Vegetable Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Sweet Corn
Our usual June flight has begun, and will probably remain elevated for the next couple of weeks. Sweet corn should be treated at first silk; do not let the silks and the moths get ahead of your sprayer. Our thresholds can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/insect-trapping/ecb-cew-moth/. Several states also adjust their thresholds based on temperature by decreasing a day from the spray schedule. Once the temperatures reach or go past the mid 80s, it takes less time for corn earworm eggs to hatch. For example, if a moth lays an egg on Thursday evening and it gets into the 90s on Friday and Saturday, that egg could hatch out by Saturday evening. We had a test in 2018 where we sprayed according to pheromone schedule and sprayed with a temperature adjusted threshold. The % of clean ears was greater in the ‘adjusted’ treatment, but the % clean ears and % tip damaged, or ‘processing,’ ears were the same.

We have begun testing moths for signs of pyrethroid resistance using cypermethrin treated vials. These are the same vials that have been used in southern states for years. Last year, moth survivorship between June 10 and July 2 ranged from 8 – 18%. So far, we have tested 40 moths, and our survivorship is higher at 25%. I suspect that the warm but not wet winter has something to do with this. Resistant moths are not as fit as their susceptible counterparts, and so winters that are less conducive for moth survivorship should weed out more of them. Practically, this means that sweet corn spray programs need to be a bit more conservative. Diamides (ex. Coragen) can sometimes give some inconsistent control compared to tank mixes or premixes of diamide and pyrethroids (ex. Besiege). Most of the pyrethroids have been weak by themselves in recent years. The one exception in last year’s spray trials was Baythroid XL, it performed as well as a Besiege/Warrior rotation. A big caveat must be noted in that bifenthrin (ex Brigade) was not included in last year’s high moth pressure trial as a stand alone treatment, but it seems to be a pretty solid chemical on other, sometimes hard to kill pests like stink bug. Mixing a pyrethroid + Lannate will also greatly improve control in many cases. The fourth mode of action that we have available is group 5 spinosyns like Radiant or Blackhawk. By themselves, these products do not perform well. Unfortunately, there is little data to determine if they should be mixed with pyrethroid or with Lannate to improve resistance management.

Bt varieties should be treated like conventional sweet corn with the exception of Attribute II or Attribute Plus sweet corn. To check what trait package may be in your corn, there is a useful table produced by Michigan State and Texas A&M that can be found here: https://agrilife.org/lubbock/files/2020/02/2020_BtTrai_Table_Sweet.pdf. Please note that Attribute sweet corn has only cry1Ab, while the other two have the Vip3A trait.

Please also note that a pheromone trap right next to tasseling and silking sweet corn is a much better indicator of moth activity than a trap not so similarly located. At the research farm, I often have traps just a couple of thousand feet from each other and they can sometimes catch radically different numbers of moths. Use the trap counts from the network to get an idea of population trends in your area.

Blacklight and pheromone trapping data is uploaded by Tuesday and Friday mornings and can be found here: http://agdev.anr.udel.edu/trap/trap.php. Threshold information can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/insect-trapping/ecb-cew-moth/.

Cucurbits
Squash bugs are active. Thresholds on summer squash are 1 egg mass per plant. Egg masses are going to be underneath of leaves, often in between veins near the leaf base. Once you reach threshold, wait until the eggs begin to hatch before treating. Very few insecticides penetrate egg cuticle, while small nymphs are pretty easy to control.

In southern Delaware, we may see the first squash vine borer by the end of next week. They typically emerge from the soil between 900 and 1000 degree days. We are around 800 right now in Georgetown. Be sure to scout summer and winter squash regularly for the presence of bronze, flattened eggs laid singly on the stem or leaf petiole. Suggested treatment for vine borers is to treat vines with a pyrethroid weekly once the moths are active in your area. This is where scouting is going to be very important. Just because they could be active doesn’t mean that they are in your planting.

In watermelon, spider mites and aphids are active. I have not received any reports of significant aphid activity from commercial fields, just a certain small plot that was not destined to be an aphid trial. Insects are jerks. Aphids will be controlled with neonics applied for cucumber beetle.

Spider mite activity seems to be unusually early and widespread in the region, although mites are currently at low numbers. Several of the fields that we sampled this week had mites on pollinizers near but not necessarily on the edge of the field. I suspect that with the warm winter, low numbers of mites remained active during the winter in and around greenhouses and that these mites then colonized the plants. I have not yet seen any mites in pokeweed which, for the last two years, has been a reliable indicator of mite populations building up on the edge of fields. Last year, all but one of the fields we sampled had mites move in from the edge of the field.

The table below lists the miticide active ingredients and their mode of action group for watermelon. There are generic formulations of some of the chemicals, this list is not meant to serve as an endorsement. We have not tested all of these products in the same spray trial, but Portal, Zeal, and Agri-Mek have consistently performed well.

Please note that avermectin is in the same mode of action class as Abamectin and that Magister and Portal are in the same class. Do not apply one right after the other. Also, some of these products will stop mite feeding quickly, but the mite will take a few days to die. If you have sprayed a field and see mites a couple of days later, wait another few days and resample. Also, many of these products have a long residual activity, meaning that if it is not active immediately on eggs, it should still be around once the eggs hatch. As always, read the label thoroughly for further guidance. Some products have restrictions on reapplication interval, and restrictions on consecutive applications. Abamectin should be mixed with a spray adjuvant. Good coverage is key for miticide efficacy, even on those that are translaminar. Also, be mindful of products that are in a tank-mix. For instance, Zeal may have some reduced efficacy when mixed with Boron, and stickers or ‘sticky’ fungicides like Bravo can interfere with Agri-mek. Both Agri-Mek and Magister can be very toxic to pollinators, while Portal and Kanemite are not generally considered toxic to bees. All of the others advise caution.

Potato
Continue scouting for Colorado potato beetle and also potato leafhopper. Leafhopper thresholds are 1 adult per sweep or 1 nymph per 10 leaves. If you have to target both, Imidan, Torac, Sivanto, and various neonics should be effective.