Botanical controls are needed because insecticides have not been successful in controlling intense grasshopper attacks. They also have the advantage of avoiding chemical buildup in the food chain. Researchers at Obafemi Awolowo University in Nigeria conducted the experiments using techniques they considered appropriate for the village level.
The fruit of the native aridan tree, Tetrapleura tetraptera was used as an emulsifier for neem oil. Dried aridan fruits were chopped then boiled in water until foaming. This mixture was filtered before being used as an emulsifier. Neem oil was added at different volumes.
Neem Kernel Water Extract (NKWE). The neem kernel was pounded into a coarse powder, distilled water was added, and the mixture was allowed to sit overnight. Different volumes of powder were added.
Neem Leaf Water Extract (NLWE). Young neem leaves (less than two-weeks-old) were chopped then boiled for 10 minutes in a vacuum flask. Equal parts of water and leaf were used.
These mixtures were applied to cassava in two experiments, one of which included the chemical treatments. Grasshoppers were introduced to the plants. Observations were taken at intervals to evaluate the amount of defoliation.
All neem products protected the cassava. Those prepared from seed seemed to lose their effectiveness after 11-14 days.
NOEC protected cassava for up to nine days at .05%, and was potent for 11 days permitting no feeding at 3 and 4%, but was toxic to the plant at 5%. As an oil product, NOEC is less likely to wash off, although in southern Nigeria grasshopper attacks are worse during the dry season. NOEC prepared with T. tetraptera served a double function as emulsifier and molluscicide. (T. tetraptera controlled snails on the treated farm.) It can be stored under refrigeration for up to five months.
NKWE at 5% was 100% successful for at least 12 days and was superior to the three chemical insecticides applied at field strength. All applications of NKWE were more effective than lindane for the first four days. NKWE seemed to persist longer and is the easiest to prepare, but uses more neem seeds than NOEC to be effective. The NLWE was effective for one week with no defoliation. It was also the cheapest to prepare and uses leaves, which unlike seeds, are available year round. Preparing NLWE with boiled leaves appears to be more effective than grinding them.
Results indicate that the following four neem treatments are recommended for the control of Z. variegatus on cassava in the dry season:
The study ranked NOEC as the most effective, followed by NKWE and NLWE. Although NKWE was in fact more effective in protecting the plant, NOEC was ranked first because as an oil it adheres better, and it also stores well. It can be refrigerated for up to five months.
Neem products show promise as an effective control against grasshoppers, and have many advantages over synthetic controls: they can be prepared at the village level with readily available materials, at no cost to the farmer, and with no danger of accumulating in the food chain. More study is needed of neem in on-farm trials under actual pest pressure.
For more information:
Department of Plant Science
Obafemi Awolowo University