Part of getting rid of mealy bugs is understanding life cycle. No spray that is safe for trees and safe for a human to apply will kill 100% of the insects with a single application. Also, if you have insects on one plant, it is very rare that this plant is the only plant affected. For example in my orchid collection of several hundreds of plants, if one is infected, there are other plants that have a couple of sneaky little buggers hiding out. SPRAY ALL PLANTS IN THE SAME GROWING AREA. If your maple has a problem, it is probably because the elm next door has a small population of bugs that are not doing enough damage to notice, and they spread out. The insects will often be at low levels throughout the collection and have a population explosion on only your weakest tree, or most expensive tree.
Success requires repeated spray applications.
At 65 - 70 F (about 18 C ) mealy bug go from egg to breeding adult in about 14 days, at 76 to 80 F (25-27 C ) mealy bugs can go from egg to breeding adult in as little as 7 days. When you spray, most sprays kill only adults, eggs will be unaffected. You must repeat applications of contact kill pesticides at an interval shorter than the time it takes for an egg to hatch, and the nymph to mature into an egg laying adult. So in cool weather, if it takes 14 days for the mealy bugs to mature, you need to spray at roughly a 10 day interval. Kill off the newly developing mealy bugs before they are old enough to begin laying eggs. In warm weather, you might have to re-apply at a 4 or 5 day interval to actually get rid of the mealy bugs. This is for contact pesticides, like insecticidal soaps, alcohol, and some of the chemical sprays like pyrethrum. The pyrethrum sprays are often labeled for indoor use because pyrethrum breaks down rapidly, usually in less than 24 hours.
Some pesticides have a longer residual activity. The length of residual activity will be listed on the label. This can change the length of time for re-application. The label should spell out how often you need to re-apply. You MUST READ THE LABEL, and follow the label directions for how often to apply a pesticide. Yes labels are lengthy, but they contain valuable information, both on proper use and on safe use.
If one pesticide does not work, note the mode of action group, or mode of action class number the pesticide belongs to, and switch to a second pesticide that is in a different mode of action group. Many parts of the country have developed pesticide resistant populations. Mealy bugs in particular have become almost immune to pyrethrum based sprays and Carbamate based sprays. Sevin is a group 1A carbamate. Pyrethrum is usually in group 3, or can be 3 & 27 if sysnergists are included in the formulation. Point is, different pesticides work on different insect vulnerabilities. If your group 1 pesticide seems ineffective, don't reach for another group 1 product, find a product that has a different mode of action. Alternating between different mode of action groups helpful.
Example, spray with alcohol & water for initial knock down, get your mealy bug population down from its peak. Then 4 or 5 days later, spray with your Sevin. Then 4 or 5 days later spray with your pyrethrum spray, or an organophosphate spray like Malathion. Then follow this with a return to Sevin or Pyrethrum.
Now the Carbamates (Sevin), and Organophosphates (Malathion) are fairly nasty environmentally. They are a hazard to honeybees and other beneficial insects, they are hazardous to humans, causing nerve damage if exposed to skin or inhaled mist. They are nasty chemicals. The Pyrethrum compounds are quite a bit safer for humans, and because of their short residual are less of an environmental problem. All three of these old line pesticides have significant problems with resistant insect populations. Luckily seldom will any one population of insects have high resistance to all three.
The neonicotinoids, specifically imidaproclid found in Merit and one of Bonide's products, this group is systemic, rendering the sap of the plant poisonous to the sucking insects. This is mode of action group 4. These are potent chemicals, will work well for sucking insects like aphids and mealy bug. Note, presence of imidaproclid has been shown to aggrevate, cause an increase in spider mite populations. So if you have both mealy bugs, and spider mites, using imidaproclid will kill the mealy bugs and cause the spider mites to have a population explosion. The neonics, in group 4 are very effective, but often have long residual activity, are very detrimental to honey bees, and pose some human hazard. If not applied with correct safety precautions, you can hurt yourself. Read labels and follow directions. This group will not need to be re-applied at short intervals. Read labels, follow re-application directions.
Some of the new pesticides, derived from fungi, bacteria, and some plant extracts are much better behaved, but much more specific in activity. One might be good for aphids, but not mealy bugs. One might be good for beetles, but nothing else. Spinosaids, Abemectin, bacillus thuringensis (BT), and other fermentation products can actually be registered for use in organic operations. Much reading is required to use these, because they often are very specific as to which pests they will kill.
Juvenile Growth hormone disruptors - group 7 - work by interfering with the insect growth hormone systems. These generally are designed to be used in a mix with other pesticides. When you blend 2 or more pesticides into your spray solution, this is referred to as a "tank mix" in the pest business. The group 7 products are not intended to be used by themselves. They are always combined with a "knock down" or "quick kill" pesticide. One product I've used is Enstar II, it is quite expensive, over $100 for 4 ounces. However, only drops are required, the 4 ounce bottle will adequately treat a 250 tree collection 4 to 6 times. It is too expensive for small collections. But something to consider, there may be cheaper versions out now as these have been on the market long enough for patents to begin to expire and generics are becoming available.
Tank Mixes - for example, I have used Decathalon WP (a synthetic pyrethrum) blended with Enstar II (hormone distruptor) and Pentac (miticide) in a single mix. With proper personal safety equiptment. I found I could spray twice, at a 10 day interval, and completely clean a 1000 orchid collection of mealy bugs, aphids, scale and spider mites with no more than 2 or 3 applications, 10 days apart. Do it in the autumn, the collection would stay clean until spring or summer. I went from spraying almost weekly to just a clean up in autumn. Or at worse, a summer series, and then a late autumn series. But this was for a large, valuable orchid collection.
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