Studies on the effectiveness of various insecticides and drying in controlling granary weevil (Sitophilus granarius L.) in stored wheat seed : a thesis presented in partial fulfilment of the requirement for the degree of Master in Agricultural Science (Seed Technology) at Massey University, New Zealand
This study was designed to examine the effects of various insecticidal materials and of repeated seed drying during storage on the population dynamics and survival of Granary Wevil (Sitophilus granarious L.) in wheat. The study was conducted in three parts. The first experiment examined the immediate and longer term effectiveness of the contact insecticide malathion and the fumigant phosphine along with influence of repeated drying to safe moisture contents on granary weevil infestation. Wheat seed was stored for 165 days at 25 C and 80% R.H. Treatments were examined every 30 days to study the developrrent of Sitophilus granarius populations and associated damage to wheat seed and evaluation made of the effects of drying, malathion and phosphine on the established infestation and on seed quality. Granary weevils increased by about a factor of X 10 every 60 days and did extensive damage to chemically untreated seed. Repeated drying reduced the rate of increase but did not eliminate the insect population. Malathion dust added to infested seed severely checked insect development and when combined with drying destroyed the infestation completely. Malathion also displayed considerable residual effect and had no adverse effect on seed viability. Phosphine was found to be totally effective in eradicating an established population of granary weevils from seed without affecting seed quality.
In a second experiment malathion was sprayed onto jute squares at
2.5% and at one half and one quarter of this rate. Treated squares were stored for 90 days at 20 C, ambient RH of 70 - 90% or 30 C, ambient RH of 60 - 80% and the residual toxicity of the deposit was assayed with live insects at intervals after treatment. Malathion was also applied at 2.5% concentration to the outside of grain filled sacks which were then placed individually into large plastic bags into which adult granary weevils were introduced at 7 day intervals. After
56 days storage, counts were made of live and dead insects inside the sacks to assess protective effect of the malathion treatment. On jute squares malathion was completely effective at all concentrations and at both storage temperatures (20 C and 30 C) immediately after application and also after 7 days. Thereafter, it lost its effectiveness slowly over the next 90 days storage. In whole sack treatment malathion was found to provide only immediate protection at both temperatures and was inadequate after only a few days.
In the third experiment wheat seeds, uninfested and infested with Sitophilus granarius, were mixed with ground neem seed of each of two species of neem ( Azadirachta indica and Melia azaderach) at 1 g per
20 g wheat and were stored at 25 C and 80% R.H. Seeds were examined for live and dead insects and germination assessed after 90 days storage. Little or no direct mortality of adults was recorded but there was indirect evidence of suppression of egg laying particularly with Azadirachta indica. Neem seed powder did not affect the viability of the wheat seed.
This study has clearly shown the short term residual effectiveness of malathion, the immediate eradicant action of phosphine and the poor performance of the natural insecticidal chemical in neem seed on granary weevil infestation in wheat. The results also show the maintenance of low seed moisture contents in wheat to be a practical method of reducing insect populations. The role of granary weevil in damaging seed was clearly seen by X-ray photography and by the extent of types of abnormal seedlings found in positional germination tests. In the absence of effective control Sitophilus granarius has the potential to devastate wheat seed quality in terms of both purity and germination in as little as 90 days.