Larger grain borer (LGB)

Damage

The larger grain borer was accidentally introduced from Central America into Tanzania in the late 1970s, and spread to other countries in the region. In West Africa it was first found in Togo in the early 1908s. It has now spread to many African countries becoming the most destructive pest of stored maize in both West and East Africa.

The larger grain borer is a serious pest of stored maize and dried cassava roots, and will attack maize on the cob, both before and after harvest. Adults bore into the cassava or maize husks, cobs or grain, making neat round holes and tunnelling extensively producing large quantities of grain dust as they tunnel. The adults prefer grain on cobs to shelled grain, thus damage on unshelled maize is greater than on loose, shelled maize.

Maize cob damaged by the larger grain borer.

© J. Maundu, icipe

When infesting stored maize cobs with husk intact, the adults frequently begin their attack by boring into the maize cob cores, and eventually gain access to the grain at the apex of the cob by crawling between the cob and husk. They may also bore directly through the husk. They cause considerable losses in stored maize; weight losses as high as 35% have been observed after only 3 to 6 months storage in East Africa. Losses in dry cassava can be very high too; the dried roots may be readily reduced to dust by boring adults. Average losses of 19% have been recorded after 6 months storage and as much as 30% in some cases. 

The larger grain borer is spread over longer distances almost entirely through the import and export of infested grain. Local dispersal is through the local movement of infested maize and dried cassava and by flight activity of the adult beetles. 

Although the larger grain borer develops best at high temperature and relatively high humidity, it tolerates dry conditions, and may develop in grain at low moisture in contrast to many other storage pests, which are unable to increase in number under low moisture conditions. For this reason, infestations of the larger grain borer usually found together with other storage pests, is the predominant under dry conditions. 

Attack by the larger grain borer is sporadic. Pest incidence may be low for several years and then suddenly increase in a "bad" year. 
 

Symptoms

Adults tunnel through stored maize grain or other starchy products, such as dried cassava chips, creating large quantities of dust. 
 

Host range

The larger grain borer is reported to breed only in maize and cassava. The adults can, however, live in and damage many stored products such as bulrush millet sorghum, yam, and wheat, as well as structural wood and wooden utensils.

Adults also bore into a wide range of foodstuffs and other materials such as bamboo, gourds, plastic and soap. In heavy infestations, wooden storage structures may be become damaged and act as reservoirs of infestation from which the new harvest may be attacked. The larger grain borer also occurs in the natural environment. It is able to breed on dead, dry wood of a range of trees, as well as dried stems of cassava and maize plants. Studies of this pest using pheromone traps showed that it was widespread in the natural vegetation in the Tsavo National Park, Kenya (Nang'ayo et al., 1993, 2002, Nansen et al., 2004).

 

Symptoms by affected plant part 

Seeds: holes, large quantities of dust.

 

Affected plant stages 

Post-harvest. 

 

Affected plant parts 

Seeds. 
 

Eggs are laid in tunnels and chambers bored by the females in the food source. Larvae hatch from the eggs after 3 to 7 days

The larvae are white, fleshy and sparsely covered with hairs and have three pairs of legs. They develop within the grain or in the flour that accumulates by the feeding action of the adults. They pupate inside the food source. 

The adult beetle is 3 to 4.5 mm long and dark brown in colour. It has a cylindrical body shape, when viewed from above the rear of the insect is square shaped. The thorax bears rows of teeth on its upper front edge and the head is turned down underneath the thorax so that it cannot be seen from above. The female lays 30 to 50 eggs into the produce (maize, cassava, etc).

The lifecycle can be completed within 25 to 26 days at optimum conditions - this is high temperature (about 30°C) and relatively high humidity (about 70% RH, + 13% grain moisture content). Development takes longer under cooler or drier conditions. 

The larger grain borer develops more rapidly on maize grain than on cassava. 
 

Pest and disease management: General illustration of the concept of Infonet-biovision

This illustration shows the methods promoted on infonet-biovision. The methods shown at the top have a long-term effect, while methods shown at the bottom have a short-term effect. In organic farming systems, methods with a long-term effect are the basis of crop production and should be of preference. On the other hand methods with a short-term effect should be used in emergencies only. On infonet we do not promote synthetic pesticides. 

Further below you find concrete preventive and curative methods against Larger grain borer.

Cultural practices

Detection and inspection methods

Except when populations are very high is not possible to detect the pest by visual inspection. The immature stages develop within the food source, and therefore they are not normally seen. Traps baited with the chemical attractant (pheromone) produced by the male beetle are useful to detect and monitor adult beetles. This pheromone is synthesised in the laboratory and loaded into plastic capsules, which then release the pheromone slowly through their walls. A pheromone capsule is then placed in a suitable trap.

Flight traps, such as funnel, delta or wing traps baited with the pheromone are considered the best for monitoring the larger grain borer. These traps are suspended about 1 to 2 m from the ground outside the store or the standing maize crop. They should be placed at least 100 m from stores or from the field to avoid attracting the beetles to these food sources. The traps are useful for researchers and for plant protection authorities. Traps are an important tool for phytosanitary purposes and for warning farmers about impeding attack by the larger grain borer. 
However, small populations already feeding on maize or cassava in a store cannot be detected by pheromone traps because the pest does not react to the pheromone until dispersing from its food source. Only when the population has increased to an extent whereby the infestation is obvious and the beetles are starting to disperse will the traps catch beetles. Presently, the only means of assessing infestations in store is by manual sampling of the produce. 

A detailed leaflet giving recommendations on the use of pheromone traps to monitor the larger grain borer has been prepared by Hodges and Pike (1995). Although the traps and pheromones are available commercially, they are expensive and not easy to get. See under reference addresses.

Store hygiene

• Good store hygiene is very important in limiting infestation.
• Clean store thoroughly between harvests.
• Remove and burn infested residues before the new stock is stored.Immerse used sacks in boiling water to eliminate residual infestations.
• Eliminate residual infestation in the wooden structure of the store by removing timber or by fumigating the whole store under a gas-tight sheet.

For more information on storage pests click here 

 

Harvest timely

When maize is ready for harvest, do not leave it for too long in the field; the larger grain borer or other storage pests could attack it. Studies in Benin have shown that maize harvested 3 weeks after physiological maturity gave better economic returns when stored for 8 months than maize harvested 1 or 7 weeks after physiological maturity. 
Leaving the maize in the field for extended periods after physiological maturity resulted in severe grain losses after 8 months of storage, mainly due to damage by the larger grain borer. However early harvested maize had a higher proportion of mouldy grain (Borgemeister et al., 1998).

 

Post harvest

In locations where the larger grain borer is a problem, shell infested cobs as soon as possible before storing and dry completely to below 12% moisture (safe for bagging); when the kernels are too hard to bite through with the teeth they are usually dry enough for bagging. Treat the grain with a botanical pesticide. Traditional varieties with good husk cover are much less likely to be attacked, thus when storing these varieties on the cob, reject any cobs with damaged or open sheathing leaves. (Meikle et al, 2002; Borgemeister, et al, 2003).

In the case of cassava, leave roots in the ground for as long as possible to reduce the storage period in order to minimise losses. After harvest, sun dry the cassava and immediately transfer it to sealed containers. 

 

• Store only clean produce. Carefully inspect the store before the newly harvested maize or cassava is placed inside and sort out infested cobs or roots for immediate use.
• Store the grain in a suitable container. The larger grain borer easily attacks grains stored in gunny bags or guards. Moreover, this pest also damages guards. The most suitable containers are those that can be sealed such as metallic containers, old oil drums or mudded cribs or baskets. They provided a very effective barrier to pest attack and can be used provided the stock is sufficiently dried so that ventilation is not required.
• Use brick stones to construct the granaries; wood and grass would encourage breeding and multiplication of the larger grain borer.
• Prefer iron sheet roof for the stores to avoid harbouring the pest. If using grass thatched, it should be a thick layer and cone shaped; the roofing should be replaced after a certain interval period to minimise leaking.

 

Sell the maize stock within 3 months

Sell the maize within 3 months since the extent of larger grain borer infestation during the first 3 months of storage is generally low. Alternatively split the maize harvest into two portions. One portion, destined for consumption by the familiessince the extent of large brain borer infestation during the first 3 months of storage is generally low. Alternatively split the maize harvest into two portions. One portion, destined for consumption by the families, should not be kept longer than three months in the store. The other portion to be kept longer in the store should be treated if larger grain borer was observed the previous year. If not, the stock should be regularly inspected. If the pest is found subsequently then grain shelling and treatment either with a botanical or with an inert dust is required.
 

Biological pest control

Natural enemies

The beetle Teretrius (formerly Teretriosoma) nigrescens, which is a specific predator of the larger grain borer in Central America, has been introduced into Africa. The adult and the immature stages of this predatory beetle feed on eggs and larvae of the larger grain borer. 

The predatory beetle has been released in Benin, Ghana, Guinea-Conakry, Kenya, Malawi, Tanzania, Togo and Zambia. It became well established and spread in most countries. However, despite the successful introductions, there are still regular outbreaks of the larger grain borer and farmers still suffer losses. Nevertheless this predator has a role to play in the management of the larger grain borer, as it is able to reduce the density of the pest.

Biopesticides and physical methods

Effect of Neem on storage pests

Several plants have been reported to control the larger grain borer. See table below:
 

Plant	Plant part	Product/ concentration	Effect on damage
Castor beans	Seed	10% ethanolic extract	-
Neem	Seed	5-10% slurry	< 10% damage
Neem	Oil	1.5% (vol/vol)	< 16% damage
Pyrethrum	Flower	0.5% powder (w/w)	Highly effective
Velvet leaf	Leaf, root	2.5-10% slurry	< 10% damage

                                                                                                                                                                              (Source: Modified from G. Stoll, 2003)

Using plant material in the form of slurry has given better results than plant powders. The slurry can be prepared by weighing out powder into 150 ml containers and adding sufficient water to give a 10% concentration (w/w), and stirring until a smooth paste is obtained. Then, the grain is poured into prepared slurries and stirred with a rod until all grains are coated (Tierto, as cited by GTZ, and Stoll, 2003).

Neem shows considerable potential for controlling pests of stored products. Jute sacks are also treated with neem oil or neem extracts to prevent pests - particularly, weevils and flour beetles- from penetrating for several months. 
However, neem products are not as effective for protection of maize grain against the larger grain borer as against grain weevils. Pyrethrum is much more effective. Since these two pests are usually found together, a mixture of neem and pyrethrum known as ("Nimpyr") seems as a better option to protect stored maize. Trials in Tanzania showed much lower grain damage in maize treated with "Nimpyr" (0.5 - 6% kernel infested) compared to untreated maize (17% to over 90%) 6 months after treatment.
 

But there are some shortcomings to the use of this mixture, namely:

• A relatively large amount of the mixture is needed to protect grain (2 to 3 kg/ 100 kg grain).
• The labour input needed to manufacture "Nimpyr" is considerable.
• The active principles of pyrethrum deteriorate relatively rapid on exposure to heat and/or light.
• Pyrethrum has an unpleasant odour, whilst neem has a bitter taste (although this can be eliminated by soaking and washing the grains in water for a sufficient period).
• The mixture is unlikely to give protection in maize stored in cobs, since the pests are protected under the husks.

How to prepare and use "Nimpyr"

• Collect ripe neem fruits; tease out the seed kernel, wash and dry in the shade for 3 to 5 days. Then pound the kernels into a fine pulp, dry for a further 1 to 2 days, crush with the fingers and sieve. Pound and sieve again until a fine powder is obtained. Do not use the same mortar as for food processing because of the bitter taste of the neem seeds.
• Pluck the dry florets of pyrethrum, pound in a mortar and sieve. Florets and powder quickly lose their effect in the sunlight. Therefore, store florets away from the light if they cannot be processed immediately. Use powder straight away.
• To treat 100 kg of grain, mix 1.5 to 2 kg of neem seed powder with 0.5 to 1 kg of pyrethrum floret powder. When the grain is well dried, distribute this preparation over it and mix in carefully. Store without delay.
• Grain intended for consumption must be soaked and washed in abundant water before use to eliminate any unpleasant odour or taste.

(Source: GTZ- Plant derived products as protectants against the larger grain borer and other stored-food pests).

Neem oil is an extremely effective and cheap protection for stored beans, cowpeas, and other legumes. It keeps them free of bruchidbeetle infestations for at least 6 months, regardless of whether the beans were infested before treatment or not. This process may be unsuitable for use in large-scale food stores, but it is potentially valuable for household use and for protecting seeds being held for planting. The treatment in no way inhibits the capacity of the seeds to germinate.

Neem has also been used in India to protect stored roots as well as tubers against the potato moth. Small amounts of neem powder are said to extend the storage life of potatoes for 3 months. (OIA 1992).

For more information on neem in pest control click here

The use of diatomite earth for control of grain boring insects during storage has in many cases been successful. Mix diatomite powder with grain before storing in bags or dust newly harvested dry cobs before storing them with diatomite. Use 1 kg diatomite per bag of maize or grain. 

Some confusion exists on the use of diatomite earth, as finer ground diatomite products commonly used for sifting beverages is not effective as insect control. However unprocessed products such as 'Kensil Lagging' work on the same principles as laterite mentioned below, by dehydrating the insects and by destroying the insects' articulations.