Weekly-mum producers have seen higher-than-normal spider mite levels coming in on cuttings from the U.S. recently. This might impact seasonal potted-mum growers as well.
Here’s some tips and tricks on two spotted spider mite control within a chrysanthemum IPM program.
Source of the Problem:
We’ve long known that spider mite problems in floriculture crops often arrive from imported plant material from from the U.S., including California and Florida. The above graph (courtesy of the Buitenhuis Lab, VRIC) demonstrates that up to 92% of batches of chrysanthemum cuttings can be infested with either spider mites, thrips, or both (data collected in 2015).
Typically, spider mite densities seem to range between 5-20 mites/100 cuttings, but current observations suggest this year’s pressure to be even higher.
Preventing a Problem:
Although we likely can’t prevent the spider mites from coming in the door, we CAN help reduce their numbers immediately.
Dipping cuttings in “softer” chemicals, such as oil or soap can be a reliable way to reduce certain pests from entering your greenhouse without unnecessary pesticide residues that can affect biocontrol programs.
Dipping mum cuttings in low-risk chemicals has become a popular method to reduce initial pest populations on mums in Ontario in recent years. To reduce thrips adults and larvae on plant material, cuttings can be dipped in 0.5% landscape oil. Always check the label for more information on how to limit phytotoxicity. The most current label can be found here (click on the registration number). Also, watch this video on proper dipping technique.
Although we don’t yet have specific data on the ability of oil dips to reduce spider mite populations, this method may kill a significant of adult and/or nymphal mites, along with thrips. (Or at the very least, will help reduce your thrips issues so you can concentrate on those darn spider mites!).
In terms of preventative biocontrol agents, Neoseiulus fallacis and Neoseiulus californicus are good options. Both can subsist on low levels of spider mites, and supplement their diet with other prey or pollen. Fallacis or Californicus can be applied as soon as the misting phase is complete. Given the high mite pressures this year, you may want to think about adding this into your program.
( Additional Note: as suggested by Ronald Valentin of Bioline AgroSciences in the comments below, this year, perhaps we treat our mums like we do poinsettia — i.e. we don’t wait to FIND Bemisia, we just ASSUME it’s there and add the most effective bios right away. In that case, applications of the predator P. persimilis immediately after dipping might be the better call. See more on this natural enemy below).
Reigning in a Problem: Bios are Best
Once spider mites are detected, Phytoseiulus persimilis, a specialized spider mite predator, should ALWAYS be the first thing you reach for.
Persimilis feeds on ALL stages of spider mite, are quite cost-effective, and can clean up a problem quickly (even a nasty outbreak). Release rates range from 1–4 mites per ft2 per week until the situation is under control. Generally, releases are only initiated once spider mite populations are detected, since the downside of this bio is that it can’t survive long in the absence of spider mites.
What’s the best way to detect spider mites early? By the time the characteristic “bronzing” or webbing appear, your spider mite problem is usually larger than you think. Young colonies rarely show these signs.
Your best options are to have employees check a random sample of cuttings for mites as they come in the door, and frequently tap plants onto a white sheet to check for live mites. Two spotted spider mites are easy to distinguish from beneficial mites with just the use of a 10x hand lens.
Although Persimilis works great this time of year, it prefers conditions between 20-28ºC and 75% RH. When temperatures climb to 30ºC, Persimilis tends to seek shelter, and cannot keep-up with the reproductive capacity of TTSM. If spider mites are still an issue, you can also switch to N. californicus, which does better at higher temps and lower humidity.
Reigning in a Problem: Chemical options
We also have a LOT of chemical control products registered for spider mites. Choices will likely depend the severity of the problem, how close to shipping you are (do you have TIME for Persimilis to work?) and whether you’re using other bios for management of other mum pests. Products with long residuals (like Avid and Pylon) should be avoided if natural enemies are a big part of your general IPM program. Generally, Vendex and horticultural oils are your “softest” pesticide choices, but re-application will likely be necessary.
A table of registered products for floriculture use in Ontario can be downloaded below (products are listed alphabetically by pest issue). This table has information like potential phytotoxicity risks and compatibility with natural enemies to help make your choices for pest control – including spider mites – easier.
Considerations BEFORE you spray:
Controlling spider mites with pesticides alone can be a challenging task. Here are some things you should think about before reaching for the pesticide applicator.
- Most products registered for spider mite control are NOT compatible with predatory mites for thrips . Expect to make re-releases of predatory mites after pesticide applications. Sachets are unlikely to protect predatory mites from pesticide effects unless applied as a drench.
- Products like Avid and Pylon are also highly toxic to parasitoids (e.g. Encarsia, Diglyphus, and Aphidius species) used in greenhouse biocontrol. Avoid their use, especially if leafminer is a concern in your greenhouse, since Diglyphus can take months to re-establish once these chemicals are applied.
- Spider mites can develop insecticide resistance rapidly. So, if you are relying on pesticides for control, make sure to rotate between chemical classes (i.e. Forbid and Kontos are both under class 23, and would NOT be good choices for back to back applications). Cross resistance between chemicals is also a possibility.
- Most insecticides/miticides will not kill the egg stage, so two applications 7-14
days apart may be necessary, depending on pesticides to be used.
- A spreader-sticker or wetting agent will improve the effectiveness of most miticides, particularly on waxy leafed plants. However, the possibility of
phytotoxicity may increase, so spray a small area before doing any large-scale applications.
Other Tips, Tricks and Random Facts:
- Monitor for TTSM more closely in areas of the greenhouse that are warmer and drier, since spider mites thrive in high temperatures and low humidity. This includes around heating pipes, south-facing walls and open vents/doorways.
- Always confirm spider mite infestations using a hand lens: in plants such as ivy geranium, damage caused by thrips feeding and edema can look similar to damage caused by twospotted spider mites. No point spraying if you don’t need to.
- Over-fertililized plants can be more susceptible to twospotted spider mite. Fertilizers promote succulent new growth which is more attractive. Many ornamental plants are over-fertilized in the greenhouse, so check your rates and see if you can back off.
- Twospotted spider mite populations may be higher in greenhouses that use only drip irrigation, since this keeps foliage dry (which spider mites like!). The use of occasional overhead irrigation will wash mites off plants.
- Spider mites are easily transferred around the greenhouse on clothing. If you have a hot spot, mark it with flags and make sure workers always visit this area last
- Bush beans can be an effective indicator plant for low spider mite infestations, as they show damage easily. They can also act as trap plants, attracting spider mites away from your crop. More about this trap plant method can be found here.