David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
We found our first two spotted spider mites in watermelon, cantaloupe, and soybean last week. Spider mites move into fields in 3 ways: hitching a ride on equipment or workers, crawling over soil, or by being transported by wind. When host quality declines, TSSM will move to high objects and stand on their ‘tip toes’, waving their first pairs of legs until a passing air current catches them.
Spider mites quickly reproduce with the warm weather that we have had. Mites can develop from egg to adult in as few as 8 days, and lay over 140 eggs during their 2 to 3-week life span. Eggs take about 6 days to hatch. Even with the wet weather we have had, it is possible that spider mites may need treating at some point in your fields later this season. Humid weather favors entomopathogenic fungi which can reduce mite numbers, but many fungicides slow down or prevent fungal growth.
Action thresholds for watermelon are 20 – 30% infested plants with 1 – 2 mites per leaf. For R-stage soybean, thresholds are 20-30 mites per leaflet and 10% of plants with one third of the leaf area stippled. For tomato, NCSU entomologist Dr. James Walgenbach recommends an action threshold of 2-4 mites per leaflet. On tomato, spider mites can also be a contributing cause of ‘gold-fleck.’
NCSU research indicates that two spotted spider mite resistance is unstable. If mites are resistant at the end of one season, that resistance will not be as strong at the beginning of the next. However, resistance is still present and can be rapidly selected for. There is limited population mixing from one farm operation to another. What this means is that a population of mites on one farm does not spread to others. This means that miticide rotation is extremely important. Miticides tend to be most active on certain demographic groups, meaning that there is likely a subset that will not be killed from the application. Given their life span (6 days as eggs, 8 as juveniles), if you need to spray, anticipate a second application.
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.
| Miticide | Active ingredient | MOA group | Life stage active | Applications/season |
| Agri-Mek | Abamectin | 6 | Mobiles (translaminar) | 3-5 |
| Gladiator | Zeta-cypermethrin + avermectin | 3 + 6 | Mobiles (translaminar) | 3 |
| Acramite | Bifenazate | 25 | Mobiles, some ovicidal (contact) | 1 |
| Kanemite | Acequinocyl | 20B | Mobiles, some ovicidal (translaminar) | 2 |
| Oberon | Spiromesifen | 23 | Eggs and juveniles (contact) | 3 |
| Portal | Fenpyroximate | 21A | Mobiles (contact) | 2 |
| Zeal | Etoxazole | 10B | Ovicidal, juveniles (translaminar) | 1 |
Please note that avermectin is in the same mode of action class as abamectin. 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. This year, we have plans to test these products at our research station.
For soybean, three materials have been previously recommended: bifenthrin (a pyrethroid), Agri-Mek 0.7SC, and Zeal 2.88SC. Generally speaking, pyrethroid resistance can be an issue with spider mites. This class will also remove beneficial predatory insects.
As always, read the label thoroughly for further guidance. Some products have restrictions on reapplication interval, and restrictions on consecutive applications. There are also requirements on some for spray adjuvants to avoid illegal residues. Good coverage is key for miticide efficacy, even on those that are translaminar.