This article is from ECHO Asia Note # 37.
[Editor’s Note: The following article comes out of research conducted by ECHO Asia staff and is a condensed version of an article that was recently published in Agronomy for Sustainable Development, an international peer-reviewed journal. The original article can be read on the Agronomy for Sustainable Development website.]
Introduction
Seed saving in sub-tropical and tropical climates is challenging. Without equipment designed to maintain dry and cool environments, the quality of seeds may quickly deteriorate. High temperature and humidity during storage increase seed metabolism and encourage the proliferation of seed-eating insects (Lale and Vidal, 2003; Upadhyay and Ahmad, 2011). Technologies such as refrigerators, dehumidifiers, and pesticides can help prevent these seed-damaging conditions, but may not be available to smallholder farmers in the tropics. Traditionally, many locally available treatments have been used to prevent insect pests. These treatments, typically added to seeds prior to storage, are meant to poison, damage, or discourage movement of insects around the seeds. Some treatments may effectively reduce insect growth, but they may also damage seed viability; it is important to identify which treatments are effective and appropriate for use by farmers. ECHO Asia research staff analyzed five low-cost treatments to determine their effectiveness in preventing the growth of a common seed storage pest called cowpea bruchids (Callosobruchus maculatus) in stored Lablab bean seeds (Lablab purpureus L.). In keeping with previous ECHO research by Croft et al. 2012, each treatment was also analyzed with and without vacuum sealing.
Seed pests like cowpea bruchids are difficult to detect, since they lay eggs on developing seeds in the field (Figure 1A) and hatch during storage to consume the mature seeds (Chauhan and Ghaffar 2002). The bruchids rapidly multiply under warm and humid conditions. Within a short period of time, they can consume large amounts of stored seeds, which otherwise would have been used for food or for planting the following year (Figure 1B). However, like all insects, bruchids cannot complete their life-cycles without oxygen (Ahn et al. 2013). Vacuum sealing can be used to reduce available oxygen to insects during storage (Van Huis 1991; see Figure 1C). The goal of this experiment was to explore low-cost seed treatment options along with vacuum sealing, to determine how they affect the growth of bruchid insects and whether or not they maintain seed viability.
How We Set Up Our Experiment
We evaluated six different low-cost control options that are used when storing seeds in tropical environments; each of the treatments was suggested to us by an ECHO network member. For each treatment, some bags of seed were sealed under vacuum and others were sealed inside plastic bags without vacuum. The treatments were also compared to seeds without any additional treatment, both vacuum sealed and not, as controls.
The treatments included:
- 10% bleach solution, used to wash the seeds prior to storage;
- Powdered galangal root (Alpinia galanga (L.) Willd.), mixed with seeds prior to storage;
- Locally purchased carbaryl, combined with seeds prior to storage;
- Pulverized bamboo charcoal, mixed with seeds prior to storage;
- Powdered laundry detergent, mixed with seeds prior to storage;
- Vegetable oil commonly used for cooking, mixed with seeds to coat them prior to storage.
We divided lablab seeds into plastic bags and applied the different control options. Over the following year, we evaluated the bags of seeds for bruchid presence and viability approximately every two months. We measured insect presence, or total pest load, by counting and adding the numbers of bruchid eggs, larva, adults, and insect holes called ‘windows’ on damaged seeds. We also tested seed viability each time, by testing the germination rates of the seed. We measured seed vigor by counting how many days it took until 50% of seeds germinated (this is important because uniform emergence of seedlings in the field matters to farmers, who frequently rely on specific periods of rainfall to plant their crops).
Results of the Experiment
Vacuum sealing proved to be overwhelmingly effective (Figure 2, A and B; results for vacuum sealing are in blue, beside the red-striped bars). Vacuum-sealed bags consistently prevented bruchid eggs and/or larvae from maturing and subsequently damaging the seeds. Vacuum sealing also maintained seed viability; after one year of storage, vacuum-sealed seeds maintained germination levels of 75-80%, while seeds that were not vacuum-sealed, and kept within plastic bags had germination rates of 65-70% (Figure 3A, page 4).
The low-cost treatments that were not vacuum-sealed showed varying levels of effectiveness (P < 0.05) against bruchid presence (Figure 2B). Bleach and laundry detergent were the least effective, while galangal powder, carbaryl, charcoal, and oil held bruchid populations at lower levels than the control. Only the oil treatment was statistically significantly lower (P < 0.05) than the control. The oil treatment reduced bruchid numbers equally well when samples were vacuum-sealed and when they were not vacuum-sealed. However, because of the oil’s negative effect on seed viability, is not a recommended control strategy (Figure 3B, page 4).
Seed viability did not differ significantly (P > 0.05) between vacuum-sealed and non-vacuum-sealed bags until the final sample month (Figure 3A). However, by the conclusion of the study, vacuum-sealed bags maintained initial viability, while non-vacuum-sealed bags showed some reduction (Figure 3A). The six treatments showed definite differences (P < 0.01) in seed viability
throughout the study (Figure 3B). The bleach treatment reduced seed viability and also did little to prevent bruchid growth. While the oil treatment effectively prevented bruchid growth, it greatly reduced seed viability. All other treatments were comparable to the control, and did not cause any noticeable change in seed viability.
Both non-vacuum-sealed and vacuum-sealed seed samples had a similar pattern for seed vigor, measured in days that seeds took to reach 50% germination (Figure 4A, page 4). The spike at month 6 (M6) might be due to the cooler weather and to seasonal changes of less light and humidity. The subsequent downward trend, to fewer required days of germination, makes sense because warmer, springtime conditions occurred in the later portion of the trial. Individual treatments had little effect on the mean number of days it took for seeds to reach 50% germination. Seeds from all the individual treatments were affected by seasonal weather conditions in month 6 (Figure 4B, page 4).
Conclusions
Storing seeds by vacuum sealing them is a very effective means of preventing seed loss. The vacuum environment maintains seed viability over time. It also prevents the growth of insect populations that feed on stored seeds. One can create a vacuum inexpensively using appropriate technologies such as a bicycle pump, which can be reconfigured to pull air from containers. If vacuum sealing is not a feasible solution in a particular context, several of the treatments described in this study appear to reduce bruchids while maintaining seed viability; these include carbaryl, charcoal, and galangal powder. The oil treatment also effectively prevented insect growth, but is not recommended as it greatly reduced seed viability.
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