This week, my PhD student, Sean Boyle, observed pickleworm holes in our zucchini and squash in Whitethorne, VA near Blacksburg. This is the first that we’ve seen this pest in 2020. If you have noticed this pest in your area, please let me know – email tkuhar@vt.edu. The pickleworm, Diaphania nitidalis (Stoll) is a tropical moth pest of cucurbit crops including pumpkins, squash, and cucumbers (Fig. 1). It is typically a pest in the southern U.S. and does not overwinter in Virginia. The past few years, the pest has made its way northward in late summer on wind and storm fronts. Several pumpkins growers in Virginia have suffered damage from this pest in since 2017 usually following some August summer storms.   

Moths fly to flowering pumpkins, squash, or cucumbers and deposit their eggs. A single female moth can lay up to 400 eggs usually on cucurbit flowers.  Larvae feed on flowers (Fig. 2) and bore into fruit leaving a characteristic perfectly round hole often with sawdust-like fecal material around it as well.     

Management. Pickleworm is very difficult to predict or monitor for as eggs are very tiny, moths fly at night, but are not attracted to lights, and there is no commercially-available pheromone lure.  As a result, cucurbit growers in the South often apply insecticides weekly during the fruiting stages until final harvest.  Pyrethroid insecticides can be effective at controlling this pest if sprayed in a timely manner (i.e., lambda-cyhalothrin, permethrin, bifenthrin, Baythroid XL, Mustang Max, etc.). Pyrethroids are often used because of their low cost and because they also control squash bugs and cucumber beetles, but they are not IPM compatible and can result in outbreaks of secondary pests such as aphids.  Usually two or more sprays of pyrethroids in late summer can cause severe aphid problems leading to honey dew build up on plants. Other insecticides that control pickleworm include: the spinosyn productss, Radiant and Entrust, the diamide insecticides, Coragen and Harvanta, the insect growth regulator (IGR) Intrepid, and the lepidopteran-targeting insecticide Avaunt eVo.  All of these products will have less nontarget impacts than pyrethroids and will also control pickleworm.                                                                                           

This article was co-authored by Virginia Tech Entomology Ph.D. student, Kadie Britt.

Here are the corn earworm moth catch numbers from a handful of pheromone traps that we have set up in hemp fields and one sweet corn field around Blacksburg, VA. Trap catch appeared to peak around mid-August with moths emerging from cornfields and has subsided a little toward the end of August. Crops such as soybeans, hemp, tomatoes, and sweet corn are still at risk to this important pest.

Corn earworm (CEW) is the insect pest of greatest concern to hemp grown outdoors in Virginia and other states. For more information about this pest’s biology and behavior in hemp, see this factsheet: https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ento-328/ENTO-328.pdf.

We have already started to see worms feeding on hemp throughout Virginia. CEW feeding on hemp causes mechanical damage to buds, allowing environmental pathogens to enter crop material, ultimately leading to bud rot. Bud rot is visible and present in hemp crops right now but is not prevalent just yet. Managing populations early on will be key to reducing crop injury this season.

Strict regulations on pesticide applications to hemp prevent the use of many available insecticides. We have conducted a laboratory experiment to evaluate the efficacy of some products that are currently allowed for use in hemp in Virginia as well as Pyganic, which is not labeled on hemp, but is a widely used organic insecticide.

Treatments included in the August 19, 2020 bioassay included:

1. Untreated control (UTC): water
2. Agree: Bacillus thuringiensis subspecies aizawai strain GC-91
3. Spear-Lep + Leprotec: GS-omega/kappa-Hxtx-Hv1a + Bacillus thuringiensis variety kurstaki
4. XenTari: Bacillus thuringiensis subspecies aizawai strain ABTS-1857
5. PyGanic: Pyrethrins
6. PyGanic + PBO (insecticide synergist): Pyrethrins + Piperonyl butoxide

To conduct this bioassay, corn earworm larvae (3^rd to 5^th instars) were collected from untreated sweet corn grown at Kentland Research Farm in Whitethorne, VA. Untreated hemp seed heads were collected from grain hemp, variety ‘Joey’, grown at the Urban Horticulture Center farm in Blacksburg, VA. Seed heads were dipped in spray tank concentrations of each insecticide at the high labeled rate and placed into 1 oz. diet cups. Larval corn earworms were placed directly on top of treated material. Mortality was evaluated at 1, 2, 3, and 4 days after the experiment was set up (Figure 3). PyGanic + PBO provided significantly greater efficacy against CEW than all other products tested with only 6.5% of worms surviving after 4 days. The addition of the synergist is needed for effective control with Pyganic due to pyrethroid resistance development in this pest.  Unfortunately, Pyganic is not currently labeled for use on hemp in Virginia.  Agree, XenTari, and PyGanic without PBO all performed similarly with 50.3%, 53.5%, and 59.9% of worms surviving after 4 days, respectively.

Looking ahead this season, insecticide research trials in CBD hemp will be conducted in Blackstone with all of the aforementioned products plus many more. We will continue doing lab bioassays with CEW and other insecticides that are allowed for use in hemp at this time. Results will be shared as they become available.