Biorational control methods for protection of stored grain legumes against bruchid beetles

Bruchid beetles such as Callosobruchus maculatus are known to the storage pests of grain legumes and cause tremendous damage. The main method to protect from them is chemical insecticides or fumigants. But, they would cause some problems such as environmental pollution, hazard to health. So, it is necessary to develop the grain legumes protection methods or systems using less chemical insecticides or fumigants from the perspective of integrated pest management (IPM). In this paper, I review the works of legumes grain protection methods without chemical insecticides or fumigants especially for the natural parasitoids, essential oils and other methods recently developed.


INTRODUCTION
Chickpeas (Cicer arietinum L.) and cowpea (Vigna unguiculata Walp.) are important grain legumes grown in Asia, the Mediterranean, Australia, Canada, the USA, and Africa [1] and stored dry.However, they become infested by many insect pests both in the field and during postharvest storage.The most important pests of stored grain legume seeds are bruchid beetles (Coleoptera: Chrysomelidae: Bruchinae), such as Callosobruchus maculatus (F.) and Callosobruchus chinensis (L.) [2,3].Even low initial infestation rates can lead to tremendous damage because of the beetles' high fertility and short generation times [4].Each emerging female C. maculatus quickly finds a mate and, if food is readily available, produces about 100 offspring.After three or four generations, each of which takes only about a month, losses are severe [5][6][7][8].Most subsistence farmers in developing countries rely on traditional storage structures, which are especially vulnerable to bruchid attack [9,10].The use of insecticides or fumigants to protect the seeds from bruchids is effective, but these chemicals pose health hazards to farmers and consumers, cause environmental pollution, and cost money.Furthermore, insects develop resistance to insecticides, necessitating the application of larger amounts [11].These problems can be reduced through the use of integrated pest management.This paper reviews alternative methods to protect against bruchid beetles.Breeding of crops for resistance to storage insects, especially bruchid beetles, has already been reviewed [12].This paper focuses on parasitoids, essential oils, and recent new methods.

Natural Parasitoids
Some wasps parasitize bruchid beetles (Table 1).Females of the wasps oviposit on the eggs, larvae, or pupae, and the emerged larvae feed on them.Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae) parasitizes lateinstar larvae and pupae of a wide range of beetle species, having a stronger impact on beetle populations than other species such as Eupelmus vuilletii (Crawford) (Hymenoptera: Eupelmidae) [18,22].Uscana lariophaga Steffan (Hymenoptera: Trichogrammatidae) parasitizes eggs of C. maculatus and Bruchidius atrolineatus Pic (Coleoptera: Chrysomelidae: Bruchinae).The release of D. basalis adults in suitable numbers and under suitable conditions reduced populations of C. maculatus drastically and halted seed weight loss for 6-7 months in West Africa [20,23,24], and controlled Acanthoscelides obtectus (Say) in Colombia [25].Uscana lariophaga significantly reduced the numbers of C. maculatus adults, the rate of damaged beans, and seed weight loss in a traditional storage system in Niger for 3 months [26].Tracking of the phenological relationship between A. calandrae and C. maculatus populations over 5 months suggested that A. calandrae reduced the C. maculatus populations [27].However, these parasitoids need suitable temperature and humidity to be effective as biological control agents.And within closed storage systems, populations of both hosts and parasitoids can reach high densities, which could lead to high intra-and inter-specific competition among parasitoids for the host resource as, for example, among E. vuilletii females [28] and between E. vuilletii and D. basalis [18,22,29].Such competition may reduce the parasitoids' effectiveness at biological control, although the coexistence of U. lariophaga did not change the ability of D. basalis to suppress C. maculatus and damage to beans [30].Before parasitoids are used as biological control agents, it is necessary to take into account the costs and benefits associated with the optimum environmental conditions, numbers, kinds, and combination of parasitoid species.

Essential Oils
Several plants and constituent bioactive substances, also called "insecticides of plant origin" or "botanical insecticides", have been tested against seed beetles.The effects of essential oils from some plant species on adult mortality, oviposition, F 1 adult emergence, and other behaviors of bruchid beetles, especially C. maculatus, by contact and fumigant treatment have been reported (Table 2).Essential oils comprise mainly monoterpenes, sesquiterpenes, and low-molecular-weight aromatic compounds [41].Some monoterpenoids affect mortality at all developmental stages and oviposition [42]; and 1,8cineole from Alpinia calcarata (Rosc.)(Zingiberaceae) affects adult mortality, oviposition, and F 1 emergence [31].However, essential oils are likely to affect predators as well.For example, essential oils of Cymbopogon nardus (L.), Cymbopogon schoenanthus (L.), and Ocimum basilicum (L.) increased the mortality of D. basalis larvae and pupae that parasitized C. maculatus, as well as C. maculatus [43].In contrast to genetically modified legumes expressing α-amylase inhibitor-1 of Phaseolus vulgaris L. or cysteine protease inhibitors, which are resistant to several bruchid species and should be compatible with the use of parasitoids [44], essential oils might not be compatible with the use of parasitoids.

New Methods
Temperature management is one of the most promising tools for controlling pests of stored grain [45].Studies of the effects of both low and high temperatures on mortality of C. maculatus at different life stages give varying results, depending on the methods [46,47].Although temperature control offers a way to disinfest beans, potential problems include the effect of the treatment on the beans and the cost of the equipment.Although low temperatures do not harm seeds, high temperature stimulated the germination of mung beans [48].Further research of the effects of temperature, especially high temperature, is needed.
Hermetic storage bags (SuperGrainbags; GrainPro, Concord, MA, USA), which reduce internal oxygen levels, increased C. maculatus mortality and reduced invasion from outside when used to store cowpea [49].

CONCLUTION
Several methods may protect stored beans from bruchid beetles without the need for chemical insecticides, but adequate control might depend on a combination of methods.Combining methods would also reduce the likelihood of the development of resistance [11].Neem oil could be combined with resistant cultivars for the management of C. maculatus [50].The combination of arcelin, a protein associated with insect resistance in P. vulgaris, with biological control by D. basalis increased the mortality of A. obtectus relative to D. basalis alone [51,52].Such combinations would support integrated pest management of bruchid beetles while reducing the use of chemical insecticides or fumigants.

Table 2 .
Sources of essential oils, target species, and effects and types of treatment.