical regions. The 2018-2019 winter was mild, especially in the southern half of the state, where temperatures didn’t dip below a light frost. Here we provide a crop-specific breakdown of which pests will benefit from these conditions.

Rice: The rice water weevil and the rice stink bug aren’t heavily influenced by winter temperatures and are consistent threats to rice year in and year out. Populations of stem borers may be above average, particularly in southwest Louisiana. South American rice miner populations also may be heightened. Warm winters in the past have led to issues with this pest in late-planted rice. Rice scouts should be on the lookout for the rice delphacid, a plant hopper from Central and South America. Although this pest is not thought to be in Louisiana, it has been damaging ratoon rice fields in Texas in recent years. More information on all rice insects is available at https://www.lsuagcenter.com/topics/crops/rice/insects.

Sugarcane: The primary pests of sugarcane, the sugarcane borer and the Mexican rice borer, will have survived well this winter. Plenty of weedy grasses and unharvested sugarcane in some areas have remained green through the winter, providing a good overwintering environment. Spring populations of borers are not a major threat to cane, but if spring conditions are favorable as well, we may be looking at elevated borer populations this summer. Green cane in the field also benefits the West Indian canefly. Canefly outbreaks followed mild winters in 2012 and 2016. Aphid populations can increase rapidly and tend to respond to environmental conditions in the spring and summer rather than winter.

Soybeans: The primary true bug pests of soybeans are southern green stink bugs, brown stink bugs and redbanded stink bugs. Redbanded stink bugs are an invasive species from South America and do not overwinter well when temperatures are abnormally cold, as they were in the winter of 2018. However, agriculture professionals should be mindful of potentially large populations of redbandeds moving into soybeans this year due to a mild winter. Southern green and brown stink bugs, which are native to Louisiana, are consistent threats to soybeans from year to year. Worm pests such as velvetbean caterpillars and soybean loopers do not overwinter in Louisiana, and winter temperatures have little influence on pest populations during the growing season.

Corn: Corn earworms and fall armyworms are the primary ear feeders that affect Louisiana corn. Corn earworms overwinter in Louisiana. Mild winter temperatures typically promote earlier emergence, which can lead to larger populations in the summer. Fall armyworms do not overwinter in Louisiana, and migration occurs from southern Texas and Florida. Louisiana winter temperatures have little impact on fall armyworm populations during the growing season. Southwestern corn borers and sugarcane borers both overwinter in Louisiana in crop residue. These pests are consistent threats to corn year to year.

Cotton: Tarnished plant bugs are the primary true bug pest of cotton. Tarnished plant bugs are native to Louisiana, and winter temperatures do not significantly influence population numbers during the growing season. Bollworms are the primary worm pest of cotton, and mild winter temperatures typically promote earlier emergence, which can lead to larger populations in the summer. Spider mites and cotton aphids are secondary pests that overwinter in Louisiana, making them consistent threats to cotton every year.

​Carefully select cotton nitrogen rates

By Dan Fromme

Once the cotton stand has been established, nitrogen applications should be made for the upcoming season. Recommended nitrogen rates are 60 to 90 pounds per acre for coarse-textured soils and 90 to 120 pounds per acre for high clay soils. The lower recommended rates should be used on fields that are following soybeans, corn and legume cover crops as well as those with a history of excessive stalk growth.

Caution should be used to not apply more nitrogen than what cotton plants will require. Applying too much nitrogen can lead to very tall and rank cotton. This increased vegetative growth will hinder reproductive growth and, ultimately, yield. Furthermore, to limit this excessive growth, producers will have to rely heavily on mepiquat chloride applications. This creates the potential for making the cotton plant harder to defoliate at the end of the season. Excessive nitrogen, especially in combination with high amounts of late-season rainfall, can delay maturity; reduce harvesting and ginning percentages; and promote boll shedding and boll rot.

Best management practices suggest making split applications of nitrogen, especially on sandy soils with a high leaching potential and soils with a high saturation potential, due to denitrification losses. For split nitrogen applications, a third to half should be applied at planting, with the remainder being applied by early bloom at the latest.

Table 1. Nitrogen rates for cotton in Louisiana

​Look for corn nutrient deficiency symptoms

By Dan Fromme

Deficiencies are not always due to low nutrient amounts in the soil. Factors such as poor root development, root damage, poor soil conditions (dry, water logged or compacted), and unfavorable weather and growing conditions can induce nutrient deficiency symptoms.

This year’s growing conditions have been cool and wet, which has contributed to early season deficiencies of nutrients including zinc, sulfur and phosphorus. Signs of these deficiencies are showing up frequently in corn fields.

Figure 1. Leaves of young phosphorus-deficient plants are bluish-green and slightly narrowed, turning reddish-purple starting at the tips and along the edges. Photo provided by the International Plant Nutrition Institute.

Figure 2. Symptoms of zinc deficiency appear as yellow to white bleached bands on the lower part of leaves. The mid-vein margins and tip remain green. The deficiency is favored by high soil phosphorus; high pH; cool, wet soil; and low organic matter. Photo provided by the International Plant Nutrition Institute.

Figure 3. Sulfur deficiency causes stunted, slow-growing and yellow plants. Yellowing occurs between the veins, especially in upper leaves. Photo provided by arkansas-crops.com.

Use leaf collar method to track corn growth

By Dan Fromme

Vegetative (V) stages are determined by the total number of leaves with visible collars, beginning with the first leaf. A collar is the off-white band at the base of the leaf blade, where it extends away from the stalk (Figure 1). Throughout the vegetative stage, a new leaf will appear approximately every three to five days.

At right, Figure 1 shows a collar, which is the off-white band at the base of the leaf blade.

As the season progresses, lower leaves will be lost. These leaves must be included in your leaf count, or development stage will be misidentified. For an accurate assessment of the leaf stage, the stalk must be split lengthwise (Figure 2).

Each leaf will be attached to a single node, and the nodes are visible as lines across the split stalk. The first four nodes are usually indistinguishable within the crown. The fifth node is less than one-quarter inch above the area that contains the first four nodes.

The node corresponding to the uppermost leaf with a visible collar defines the vegetative stage.

Below, Figure 2 shows a corn stalk that has been split lengthwise. Splitting the stalk is necessary to accurately assess the leaf stage.

Dealing with hail damage to young corn

By Dan Fromme

Evaluation of hail-damaged fields should not be attempted the day after the storm. It can be difficult to predict survivability of plants by simply looking at the damage itself (Figure 1). Young corn has an amazing capacity to recover from early-season damage, but patience is required to allow the damaged plants enough time to visibly demonstrate whether they will recover.

Damaged but viable plants usually will show noticeable recovery from the whorl within three to five days with favorable temperatures and moisture conditions. Initially, the growing point (one- to four-leaf collar stage) is located below ground and is protected from hail damage (Figure 2). Not until about the five- to six-leaf collar stage is when the growing point begins to elevate above ground.

The growing point region is important because it is responsible for creating all the leaves and the tassel of a corn plant. Healthy growing points will be white or cream-colored; dead points will be dark and/or flaccid. Therefore, corn plants damaged by hail early in the season (before the seven-leaf collar stage) usually recover from the tie whorls and bruised stalks.

Figure 1. Hail-damaged corn. Photo by Bruce Garner/LSU AgCenter.

Figure 2. At the four-leaf collar stage and earlier, the growing point on corn is below ground. LSU AgCenter photo

LSU AgCenter Specialists