Understanding IGRs

Insect growth regulators, or IGRs, are materials that act on the hormonal systems of immature insects. Insects pass through several stages on their way to adulthood. Each step in the process in controlled by chemicals that the insects naturally produce. These chemicals determine whether the insect molts into another immature stage or becomes an adult, is able to successfully shed its old skin, or can produce chemicals necessary for its tough outer shell, or exoskeleton.

In the not-so-old days, most insecticides generally acted upon insects’ nervous systems. Unfortunately, this is an area where humans and insects share much in common, which partly explains why many insecticides were also toxic to us. These days, research has taken pest management to a new molecular level in which many current products, including IGRs, capitalize on insects’ unique physiology as a point of potential vulnerability.

Insects’ inability to mature doesn’t grant eternal youth; instead it results in a much-reduced life span. Immature insects exposed to IGR treatment may simply die or be unable to emerge from their old shell. Sometimes, eggs fail to hatch.

IGR types
There are three main types of IGRs:

Juvenile hormone mimics act like the so-called juvenile hormone in immature insects, the presence of which assures its next molt will be to another immature stage. Distance and Enstar II fall into this group.

Chitin biosynthesis inhibitors, the second group, affect the formation (synthesis) of chitin, an important chemical providing hardness and flexibility to the hard exoskeleton of insects. Insects exposed to a chitin synthesis inhibitor may not be able to molt properly. Pedestal, Dimilin, Adept and Talus are in this group. Citation is thought to interfere with chitin synthesis, but its true mode of action is unclear.

Click the picture above to view to full table.

A third set of IGR’s, the ecdysone receptor agonists (antagonists), interfere with the normal operation of ecdysone, an important hormone in the molting process. Treated insects may molt abnormally or molt too soon. The products Confirm and Mimic are in this group. Azadirachtin products (Azatin XL, AzaGuard, Aza-Direct, Ornazin, Molt-X, etc.) are sometimes included in this third set.

In addition to this general classification, pesticides are now assigned to various modes of action (MOA) groups based upon the way they work to control insects and mites. You will see the MOA group number displayed on most labels now, and they are shown in Table 1. These group numbers help in selecting and rotating among products to reduce the risk of developing pesticide-resistant populations.

IGRs in a pest management plan
There is an ongoing discussion among entomologists about where IGR products fit into a pest management plan. I emphasize use of cultural and non-chemical strategies first, such as removing flowers (during early crop stages to reduce Western flower thrips) or foliage (lower leaves on established poinsettia cuttings to eliminate whiteflies), controlling weeds that may be alternate hosts, and discarding infested material as soon as possible.

Next, consider whether biological control might be a cost-effective or appropriate option, particularly for a long-term situation. Growers might consult the Koppert or Biobest side effects lists at their respective websites to determine compatibility of any IGR or other pesticide with the natural enemies that are or will be used.

IGRs fit well into conventional programs, of course. It is important to remember that IGRs don’t directly affect adult insects, so these products alone are unlikely to quickly eliminate an existing infestation. Remember that greenhouse pest populations are typically composed of mixed stages (adults, immatures, eggs) at any one time.

As with any material, verify that it is likely to be effective for the target pest(s). This changes over time, so contact an extension specialist or consultant for the most up-to-date information. Table 1 shows most of the IGRs used for greenhouse pests and what they are labeled for. Be sure to read labels before applying any product and note sensitive plants, whether the product and uses are approved in your area, and other application restrictions.

Evaluating IGRs
I have had much experience with evaluating IGRs over the years and working with growers who use them. These products have been highly effective in many cases. For example, in Long Island trials, Distance, Adept and Citation have performed very well against fungus gnats as drenches to control the immature or larval stage in media. Talus was highly effective against greenhouse whitefly, and in the days before Q-biotype whitefly arrived, Distance was quite effective for sweet potato whitefly (and it may still be where populations are susceptible). Enstar also worked very well against sweet potato whiteflies when that pest first appeared.

Though cabbage looper does not appear on the label, Pedestal worked quite well to control that pest in ornamental kale, and in at least one trial it provided significant control of Western flower thrips. The azadirachtin products are widely used by growers, and some tested in my trials have been at least moderately effective against fungus gnats. Recent data from another study showed efficacy against Western flower thrips as well. Though labeled, one product did not control cabbage loopers or damage to plants with weekly applications. As with any product, learn about the strength and weaknesses of IGRs and where they may fit into your production plans.

Daniel Gilrein is an entomologist, Cornell Cooperative Extension of Suffolk County; dog1@cornell.edu.