Author: kcook8@illinois.edu

Average winter temperatures in Illinois this past winter were very similar to what we saw during the winter of 2020-2021. (Figure 1). Warmer temperatures during the months of December, January and February favor increased survivability of the corn flea beetle and subsequently, the early season prevalence of Stewart’s Wilt, particularly in the southern half of the state.

Figure 1. Average winter temperatures of 2020-2021 and 2021-2022.

 

Corn flea beetles are the primary vector of Stewart’s Wilt. Erwinia stewartii, the bacterium that caused Stewart’s wilt, survives the winter in the gut of the corn flea beetle and the survival of the corn flea beetle is dependent on winter temperatures. Warmer winters result in greater survivorship of corn flea beetles, thus increasing the potential for Stewart’s wilt, while colder temperatures adversely affect it. Using the average temperatures of December, January, and February, the potential for Stewart’s wilt can be predicted (Table 1).

Table 1. Projected risk of Stewart’s wilt based on the average temperatures of December, January, and February.

Average temperature of December, January, & February	Probability of early season wilt	Probability of late season blight
<27° F	Absent	Trace, at most
27-30° F	Light	Light to Moderate
30-33° F	Moderate	Moderate to Severe
>33° F	Severe	Severe

 

 

 

Corn flea beetles become active in the spring when temperatures rise above 65°F, and they feed on and transmit Stewart’s wilt bacteria to seedling corn plants. The bacterium can spread systemically throughout the plant. Although most commercial field corn hybrids are resistant to Stewart’s wilt, the disease is still a concern for susceptible seed corn inbreds and many sweet corn hybrids.

 

There are two phases of Stewart’s wilt: the seedling wilt phase and the leaf blight phase. The seedling wilt stage occurs when seedlings become infected at or before the V5 stage. The vascular system becomes plugged with bacteria, causing the seedling to wilt, become stunted, and die. Infections of older corn plants usually result in the development of the leaf blight phase of Stewart’s wilt. This phase is characterized by long, yellow to chlorotic streaks with wavy margins along the leaves. When the late infection phase or “leaf blight phase” of Stewart’s wilt occurs after tasseling, it is generally not a concern in sweet corn because ears are harvested before damage occurs.

Based on the recent winter temperatures from the Midwest Regional Climate Center, estimates of early season Stewart’s wilt are shown in Table 2. Remember, however, that these are only predictions; numbers of surviving corn flea beetles are not known.

 

Table 2. Early season Stewart’s wilt predictions, 2022

Location	Average temperature December 2021-February 2022	Early Season Wilt
Freeport	23° F	Absent
DeKalb	24° F	Absent
Monmouth	26° F	Absent
Peoria	28° F	Light/Moderate
Champaign	30° F	Light/Moderate
Springfield	30° F	Moderate
Newton	35° F	Severe
Belleville	35° F	Severe
Mt. Vernon	37° F	Severe
Carbondale	38° F	Severe
Dixon Springs	38° F	Severe

 

 

 

We are happy to announce that the 6th annual Illinois Invasive Species Symposium will be held on May 23 at the Champaign County Extension Auditorium in Champaign, IL. Mark that date in your calendars because this event will provide an opportunity to learn about projects and programs underway to address all taxa of invasive species that are impacting Illinois’ natural lands and native species.

Registration will open for the symposium in late April.

We are now accepting abstract submissions for presentations.

Lots of questions about the “brown moths” that have been very active around sunset across several central Illinois towns this week. These are armyworm moths. Armyworm moths migrate from the south each spring. They mate and lay eggs in grass and weedy areas. Primarily agricultural pests, their offspring feed in corn, wheat, and other plants. The larvae progress through several larval stages before pupating and emerging (what everyone is seeing now). We generally have 2-3 generations of this insect pest in Illinois. They primarily feed on grasses, but you might see them on a few vegetables. Adults feed on nectar of various flowers.

Several reports have come in detailing severe defoliation of viburnum. Upon closer inspections, homeowners found the culprit to be small larvae on the undersides of the leaves.

The viburnum leaf beetle, a native to Europe, was brought to North America on infected viburnums. While it is established in several northeastern states, we’ve only confirmed this pest in a handful of counties in northeast Illinois. If your viburnum plants are showing signs of defoliation, please keep your eyes open for larvae now or Viburnum Leaf Beetle adults during the summer.

Viburnum leaf beetles overwinter as eggs on twigs of the host plant. In May, larvae hatch from the overwintered eggs and begin feeding on host plants. The larvae are greenish-yellow and develop dark spots as they age. They are usually found feeding in groups. Between early and mid-June, larvae drop to the ground and pupate. They remain in the ground for about 10 days. Adult emergence generally occurs from mid- to late July. The adult beetle is small, ¼ to 3/8 of an inch long and is a golden brown color with sheen when in sunlight. Adults will remain active until the first frost. Development from egg to adult takes eight to ten weeks. In late summer and fall, females will begin laying eggs. They chew holes in the bark of twigs to deposit eggs and then cover them with excrement and fragments of chewed bark. A female can lay up to 500 eggs.

Feeding is limited to species of viburnum. The viburnum leaf beetles have a preference for viburnums with little hair (pubescence) on the foliage, including the European cranberrybush viburnum, arrowwood viburnum, and American cranberrybush viburnum. They also feed on wayfaringtree viburnum, Rafinisque viburnum, mapleleaf viburnum, nannyberry viburnum, and Sargent viburnum. There are several resistant varieties, including Koreanspice viburnum, Burkwood viburnum, doublefile viburnum, Judd viburnum, lanatanaphyllum viburnum, and leatherleaf viburnum.

Table 1. Preliminary list of viburnum that are relatively susceptible or relatively resistant to viburnum leaf beetles (compiled by Dr. Paul Watson, Cornell University).

Highly susceptible: V. dentatum complex, arrowwood viburnums V. nudum, possum-haw, smooth witherod viburnum V. opulus, European cranberrybush viburnum V. opulus var. americana (formerly V. trilobum), American cranberrybush viburnum V. propinquum*, Chinese viburnum, Taiwanese viburnum V. rafinesquianum, Rafinesque viburnum Highly susceptible species are the first to be attacked, and are generally destroyed in the first 2-3 years following infestation.

Susceptible: V. acerifolium, mapleleaf viburnum V. lantana, wayfaringtree viburnum V. rufidulum, rusty blackhaw, southern black-haw V. sargentii, Sargent viburnum V. wrightii, Wright viburnum Susceptible species are eventually destroyed, but usually are not heavily fed upon until the most susceptible species are eliminated.

Moderately susceptible V. alnifolium (syn. V. lantanoides), hobblebush V. burkwoodii, Burkwood viburnum V. x carlcephalum, Carlcephalum viburnum V. cassinoides, witherod viburnum V. dilatatum, linden viburnum V. farreri , fragrant viburnum (except ‘Nanum’, which is highly susceptible) V. lantanoides (syn. V. alnifolium), hobblebush V. lentago, nannyberry viburnum V. macrocephalum, Chinese Snowball Viburnum V. x pragense, pragense viburnum V. prunifolium, blackhaw viburnum V. x rhytidophylloides, lantanaphyllum viburnum V. tinus*, laurustinus viburnum Moderately susceptible species show varying degrees of susceptibility, but usually are not destroyed by the beetle.

Viburnum most resistant to the viburnum leaf beetle: V. bodnantense, dawn viburnum V. carlesii, Koreanspice viburnum V. davidii*, David viburnum V. x juddii, Judd viburnum V. plicatum, doublefile viburnum V. plicatum var. tomentosum, doublefile viburnum V. rhytidophyllum, leatherleaf viburnum V. setigerum, tea viburnum V. sieboldii, Siebold viburnum Resistant species show little or no feeding damage, and survive infestations rather well. Most species in all susceptibility groups exhibit more feeding damage when grown in the shade.

(Source: http://www.hort.cornell.edu/vlb/suscept.html)

The most effective means for controlling VLB is to prune infested branches in the fall. However, reducing the larval population will ultimately reduce the adult population that lays the eggs.  Products containing carbaryl (Sevin) as the active ingredient or one of the pyrethroids (cyfluthrin, permethrin, resmethrin) are effective as foliar sprays.

In regards to other management options, Cornell has a very helpful management guide for homeowners (http://www.hort.cornell.edu/vlb/manage.html). At this time of year, for homeowners who are experiencing defoliation, the best option is going to be pesticides. It is important to make sure larvae are present and to make a thorough application so the pesticide comes in direct contact with the larvae. Spraying adults or eggs is less effective. There is some information on the use of horticultural oils and insecticidal soaps (http://www.hort.cornell.edu/vlb/newtools.html).

 

Lee County recorded a significant moth flight this week. It is important to note that several counties recorded an increase in black cutworm moth activity after storms passed through Illinois mid-week, even though only one recorded counts high enough to be deemed significant. Projected potential cutting dates are identified in the map below. Remember, these dates are just estimates, scouting should occur before and after the potential cutting dates.

True armyworm numbers remain low; these numbers are increasing as well.

	Weekly Moth Total (April 28-May 4)
Northern (Lee County)	0
West Central (Warren County)	19
East Central (Champaign County)	20
Southern (Madison)	30

Spring insect activity is off to a slow start, but is expected to pick up with the recent warming trend.

Piatt, Madison, Montgomery, and Sangamon counties all recorded significant moth flights this week. Projected potential cutting dates are identified in the map below. Remember, these dates are just estimates, scouting should occur before and after the potential cutting dates.

True armyworm numbers have been very low. Armyworm traps are just getting going, so reports for this week are broken down into a general area of the state.

	Weekly Moth Total (April 21-27, 2018)
Northern (Lee County)	4
West Central (Warren County)	34
East Central (Champaign County)	53
Southern (Madison)	48

 

Matt Montgomery, Pioneer, reported alfalfa weevil activity in Shelby county this week. Degree-day accumulations indicate that early instar alfalfa weevils may be present in the southern third of the state. Injury may consist of pinhole size feeding on the leaves.

 

As the growing season gets underway, we encourage you to share field observations with both Nick Seiter (nseiter@illinois.edu) and Kelly Estes (kcook8@illinois.edu).