Resource Guide for Organic Insect and Disease ... - Cornell University

may cause an insect to stop feeding due to secondary physiological effects. As an egg-laying
deterrent, volatile compounds from neem may repel some insects from depositing eggs on a
plant surface.
There is evidence that other compounds found in neem have insecticidal attributes that contribute
to a given product’s efficacy. A study conducted at Washington State University, in conjunction
with the W.R. Grace and Company (manufacturers of the neem product Margosan-O at
the time), found that products containing both azadirachtin and neem oil have greater efficacy
in controlling aphids than either ingredient alone (Stark & Walter 1995). They hypothesize that
neem oil may help spread the chemicals on both plant and insect surfaces and allow them to
penetrate into the insect more effectively. Neem seed oil is formulated and used somewhat like
other horticultural oils and controls some foliar diseases as well as certain insects and mites.
The oil is also made into an insecticidal soap, which probably acts similarly to other insecticidal
soaps by disrupting insect membranes (see Soap chapter). At this time, there are no known
OMRI-listed neem/soap products.
The mechanisms for neem’s effects on mites (Miller & Uetz 1998), snails (Mostafa & Abdel-Megeed
1996), and disease organisms (EPA) have not been reported.
Active neem constituents can be absorbed through plant roots and systemically move upward
through the plant in xylem tissues (Gill & Lewis 1971; Larew 1988; Nisbet et al. 1993; Osman
& Port 1990). This uptake works best when sufficient quantities are applied to the root zone.
Systemic effects are much less apparent from foliar sprays and may differ widely depending on
plant species. Neem constituents last much longer within the plant than when sprayed on the
leaves. Over time, however, they become diluted by plant growth.
APPLICATION GUIDELINES:
Neem products are generally sold as emulsifiable concentrates (EC). While Copping (2001)
reports no known incompatibilities with other crop protection agents, phytotoxicity may be
a problem when combining neem oil or soap products. Read labels for specific application
guidelines including determination of re-entry interval (REI) and pre-harvest interval (PHI).
Range of efficacy will depend on the susceptibility of species in question and environmental
conditions at time of application; however, there are several other considerations for improving
the efficacy of this product:
1. Make multiple applications. Frequent applications are more effective than single sprays
because neem does not persist well on plant surfaces. Like most other botanically
derived materials, it can be rapidly broken down by sunlight and washed away by rain
(Thacker 2002).
2. Use against immature insects. Azadirachtin-based insecticides act on immature stages
of insects more effectively than on eggs or adults. To avoid a build-up of pest populations,
it is important to target insects in an early stage of their life cycle. For instance,
neem would likely have little effect on an infestation of striped cucumber beetle
adults; however, when applied to potato plants early in the season, it has been shown
to greatly reduce larval activity of Colorado potato beetle.
3. Begin applications before pest levels are high. Anti-feedant and egg-laying repellant
effects show best results in low to moderate pest populations.
4. Neem is reported to work best under warm temperatures (Schmutterer 1990).
5. Neem’s systemic properties suggest that applying it to transplants just before planting
to the field could be an effective and inexpensive way to control certain pests. Similarly,
applying neem with relatively large amounts of water, in directed sprays over the
rows of small seedlings, could be a very efficient method of application. In one study,
neem applied through a drip irrigation system significantly reduced lettuce aphids on
romaine by over 50% (Palumbo et al. 2001).
AVAILABILITY AND SOURCES:
Widely available from garden and farm supply mail order companies.
Organic Resource Guide 137