Controlling insect pests of stored organic chamomile by controlled atmospheres
Hashem, M. Y.1
Key words: Carbon dioxide, chamomile, modified atmosphere, population dynamics.
Different stages of Trupanea stellata and Lasioderma serricorne were exposed to four different gas mixtures differing in their CO2 content (20%, 40%, 60% and 80% CO2). In general, increase in carbon dioxide combined with decrease in oxygen resulted in increasing mortality. The gas mixture containing 80% CO2 was the most effective mixture to control the different stages of T. stellata (most tolerant than the different stages of L. serricorne insects). The use of this gas mixture to disinfest chamomile for 7day exposure in 30 m3 fumigation chamber under temperature range between 28.7-30.9oC, resulted in complete control.
Chamomile (Matricaria chamomilla L) is produced in Egypt using the organic farming system. Most of this product is for export to the European and American markets, in which the major constraints for exportation are the detection of either insect infestation or pesticide residues, of any other chemical. Chamomile is exposed during flowering in the field to attack by the chrysanthemum fly Trupanea stellata (F.) and during drying, processing and storage to attack by the cigarette beetle Lasioderma serricorne
The classic way to control these insects has been and still by the use of fumigants such as methyl bromide (CH3Br) and phosphine (PH3), which are not allowed for treatment of organic products. Recent work in many countries has focused on the possibility of using the inert gases (CO2 and/or N2) as an alternative for chemical fumigants. This method of treatment is commonly termed modified atmosphere (MA) or controlled atmosphere (CA) (Reichmuth 1992).
This work reports on the population dynamics of T. Stellata under field conditions and tests of susceptibility of different stages of L. serricorne and T. stellata to different mixtures of CO2, N2 and O2 under laboratory conditions and large scale conditions.
Materials and methods
The population dynamics of T. Stellata were carried out in Fayoum region in the winter season 2006/2007. Samples of 400 chamomile flowers were collected weekly, randomly for investigation. To examine the different stages found inside the flowers, each flower was examined under stereomicroscope after dissection. The number of larvae, pupae and infested flowers were recorded.
The Susceptibility of different stages of L. serricorne and T. Stellata to alterations of atmospheric gas concentration has been studied using the parental insects of L. serricorne, which were obtained from infested chamomile and reared on chamomile powder at 30oC 2oC and 70% 5% r.h. All stages except eggs were prepared for treatments. Trials were carried out on one week old adults, 3rd larval instars and 2-3 days old pupae of L. serricorne. The experimental unit for L. serricorne was 50 individual of each stage; and for T. Stellata; 50 dried chamomile flowers. Each unit of L. serricorne was prepared in a cylindrical cage (6 cm high and 1.5 cm diam.) made from 40 mesh stainless-steel wire gauze closed with rubber foam. Each 50 dried chamomile flowers was placed in small paper bag. Cages containing the different stages were introduced into a bottle of dressel flask (Hashem 2000).
The tested atmospheres were prepared from CO2, O2, and N2. This component was monitored using a paramagnetic oxygen analyzer (SERVOMEX/ England). To improve distribution of the components, the cylinders with gas mixtures were kept at room temperature for two days before starting the experiment. The gas mixtures tested were: a. 20% CO2, 64% N2 and 16% O2; b. 40% CO2, 48% N2 and 12% O2; c. 60% CO2, 32% N2 and 8% O2; d. 80% CO2, 16% N2 and 4% O2. Different stages in gastight connected dressel flasks were exposed to the gas mixtures from mixture cylinder through copper tubes and a humidifying unit containing saturated NaCl/H2O solution in flasks, to create 70% R.H. At the outlet of the containers the O2 content was determined continuously by an oxygen analyzer. After about 15 min. (time for about 10 replacements of total container volume by gas mixture) the outlet concentration became identical with the inlet concentration. After different exposure periods ranging from 1-4 days, each bottle was aerated and the insects transferred from the cages to Petri-dishes and held at 30oC2oC and 70%5% R.H. The adults were examined for mortality, and the pupae of L. serricorne were examined for adult emergence. Also the chamomile flowers were examined for adult emergence of T. Stellata. Each sample was accompanied by an untreated control. The experiments were designed to provide time-mortality regression lines for the different stages in various combinations of atmospheric gases and different exposure periods. Moralities of L. serricorne adults were corrected by Abbotts formula (Abbott, 1925), and Data was subjected to probit analysis (Finney, 1971) to calculate the slopes of regression lines and the values of LT50 and LT99,
The large scale application of the most efficient CO2 -concentration carried out in 30 m3 fumigation chamber (2.5 m high x 3 m width x 4 m long) which built at SEKEM Co. for biological products. The roof, walls and the interior side of the door were lined with aluminium sheets (150 thick) and the floor was covered with stainless-steel sheet (1mm thick). The door of the chamber has two openings; a lower opening for gas input and an upper opening for gas output. To ensure the air tightness of the chamber, all fill spouts, door margins and manholes, were sealed with duct tape. The pressure test to determine the efficacy of fumigation in the buildings, chambers and stores against stored product insect pests was applied before introducing CO2 (Reichmuth, 1992).
A quantity of infested 1500 kg chamomile (in boxes) was put in the chamber. Twelve cages of different stages (50 individuals/ cage/stage) of L. serricorne were placed in wire cages to monitor insect mortality within the treated product. The same number of additional cages (prepared as described above) was pushed in untreated products to serve as controls. Small paper bags of 50 infested chamomile flowers with T. stellata each were placed also within the treated product to monitor the insect mortality after treatment. To measure the temperature and relative humidity throughout the test, a thermo-hygrograph was installed in the centre of chamber.
After exposure, each stage of the two insects was transferred from the cages to a Petri dishes and held at 30oC2oC and 70%5% R.H. At 48 hours after exposure, the stages were examined for mortality. The criterion of dead larvae and pupae was its failure to develop to adults. At the same time, the infested chamomile flowers samples were examined to evaluate the survival stages of T. stellata.
Results and Discussion
Fig. (1) shows population dynamics of larvae, pupae/400 chamomile flowers as percentage of infestation. The obtained results show that the variations in the population density of larvae and pupae of T. stellata fluctuated from time to another.
Figure 1: The percentage of infested chamomile flowers by larvae and pupae of Chrysanthemum fly T. stellata.
As indicated from Fig. (1), two peaks were obvious for the larval stage in Feb. and April, but in the case of pupal stage three peaks of abundance were obvious. The first peak of the pupal population was recorded on January 21st, after that of larvae with nearly of three weeks and almost equal to it. The second peak of pupae was recorded in the third week of March. This can be explained that the present larvae transferred to pupae representing the end of a generation with a decrease in oviposition during this time. The infestation rates were high during the season especially on March 25th (34%). Knowing that import countries reject any product if the rate of infestation reached up to 5%, explains the importance of this pest attacking this crop.
LT50 and LT99 levels indicate the susceptibility of different stages of L. serricorne to alterations of atmospheric gas concentration (Fig. 2). At all gas mixtures, the stages of T. stellata, were more tolerant than the stages of L. serricorne. The LT99 values for larval stage (more tolerant than other staged of L. serricorne) were 4.80; 4.30; 2.82 and 0.6 days at the different gas mixtures. The LT99 values of T. stellata were 6.40; 5.60; 4.70 and 4.00 days. The LT99 values for pupal stage of L. serricorne were 3.50; 3.00; 1.60 and 0.51 days, and those for adult stage were 2.81; 2.00; and 1.00 days, respectively. Mortalities of insects exposed to the mixture containing 80% CO2, were higher than those of the insects exposed to mixtures containing 20%, 40% and 60% CO2 at all exposure periods ranging from 1 to 4 days. When, using the mixture containing 80% CO2, mortality reached 100% after 1 to 2 days for adults and after 5-7
Figure 2 (a & b): LT50 and LT99 values (in days) of treated stages of L. serricorne and T. stellata exposed to 4 different gas mixtures.
days for the more tolerant stages of both insects. Hashem and Reichmuth (1994) have shown that decreasing the oxygen content in the mixture increases the mortality in shorter exposure period. The descending order of the treated stages according to the LT50 and LT99 values was as follows: Stages of T. stellata > Larva of L. serricorne > Pupa of L. serricorne > Adult of L. serricorne (Hashem, 2000).
The large scale application of the efficient CO2-concentration for controlling stored chamomile insects was based on the results of the above mentioned tests (Fig. 2), the gas mixture containing 80% CO2 against the different stages of T. stellata was applied. Free space conditions throughout the application were 28.7-30.9oC and 65% R.H.
All treated stages of both insects were killed after 5 days exposure. Keever (1989) indicated that the pupae of the cigarette beetle are often more adversely affected during tests than the other stages.
The present findings indicate that the use of CO2 in well sealed containers may be a method for disinfecting chamomile and other products as long as the exposure period is not less than 5 days and the temperature is not less than 28oC.
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