June 1, 2002
Vol. 4 No. 5
Azadirachtin: An Old Insecticide is Reborn
Azadirachtin is a plant chemical that has been used as an insecticide for centuries in many tropical and subtropical countries. Formulations have been registered in the United States since 1985, but in the past few years interest has picked up greatly for many apple and pear growers. What do we know about this compound, and how does it fit into apple and pear pest management programs?
Azadirachtin (sometimes referred to as "neem") is derived from the neem tree (Azadirachta indica), a plant native to Southeast and South Asia. This evergreen, drought-tolerant tree can grow 60-120' high and is now widely found throughout arid tropical and subtropical areas of Asia, Africa, Oceania and the Americas; India alone is estimated to have close to 20 million trees. It has been planted for lumber, firewood, shade, windbreaks and ornamental use. Increasingly, plantations have been planted for the harvest of the olive-like fruits, from which azadirachtin is extracted.
The seeds of the neem tree have the highest concentration of azadirachtin, although the compound is also found in the leaves and bark. For centuries, crude extracts from the seeds or even crushed leaves have been used in India and elsewhere for pest control on crops, in stored grains, around the home and even as medicines. Swarms of locusts, that periodically plague countries in the region, were seen to devour all plants in their path ... except neem trees. This observation led, in 1967, to the identification and isolation of azadirachtin, which is the most active of several similar compounds found in the seeds. Azadirachtin A is the form most commonly found in insecticidal formulations. Azadirachtin is a complex molecule, classed as a tetranortriterpenoid plant limonoid. It is a relatively short-lived material, susceptible to rapid breakdown by UV light, and has very low mammalian toxicity. Neem oil is also derived from the seeds, and has insecticidal properties distinct from its azadirachtin content. It is also formulated as an insecticide, but phytotoxicity concerns limit its application on some crops.
Azadirachtin has multiple modes of action in its activity on insects, and the importance of each can vary between insect orders and even species. Azadirachtin has insect growth regulator effects with many insects, affecting molting hormone levels and, in some species, juvenile hormone levels; in many cases, treated immature insects die in a later molt. Azadirachtin can act as an anti-feedant with some insects, originally observed in dramatic fashion with locust swarms. It also can affect insect reproduction, reducing both fecundity and fertility in some species. Azadirachtin sprays are targeted for immature stages of insect pests, as little or no toxic effects are seen with adults. There is a limited amount of direct toxicity with azadirachtin with some immature insects, but most of its effects are sub-lethal or delayed lethal. The impact of azadirachtin applications may not be fully realized for weeks and multiple applications are usually required. Azadirachtin's multiple modes of action make the development of resistance to it unlikely, and using it in a pest management program could also slow resistance development for other compounds.
Azadirachtin has been used on a wide array of crops and against a lengthy list of crop pests around the world. On many crops, lepidopteran larvae have been a prime target (among them armyworms, leafrollers, European corn borer, and corn earworm), as well as plant-feeding beetles such as Colorado potato beetle and cucumber beetle. Control of lepidopteran pests often requires repeated applications of high rates that may be uneconomical; other selective materials (such as Bts) may be a better fit. Azadirachtin is also used, with varying success, against many other insect orders, including Hemiptera (true bugs, including lygus and stink bugs), Homoptera (aphids, psyllids, leafhoppers and more), and Orthoptera (grasshoppers and locusts). In Northwest tree fruits, there has been success in using azadirachtin products for the control of pear psylla and grape mealybug in pears and lacanobia fruitworm in apples. Trials elsewhere with some formulations have shown activity on tentiform leafminer and leafhoppers. Azadirachtin has generally low toxicity to spiders and mites, with little effect on predatory mites in trials. There is little risk to bees if flowering crops are not sprayed.
Many azadirachtin insecticides are certified for use in organic programs and are used mostly by organic producers. With its varied modes of action and effects on insects, azadirachtin may have a fit in selective programs where natural enemies are providing significant biological control, an important factor for most organic tree fruit producers. However, tests with azadirachtin on many classes of predators and parasites reveal mixed results. Adult predators usually tolerate high dosages of azadirachtin but their subsequent activity, longevity and fecundity may be reduced. No problems were observed with the treatment of lady beetle and lacewing eggs. Azadirachtin residues repelled a Campylomma species in one trial, but had little direct effect on two other hemipteran predators in another. Parasitoids, specially parasitic wasps, are generally less sensitive to azadirachtin than predators, but there have been sublethal effects on some wasp species when larval stages were treated. As with insect pests, azadirachtin effects are mostly sublethal and not quickly observed; multiple generation lab and field studies are needed to accurately assess effects.
The first commercial azadirachtin formulation registered on pome fruits in the US was Margosan-O in 1985. There are currently three formulations widely available in the Northwest: AZA-Direct, Ecozin, and Neemix (see Table 1). All are liquid formulations with differing amounts of active ingredient (azadirachtin A).
Table 1. Azadirachtin formulations registered on apples and pears
|Ecozin 3%||Amvac||3.0%||$3.13||12 oz.||11.3||$37.56|
|Neemix 4.5||Certis USA||4.5%||$4.69||10 oz.||12||$46.90|
**rates will vary with pest and program
These products are low-risk materials, with a 12-hour reentry interval (or less) after application and minimal personal protective equipment requirements. They can be applied up to and including the day of harvest. The product labels all emphasize the importance of spray coverage, and repeated applications are recommended for most situations. The addition of horticultural spray oil is often recommended to improve leaf penetration and insecticidal activity.
Trials with azadirachtin formulations are continuing to determine how best to include this material in apple and pear pest management programs. These trials, along with increasing commercial use by both organic and conventional growers, will help define the usefulness of azadirachtin in controlling the pests particular to pears and apples as well as its impact on natural enemies and biological control. Azadirachtin has been used for centuries in other parts of the world; it may have a good fit in the 21st century for many tree fruit growers in the Northwest.