Worms, Grubs, Borers, and Beneficials
By: Dr. Chris DiFonzo, Field Crops Entomologist, Michigan State University
Bugs have been quiet so far this year, until the last two weeks. Below I summarize a few that I’ve had calls about.
1) True Armyworms …Are at least an inch or so in size. Damage, frass, and caterpillars are more obvious. Moth flight and egg laying in weedy occurred in parts of fields and in rye cover crops back in May. All the calls and emails I’ve had to date involve infestations linked to rye. The small larvae are not troublesome or noticeable until the rye is killed and they concentrate onto the crop. Armyworms also lay eggs in wheat. I haven’t had any confirmed cases over threshold in that crop, but there are likely infested fields out there.
Now is the time to check both corn and wheat for infestation, because insect larvae consume the most food and do the most damage in the last stages of development. In corn, peak in the whorl or check the base of the plant for larvae. Larvae like to hide during the day, but tattered leaf feeding and frass in the whorl are a sign they may be present. In wheat, leaf feeding will be visible upon the plant, but larvae often are on the ground on sunny days (or get knocked to the ground as you walk through the canopy). Shake the plants and move the residue around to find larvae and frass pellets on the soil surface. The caterpillars are still small enough to be killed with a spray, thus avoiding significant damage from the bigger, late-stage dudes. That said, I can’t stress enough that it’s a waste of money to spray insecticide because ‘just because’. For example: the sprayer is going over the field anyway, or some guy at the Co-op said so, or Chris mentioned armyworm in this Fonz Facts, or my cousin from Frankenmuth said his neighbor Olaf’s friend’s son sprayed for armyworm yesterday. Armyworm infestations in corn and wheat are often patchy based on the pattern of weather fronts that carried moths north. Infestations in corn are additionally related to predictable environments (weediness or cover crops) that lead to egg laying. It is efficient to tank mix insecticide with a herbicide or fungicide application, but only if armyworms are over threshold (Corn: 25% of plants with 2 or 75% of plants with 1 larva per whorl / Wheat at heading: at least 2 larvae per square foot & watch preharvest intervals). Otherwise, you are likely just killing early-season natural enemies (see #4 below).
How about soybeans planted into rye? Do not spray. Armyworms like to eat soybeans as much as kids like to eat broccoli. See the link below for a video from Purdue showing starving armyworm in a soybean field.
For more detailed info on armyworm:
**Bulletin for CORN https://www.canr.msu.edu/fieldcropsent/uploads/files/27AWCorn.pdf
**Bulletin for WHEAT https://www.canr.msu.edu/fieldcropsent/uploads/files/26AWWheat.pdf
2) Asiatic Garden Beetle – AGB grub damage is at a high point now, and should be visible in infested fields in the AGB ‘risk zone’ of counties in southern Michigan, northern IN, and northern OH. Risk factors for infestation include sandy soil, poor marestail control, and corn following soybean or potato. In addition to stand loss, a hallmark of AGB infestation is an uneven stand, with adjacent plants varying from several inches to knee-high in height. Unevenness results in yield loss, because the short plants remain out of synch at pollination and their ears may not fill fully. If you are assessing poor or uneven stands in the risk zone the next few weeks, check for root pruning and grubs. Feeding is at or near the end, however, but you may find pupae or adults. Adult beetles, which are active only at night, start to emerge in early July. By that time, it is harder to confirm AGB as the culprit of a poor stand, but signs of infestation include round emergence holes in the soil and dead beetles on the soil surface between the corn rows. On the field edges, pull up weeds like marestail, ragweed, and Queens Anne’s lace to check for beetles hiding out during the day. As usual, I appreciate getting calls or emails about fields infested with this insect, as MSU and OSU are cooperating on a project to manage AGB. While travel restrictions shut down our work this spring, we are already thinking ahead to strip trial locations for 2021.
**For more info and pictures, see AGB Identification and Management in Field Crops posted at https://aginsects.osu.edu/sites/aginsects/files/imce/AGB%20in%20field%20crops%202019.pdf
3) European corn borer – I saw moths in wheat last week on campus, and I had a call about moths flying in SW Michigan. Most of the corn acres in the state are planted to Bt hybrids, and Bts still work well to control ECB. But there are more people growing non-Bt and organic corn than in the recent past. If this describes you, now is the time to plan to scout in the next couple of weeks. Because of a natural feeding deterrent in plants, larvae survive poorly on corn that is V6 stage or less. Survival increases on taller corn, and moths seek out older corn to lay eggs of the first generation. Thus target the earliest-planted fields for scouting, those in the V6-V16 range. [Note that SECOND generation ECB will occur later in the season, and it is attracted to the YOUNGER (later planted) corn.]
Scouting first generation ECB is easier than scouting for other caterpillar pests of corn later in the season when the corn is tall. It involves assessing consecutive plants for windowpane feeding, shotholing, and frass in the whorl, plus live larvae found by unrolling the whorl. A high percentage of small larvae in the whorl can die due to temperature extremes or heavy rainfall, so it is important to confirm that larvae are still present. A crude rule of thumb is to treat if 50% of the plants have feeding and live larvae are present. However, there are much better thresholds to manage ECB, which are too complicated to discuss here. If you are growing or scouting non-Bt corn, I suggest purchasing the ECB bulletin noted below, recently revised by corn entomologists across Midwest. You also should consider trapping for corn borer moths on your own farm, because the timing of scouting is based on knowing the date of first moth catch.
** Extension bulletin: ‘European corn borer ecology, management and associations with other corn pests’, available for $8 through the Iowa State University Extension store, https://store.extension.iastate.edu/. Search for the title or the bulletin number NCR-327.
** Trapping supplies are available from Great Lakes IPM in Vestaburg MI. https://www.greatlakesipm.com/ or (800) 235-0285
4) What are these things? Should I spray them?
I got a bunch of photos of mystery critters emailed to me this spring, with a question “should I spray this?”. You can see a few in the accompanying, labeled A-B-C. In these cases, the answer is 100% NO, these are all beneficial insects. The gelatinous creature in Picture A is a larva of a syrphid fly (are also known as a hoverfly). These larvae are common on wheat this spring. The adult flies lay eggs near aphid colonies on leaves. The small larvae ooze across the leaf searching for aphids to attack. Picture B is a juvenile ladybug, also common on wheat this year. The larvae look completely different from the adult beetle. But like the adult, they are voracious predators of aphids and other small insects. The pictures labeled C are ladybug pupae. Once the ladybug larva completes development, it attaches its butt to the leaf surface and goes thru one final molt, shedding its outer skin (you can see the black molted skin with the orange specimen). It sits quietly, attached to the leaf, for a few days and then the adult emerges.
Why is this important? Syrphids and ladybugs are valuable natural enemies. Their populations start and build upon wheat in the spring, then they move to other crops as wheat dries down. Your lack of soybean aphids or western bean cutworm caterpillars in July is partially due to the natural enemies raised in wheat right now. This is why entomologists are always nattering on about using thresholds to manage pests like armyworm or avoiding throwing an insecticide in the tank just because you are going over the field. Most insect control is natural and free, and wheat is an important source in Michigan.