An Insecticide of Merit

by: Robert B. Martin Jr.

As Written in the March issue of The Rose Parade

Janene Rosenthal, Editor,

In 1998, through the recommendation of a friend, I acquired a new insecticide for use on my roses. I am reluctant to experiment with any new insecticide during the show season, so I withheld its use until late in the 1998 season. At that time I had a substantial number of black aphids which usually visit my garden toward the middle to the end of the season. These pests are unusually resistant to Orthene®, my standard insecticide. They have also proven to be tough to control with other insecticides that I have used. So my plan was to see if this new insecticide would work on the black aphids.

To my surprise, the insecticide eliminated the black aphids, all other aphids and also proved effective on thrips, a pest that I have also found very difficult to control. I was so pleased with the results that I decided to use it as my insecticide of choice during the 1999 season. I carried through with that plan and can now report that the 1999 season was virtually aphid-free. (Including the dreaded black aphids.) And, although complete control of thrips was not obtained, there is no doubt that I had far fewer thrips in my rose garden during 1999 than during any other season.

In short, this is the best insecticide I have ever used. I am also pleased to report that it has a very low dosage rate, is relatively safe and is considered to have an attractive environmental profile. It is therefore, a product that I can now highly recommend to Southern California rosarians who elect to use insecticides for the control of aphids and thrips.

The product is called Merit® 75W and comes in a small (2 oz) container. At about $44-$53 , the little container does not come cheap. The recommended application rate for roses, however is one teaspoon per 10 gallons applied every other week. A little, therefore, goes a very long way. This one container was adequate to treat my garden of about 400 roses for an entire season with some left over. Overall it proved considerably cheaper than Orthene®. 

The active ingredient in Merit® is imidacloprid, a systemic, chloro-nicotinyl compound which was developed in 1989 by Bayer A.G. and has since become the No. 1 active ingredient in insecticides around the world. Recently Bayer and Pursell Industries, Inc. , a company with 95 years of experience in the lawn and garden business, have formed a new company, Bayer-Pursell, LLC, to co-market products using the ingredient to the U.S. consumer market. Together, they are introducing the brand, Bayer Advanced™, designed for home garden use. Please note, however, the comments at the end of this article on the rose care products offered under this brand name as Bayer-Pursell has founded its line on two chemicals that are vastly different.

The chemical works by interfering with the transmission of stimuli in the insect’s nervous system. Specifically, it causes a blockage in a type of neuronal pathway (nicotinergic) that is more abundant in insects than in warm-blooded animals (making the chemical selectively more toxic to insects than warm-blooded animals). This blockage leads to the accumulation of acetylcholine, an important neurotransmitter, resulting in the insect's paralysis, and eventual death. It is effective on contact and via stomach action. 

Imidacloprid is currently sold under a number of different trade names for different uses. These include Gaucho® for seed treatment on wheat, Provado® for tomatoes, Admire® for potatoes, Grubex® for control of grubs, and Premise® for control of subterranean termites and carpenter ants. It is also the active ingredient in Advantage®, the best selling product for the control of fleas on dogs, cats and other pets. 

Writing in the Fall 1998 issue of Rose Exhibitors’ Forum, Bill Blank of Snow Hill Maryland, a retired distributor of agricultural chemicals, reported on his experience with the use of Merit® over four and one-half seasons. There he stated that he had not had an aphid on his roses during this period. He also reported only one mild outbreak of thrips after an 11-day spray interval in 1999. This was deep in the flower and was cleaned up with an application of Merit®.

Based on Bill’s experience over four-and one-half seasons, he concluded that:
o Merit® is not phytotoxic to most modern roses.
o Merit® controls aphids systemically. 
o Merit® controls thrips, but does not translocate to the bud or bloom. 
o Merit® does not control adult cucumber beetles. 
o Merit® is generally compatible with most spray materials used on roses. 
o Merit® is the best legal material he has used in the last 20 years for the control of aphids and thrips.

Merit® has a "CAUTION" label and an Oral LD-50 of 1858. This may be compared to 
o Malathion - 5500
o Orthene - 1447
o Mavrik - 261+
This comparison is more important when it is realized that the application rate for Merit® is considerably less than that of other insecticides. For example, Orthene® 75WP is typically applied at between 3/4ths to one and 1/2 teaspoons per gallon. This is more than 10 times the rate of application of active ingredient than Merit®. In general, Merit® is effective at rates 85% to 96% lower than most currently registered insecticides. 

Merit® is labeled for use on ornamentals in commercial and residential landscapes for the control of aphids, leafhoppers, thrips and white flies. It is a systemic product and is translocated upward in the plant system. The label suggests that the addition of a nitrogen containing fertilizer in the solution will enhance the uptake of the active ingredient. I have used it with Response™, a foliar seaweed fertilizer, which appears to enhance its effectiveness.

Merit® mixes readily with water and may be used in many types of application equipment. I have used it exclusively as a foliar spray, however the label also contains directions for use as a soil drench. In this regard, Merit® is labeled for the control of white grub larvae for which such ground applications are more appropriate.

According to the label, Merit® has been found to be compatible with commonly used fungicides, miticides, liquid fertilizers and other commonly used insecticides. This is consistent with Bill Blank’s experience as he has reported that he had no phytotoxicity or leaf burn attributed to Merit® and no compatibility problems when mixed with dry soluble fertilizers, liquid pyrethroids, wettable chlorpyrifos (Lorsban/Dursban), Benlate, Systhane, Rally, Nova, Mancozeb, propiconazole (Banner), or chlorothalonil (Bravo).

Imidacloprid is relatively non-toxic to warm-blooded animals, as witness its use for flea control on pets. In fact, feeding studies on rats and dogs produced no observable effects at normal usage rates. This should be good news for rosarians with pets. It is, however, highly toxic to bees if used as a foliar application. It is also toxic to certain fish and should not be used near ponds.

Merit® is available at agricultural supply houses, including Orange County Farm Supply. Imidicloprid is also the active ingredient in the new Bayer Advanced™ line of insecticides including Bayer Advanced™ Garden Rose & Flower Insect Killer Concentrate which should become available in the near future in nurseries and garden centers. Please note that there is a difference between Bayer Advanced Garden Rose & Flower Insect Killer and the Bayer Advanced Garden 2-in-1 Systemic Rose and Flower Care Ready-to-Use Granules. The latter contains the ingredient Disulfoton (also known as Di-Syston, a 1953 Bayer product formerly found in Ortho Rose Pride. This is a highly toxic insecticide that in my opinion has no place in the rose garden. That, however, is another subject but does leave me pondering why Bayer-Pursell should stock its product line with two such dissimilar products bearing virtually the same name.