Jim Jasinski (Extension), Ashley Leach (Entomology)

It’s been cool and wet in most of Ohio slowing most planting schedules but cucurbit planting is poised to hit it’s stride toward the end of May through mid-June. This means growers need to be on the lookout for the primary early season pest of pumpkin, squash, melons, cucumbers and zucchini, the striped cucumber beetle.

Striped cucumber beetle adult.

These adult beetles are overwintering now but will begin actively searching for cucurbit seedlings to feed on, sometimes inflicting severe enough damage to outright kill plants. Recall that while seedlings can survive and outgrow minor beetle damage, it is key to avoid severe damage to seedlings in order to prevent bacterial wilt transmission while the plants are most susceptible, typically prior to the 3-4 leaf stage. Bacterial wilt infected plants will become symptomatic once there is high demand to translocate water from the roots to the shoots, such as the time of fruit enlargement. No treatments are available to reduce bacterial wilt once a plant is infected.

Bacterial wilt infected plant in foreground, healthy plant in background.

Scouting newly emerged cucurbit plantings every few days is essential to determine if enough beetles or damage is occurring to warrant treatment. Action thresholds vary from 0.5 – 1 beetle per plant for cotyledon and 1^st leaf stage seedlings to 1-2 beetles per plant for 3-4 leaf stage seedlings. Scout about 50 plants in both edge and interior areas throughout the field, flipping over leaves and especially looking under cotyledons to accurately determine beetle pressure and damage.

Severely damage cotyledon by striped cucumber beetle feeding.

Foliar insecticide recommendations for all cucurbit crops can be found in the Midwest Vegetable Production Guide (https://mwveguide.org/uploads/pdfs/Cucurbit-Crops.pdf).

To prepare for the arrival of striped cucumber beetles, consider reviewing a short but detailed video of several management options (beetles/plant thresholds, systemic seed treatment, use of transplants and in-furrow application) posted to the OSU IPM YouTube channel (https://youtu.be/RSzTT_gbma4).

A quick word about using insecticides to manage early season beetle populations. Based on your farm history with damage from this pest, field size, time of direct seeding or transplanting, you may not experience peak beetle pressure and can manage this pest by frequently scouting seedlings and treating if over threshold, the old fashioned IPM method!

If you purchased systemic insecticide coated seed (FarMore FI 400) which is very effective at controlling beetles, as evidenced by the pile of dead cucumber beetles on and near the treated plant, be aware that trace residues will accumulate in the pollen and nectar. If foraging honey bees, bumble bees, squash bees and other pollinators collect these food resources they may not be outright killed but more subtle sub-lethal effects on brood such as reduced feeding, fewer wax cells constructed, fewer eggs laid and other effects might occur. So, decisions about pesticide selection, pest severity, timing and non-target impacts should be considered before use.

Dead cucumber beetles at base of systemic insecticide treated plant. Courtesy of Celeste Welty.

If using FarMore FI 400 treated seed to raise transplants, do not treat them again with a systemic insecticide product during field setting as this will increase the residues found in pollen and nectar. If applying systemic products in-furrow, using the lowest labeled rate will still provide great beetle control for 2-3 weeks.

Have you visited your vegetable field lately and come back disappointed because you were met with wilted, drooping plants? You are not alone. Recently, we have had an uptick in reports of maggot damage in vegetable crops. The insect culprits in question are most likely either onion maggot, seedcorn maggot or cabbage maggot. These maggots are very similar, and even belong to the same fly genus, Delia. These cream-colored maggots are small (0.5-1.5 cm) and have between 3-5 generations per year. These fly species will overwinter as pupae in the soil and emerge as adult the following year to find suitable host plants. Maggot will feed on seedlings and either kill the plant before it can successfully mature or injure the plant, thus giving entry to soil pathogens (secondary infections). This past season, you may have noticed more damage from maggots than normal. And that’s not surprising; maggot damage is typically greater in cool, wet seasons and in fields with high organic soil types.

There are some differences between these species that may point to one being the cause over another. Onion maggots love alliums, and are most problematic in onion, garlic, and leek. Cabbage maggot has an affinity for brassica crops including cabbage, cauliflower, Brussel sprouts, and turnips. Seed corn maggots love just about everything and can be found in as many as 40 different plant hosts. Notable crop hosts for seed corn maggot include soybeans, corn, beans, peas, cucumber, melon, pepper, potato, and even onion. As a general rule, seedcorn maggots typically damage the seed, whereas onion/cabbage maggots often feed on seedling roots.

The bad news is that if you are facing maggot damage there is little you can do to “rescue” your planting. Your best bet is to wait it out and replant if possible. You can drench with Diazinon (Diazinon AG500), but this product won’t ultimately save you from the damage that has already been afflicted. If you decide to use this product, make sure you use enough water. Diazinon doesn’t move easily through the soil and is best applied with adequate water.

Avoid “chasing” adult flies. You may see adult flies (figure 1) in your field but using foliar insecticides to kill adult flies is not an effective option for any species. Keep in mind the damage is in the soil, so make sure you target your management decisions to strategies that will protect the below-ground tissues of the plant (I.e., seed treatments or in-furrow applications at planting/transplanting).

If you are dealing with maggot damage on your farm, consider some of the options below.

1. Prevention is key. If you know you have a history of either seed corn maggot or onion maggot, make sure you take action by preventing an infestation before it starts.
   • Rotate your crop. Flies will show up when they know food is available. So do your best to confuse the flies by rotating your crops, especially alliums and brassicas. If you want to limit future infestations, consider planting a non-host crop to decrease the likelihood of subsequent maggot problems. If you are rotating your crop to a non-host, make sure you rogue out any volunteers from the previous year. (Maggots love volunteers!)
   • Use a seed treatment (Table 1). Insecticide treated seed is one of the most effective tactics to manage maggot populations. A number of efficacious products are available including thiamethoxam+ spinosad (FarMore FI500), cyromazine (TRIGARD), and clothianidin and imidacloprid (SEPRESTO) for many vegetable seeds (table 1). Rotate products between years so you are not exposing multiple generations to the same active ingredient. For example, if you are using FarMore in year 1, rotate to a different seed treatment like Trigard or Sepresto in year 2. WHY DOES THIS MATTER? Reports from the Northeast and MidAtlantic suggest that some maggot populations may become resistant to these seed treatments.
     

     Table 1: seed treatment options to manage  maggot infestations in vegetables. Please note that efficacy of these products may differ based on maggot infestation and/or soil type.
     Product	OMRI listed?	Active ingredient	Relative control of maggot	IRAC codes
     FarMore FI500	No.	thiamethoxam+ spinosad	Excellent.	4A, 5
     Trigard OMC	No.	cyromazine	Excellent.	17
     Sepresto 75 WS	No.	clothianidin+ imidacloprid	Good.	4A, 4A
     Regard SC	Yes.	spinosad	Excellent/Good.	5

2. Exclude flies from the crop. One viable management approach is to keep female flies from finding your crop. You can isolate your crop either in space (row cover) or time (degree day modeling).
   • Consider using row covering over your susceptible crops to stop adult oviposition (egg-laying). Multiple studies have found that this is a highly effective method at limiting damage.
   • Avoid maggot damage altogether by planting later in the season to bypass peak infestation. Maggots have predictable phenological patterns, and you can use degree day models to accurately predict times in the season when maggot risk is high. The first peak of seedcorn maggot occurs earliest in the season when 200 degree days has been accumulated, followed by cabbage maggot (250 degree days) and then onion maggot (250-300 degree days).
3. Monitor, monitor, monitor. While there is little you can do to manage maggot infestations within the immediate growing season, it’s important to identify problem areas so you can plan accordingly for the following year.
   • The best way to tell if you have Delia maggots on your farm is to scout early and often. Fields with poor plant emergence or wilted seedlings (figure 1, video) should be inspected for maggot damage. Make sure you cull any infested plants.

On the heels of the VegNet blog article posted a few days ago (https://u.osu.edu/vegnetnews/2022/08/13/corn-earworm-flight-numbers-spike/), corn earworm flights continue to be very high in southwest Ohio. In the past 10 days, over 1,000 moths have been trapped at the research station in South Charleston, including 424 moths caught from Friday (12^th) through Monday (15^th).

This pile is what 424 CEW moths looks like.

The Hartstack trap (large metal mesh) used at the research station is placed near fresh silking sweet corn which is very attractive to CEW moths and known to catch more moths than a Heliothis trap (white plastic mesh). Be sure to use the CEW management chart, which is based on the Heliothis trap, when making management decisions. Traps placed away from fresh silking corn will not catch as many moths.

These large moth flights have yet to be recorded in the northern county tier of traps in the network (https://u.osu.edu/jasinski.4/pestvisualization/#linke). Growers who are running Hartstack or Heliothis type traps to manage their insecticide spray intervals are advised to check their traps every few days for the next few weeks.

CEW flights in monitoring network.

Clark County has reported a massive spike in corn earworm (CEW) this week.

386 CEW moths caught during past week.

Other trapping locations have not reported an increase in activity yet (https://u.osu.edu/jasinski.4/pestvisualization/#linke). This annual caterpillar pest of sweet corn and tomato should have growers paying special attention to these crops for infestation, especially during the latter part of the season. Flights of CEW typically increase through the summer as the moth migrates into Ohio from southern states such as Louisiana, Texas and Mississippi. The caterpillar stage is known to attack silking corn and tomato fruit, especially when other susceptible hosts are not abundant.

CEW, aka tomato fruitworm, feeding on tomato.

Moths prefer to lay eggs on fresh sweetcorn silk and can hatch and enter the ear tip as quickly as two days under ideal weather conditions, where they are no longer susceptible to insecticide treatments. Treatments for this pest should be made based on number of moths per day and daily temperature threshold of 80F. Use the chart below to determine the recommended number of days between insecticide application to protect sweet corn as it enters silking. Insecticide options can be found in the Midwest Vegetable Production Guide, remember to rotate insecticide classes to avoid resistance (https://mwveguide.org).

CEW spray interval chart.

Want to learn more about monitoring for CEW? Check out this original video that reviews how to set up a trap to monitor for CEW (https://www.youtube.com/watch?v=W6b7OtUOo8Y&list=PL0HRPaZDLHyG53DPisl9iGTezcx815u-q&index=8).

There is also an updated version of how to monitor for several key moths in sweet corn on the OSU IPM YouTube channel https://youtu.be/X1jvQxx_fpc

As noted by Sally Miller last week, bacterial wilt and yellow vine decline are being found in cucurbit fields across the state. There are two primary insects responsible for these outbreaks, the Striped cucumber beetle (Acalymma vittatum) and squash bug (Anasa tristis). I was just scouting some of my pumpkins this past week and counted 20 beetles in one flower! But sometimes looks can be deceiving as we can encounter as many as 4 different types of beetles in our cucurbit fields. It’s important to know what’s in your cucurbit since it could be the difference between making an insecticide application (or not). Below, I have included an image of different beetle species you may encounter in your cucurbit fields. As a reminder, we generally want to make an insecticide application when the striped cucumber beetle density exceeds 1 beetle/plant in a field. If you want information about specific products, check out my former post here.

Squash bugs are arguably easier to scout for since there aren’t many other insects that resemble them. However, we need to keep track of different squash bug life stages (shown below). Squash bug eggs are fairly diagnostic with a bright amber coloring. They are typically found along the midribs on the undersides of leaves. Those egg masses eventually give rise to nymphs which are powdery blue. Adults have a flattened appearance and are typically brown with alternating white and orange spots along their abdomen. Insecticide applications are warranted when squash bugs exceed a cumulative threshold of 1 egg mass, nymph or adult bug/ plant in a field.