Japanese Beetle Adult Emergence Begins in Southern Iowa

Jun 23, 2022

By Ashley Dean and Erin Hodgson

The cool spring has delayed development of many important crop pests across Iowa. However, with recent warm temperatures, Japanese beetle adult emergence has been on track with previous years. Japanese beetle adults begin emergence when approximately 1,030 growing degree days (GDD; base 50°F) have accumulated since January 1 and will continue emerging until 2,150 GDD have accumulated. Japanese beetle adults likely began emerging in the southern portion of the state last week, and emergence will likely occur within the next two weeks in northern Iowa (Figure 1).

Figure 1. Growing degree days accumulated (base 50°F) in Iowa (as of June 20, 2022). Adults begin emerging around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy

Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. Feeding will look a bit different depending on the host plant, but since beetles have chewing mouthparts, they consume plant tissue and cause defoliation. In field crops, adults tend to be denser at the field edge. Additionally, a combination of sex pheromones, aggregation pheromones, and feeding-induced plant volatiles lead to clusters of Japanese beetle adults feeding in one spot. Since feeding can look severe near the edge of fields, it is important to scout the entire field to get a representative estimate of injury.

On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf, leaving a skeletonized appearance (Photo 1). If adults are still present in the field, the treatment threshold for Japanese beetle in soybean is 30% defoliation before flowering (R1) and 20% after R1. It is important to note that defoliation can be caused by several pest species with chewing mouthparts. Estimations of defoliation should be made for the entire field and plant canopy. Humans tend to overestimate defoliation; use the Crop Protection Network’s Insect Defoliation Tool to train your “defoliation eye.”

Photo 1. Japanese beetle adults defoliate soybean and skeletonize leaves, meaning they leave behind the leaf veins

Photo 1. Japanese beetle adults defoliate soybean and skeletonize leaves, meaning they leave behind the leaf veins.

Japanese beetles do not typically feed on corn leaves, but adults have a long emergence period and remain active throughout the summer. Once corn reaches VT or R1, consider scouting for Japanese beetle. Adults can feed on both the tassels and silks. Silk clipping can interfere with pollination (Photo 2). Consider a foliar insecticide during tasseling and silking if three criteria are met: there are 3 or more beetles per ear, silks have been clipped to less than ½ inch, AND pollination is less than 50% complete.

Photo 2. Japanese beetle adults aggregating and feeding on corn silks near the field edge

Photo 2. Japanese beetle adults aggregating and feeding on corn silks near the field edge.

Source : iastate.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.