Science in Action: Development of Spinosad

          Professionals in the horticulture industry have a new but safe addition to their arsenal of integrated pest management products.  This product is a relatively new release for controlling insects such as caterpillars, thrips, leaf miners, dry wood termites and fire ants, and even some leaf feeding beetles while leaving most beneficial insects unharmed. Spinosad would be an excellent choice to use in rotatation with other organic controls for caterpillars such as Bt (Bacillus thuriengensis, Dipel™, Thuricide™).

Even though it is not obvious, this
container of Fertilom's Borer, Bagworm,
Leafminter & Tent Caterpillar Spray
contains spinosad. You would know that if
you read the label of ingredients.

Spinosad has demonstrated good control of Western flower thrips during trials at the University of Nevada's, Center for Urban Horticulture and Water Conservation.  Initial tests on grapes showed some level of activity on immature leafhoppers.  This insecticide will be listed in the ingredients of a pesticide label as Spinosad (pronounced spy’ no sid).

          Spinosad has been categorized safe enough by USDA to be used as a product suitable for organic production, labeled for about 200 horticultural plant types including landscape plants and food crops.  What is different about this product is that it launches a totally new category of products to be used for controlling insects.

          Spinosad is derived from a class of soil microorganisms called Actinomycetes.  Actinomycetes are no longer classed as fungi or bacteria, as they were previously, but have characteristics common to both.  Actinomycetes play a very major role in the decomposition of organic matter. These microorganisms convert raw material to humus and are responsible for releasing geosmin, the chemical identified with that “earthy smell” common to high quality composts.



This is also Spinosad. But you would never know
by looking at the label. This is Monterey Chemical
Company's version of spinosad. Again, to know
this you would have to look at the active ingredients
or see in the upper right hand corner of the label
that this formulation contains spinosad. 

          Spinosad is derived from fermentation products, called spinosyns, produced by one or more mutants of the naturally occurring Actinomycete, Saccharopolyspora spinosa. This microorganism was a chance discovery at an abandoned rum distillery by a microbiologist visiting the Caribbean. The vegetative base used to grow S. spinosa is submerged in an aerated liquid containing proteins, carbohydrates, oils, corn solids, cottonseed flour, soybean flour, glucose, methyl oleate, and calcium carbonate and other minerals.

          When the S. spinosa is allowed to grow aerobically in an aqueous growth medium, it produces a number of different byproducts including several different spinosyns. The spinosyns are large complex molecules containing mostly carbon, hydrogen, and oxygen. Technically, Spinosad is a combination of spinosyns A (85%) and D (15%) which creates a pesticide that does not conform to any previous class of insecticide; chemical or biological. 

          Actinomycetes are not strangers to us when talking about pest control.  Byproducts from one of the Actinomycetes, a subgroup called Streptomycetes, is responsible for giving us antibiotics such as actinomycin, tetracycline, streptomycin, and neomycin leading to the only Nobel Prize ever given to a soil scientist, Dr. Selman Waksman in 1952. 

          Byproducts from these soil microorganisms have been used by all of us to treat bacterial infections in animals and humans.  We still use a streptomycin product, Agri Strep^®, for controlling fireblight in commercial apple and pear orchards.

          Actinomycetes are also nitrogen fixers. Just like the other nitrogen fixers more common to us, the Rhizobia types associated with legumes, Actinomycetes convert atmospheric nitrogen into a form of nitrogen which can be used by plants. Actinomycetes form associations with the roots of some non-leguminous plants such as bitterbrush, mountain mahogany, cliffrose, and ceanothus and “fix” nitrogen from the air, making this nitrogen available to its host and other plants in the near vicinity.

Unlike some other pesticides, Spinosad degrades in the environment to its natural components of carbon, hydrogen, oxygen and nitrogen. However, this degradation is quite slow compared to biologicals like Bacillus thuriengensis (Bt; Dipel™, Thuricide™). The leaching potential of Spinosad is very low and so does not pose a threat to groundwater when used properly and no buffer zones are required by the United States Environmental Protection Agency. Persistence after application can range from 5 to 10 days. However, it may persist longer in aquatic environments.

          The principal method of entry into insects is orally, by ingestion. So leaf feeding insects are more likely controlled than those which typically “suck” plant juices. At the Center, we have been using weekly applications to control flower thrips and leafhopper nymphs.

Spinosad’s potential for chronic toxicities such as certain forms of cancer, is also quite low. It has been demonstrated that it is not carcinogenic, teratogenic, mutagenic, or neurotoxic.  EPA puts Spinosad in the “caution” category of pesticides but with an LD₅₀ of over 5,000 it is practically nontoxic to mammals and birds. It has wide margins of safety to many beneficial insects such as green lacewings, Lady Bird beetles, minute pirate bugs, and only slightly toxic to fish. 

Spinosad does provide some control of leaf miners. Although not classed as a systemic inseciticide it does cross the laminar covering of leaves and can penetrate to just below the leaf surface where leaf miners are feeding.

One drawback is its potential toxicity to honeybees.  Although not as toxic as many other insecticides, Spinosad has been classed in the highly toxic category but drops considerably after the product has dried on leaf surfaces. Early morning applications before bees fly or early evening applications after bees have returned to their nests would be ideal times of application.

The mode of action of Spinosad is similar to other pesticides which attack the nervous system and is very quick acting compared to other biologicals or natural products. 

Even though a low probability of resistance development was speculated, there has been insect resistance already noted in Hawaii with the diamondback moth.  Resistance management, alternating applications with other products to achieve pest control, is recommended to avoid the development of future resistance.

This article was written by Bob Morris and previously published in Southwest Trees and Turf.