Cereal leaf beetle

Cereal Leaf Beetle
Oulema melanopus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Family:	Chrysomelidae
Subfamily:	Criocerinae
Tribe:	Criocerini
Genus:	Oulema
Binomial name
Oulema melanopus (Linnaeus, 1758)

The Cereal Leaf Beetle (Oulema melanopus) is a significant crop pest, discovered by Carl Linnaeus in 1758.

Life History

One generation of this beetle is produced a year. Adults feed before winter and spend most of their overwintering time in protected areas such as wind rows, crop stubble, and tree bark crevices.^[1][2][3] Adults mate once the temperature warms to above 9-10 degrees Celsius (or 44.6 degrees F) ^[1][4] and females have a relatively long laying period (about 45–60 days) where they deposit eggs to the underside of leaves.^[1][2] The larvae hatch in 7–15 days,^[3] and start the most damaging eating of the entire life cycle. These larvae mature in 12–20 days.^[3] Larvae are the greatest threat to crops, as they eat the superficial layer of leaves, hindering the leaves' photosynthetic potential.^[2][5] According to Kon, Zabik, Webster, and Leavitt, there is also a chemical factor that attracts the beetles to leaves, and that chemical factor is directly related to hydrophobicity.^[2][6] Pupation occurs in the ground, with the adults emerging in 20–25 days,^[3][7] ready to start the cycle again.^[3][8]

Eggs are cylindrical and round, approximately 0.9mm long and 0.4mm wide. Eggs are often laid along the mid vein on the underside of leaves. They are bright yellow at first and darken over time, and are partially black when the larvae emerge.^{[1][2][9][10][11]}

The larvae appear white or yellow, are hump-backed, and have a black head and six small legs. They have a defense mechanism in which they smear excrement on their bodies to mask their vibrant color and to deter predators. This gives them a shiny black appearance, as opposed to yellow or white.^{[1][2][3][9][10][12]}

Adults, on average, are about 5-mm long and have dark blue wing covers and red legs. Their thoraces range in color from red to orange to reddish brown.^{[2][3][10][12]} The biggest distinction between male and female O. melanopus is the difference in shape of the first abdominal segment. The male’s segment is more narrowly rounded and flat or concave, and it is more broadly rounded and convex in the female.^[7]

Pupae are rarely seen in fields, as they are dormant at that time. Pupae are approximately 5mm in length and are yellow, also darkening with time.^[9]

Habitat

O. melanopus is native to Europe and Asia, however it has become much more common in the world today.^{[2][3][9][10]} In North America, it was first spotted in Berrien County, Michigan in 1962.^[2][13][14] Since then, it has spread through the Midwest to the east coast, and is making its way westward.^[2] Significant populations are found in Virginia, North Carolina,^[3] Utah, Montana, North Dakota,^[10] and Missouri and Iowa.^[15] Envoronmental conditions such as temperature and humidity drastically affect the O. melanopus population for that year. A cold spring will favor the host plant, whereas a warm spring will favor the leaf beetle.^[16] Scientists are worried that an increase in the general temperature of the planet would allow O. melanopus to spread into Canada.^[17] More recent surveys have shown that O. melanopus has indeed arrived in Canada, in 2006 it was present in 11.1% of fields, and in 2009 in 33.3% of fields in southern Alberta.^[18]

Locally, adults spread to the exterior of fields during their overwinter. Larvae like to stay on the outside of crop fields, but are also known to be in the center also. Local populations are never homogenous, there are hotspots and empty places in each field.^[14][19]

Food

O. melanopus consumes nearly all cereal crops; however has a strong affinity for oats, barley, rye, and its favorite host is wheat.^[1][13][20] Alternatives are corn, sorghum and sudangrass for adults, and wild oats, quackgrass, timothy, canary grass, reed canary grass, annual and perennial ryegrass, foxtail, orchard grass, wild rye, smooth brome, and fescues for the whole lifespan.^[5][10][20] The physical symptoms of the plant caused by the are thin long lines where the upper epidermis of the leaf has been eaten. Since the beetle is migratory when it eats, it is not consistent within a field. There may be spots where higher concentrations of beetles reside and another where few live. It is reported in Michigan that entire fields are rarely affected and the situation is most likely the same in the Midwest.^[2] A field of plants looks weathered and old, but is never completely destroyed. Damage is usually no more than 40% total.^[9]

When an herbivore consumes a plant, it releases Volatile Organic Compounds, or VOCs. Because VOCs are a chemical signal that attract some predators, the female O. melanopus is repelled by the chemicals for her own safety and the safety of her progeny. Males are deterred, but will not change eating habits .^[21][22][23]

To determine the eating habits of O. melanopus, scientists conducted an experiment using different media. They took agar gel infused with pea and barley extracts, and noted the resulting reaction. It was found that when used alone, both pea extract, which is not a desired food for the beetle, and barley extract, which is an alternate source, produced a weakening or halting of consumption. A combination of both, however, incited a small feeding response. The results were that there is most likely a secondary compound in barley that incites an eating response, perhaps when a lack of desired food is present.^[6]

Human control efforts

Biological solutions

Scientists have had success in labs with a few types of nematode, Steinernema feltiae B30, S. carpocapsae C101, and Hetero-rhabditis bacteriophora D54. Nematodes attack the adults that overwinter in the soil, preventing them from reproducing in the spring. The result found that S. carpocapsae C101 was the most effective, and even proved to be an effective alternative to pesticides. Each of the nematodes, however, was sensitive to temperature, implying that some strains would work better than others due to the temperature. Since the spread of O. melanopus is so great, a consistently important factor to consider is the temperature when selecting which strain is the most effective for the desired location.^[8]

As a more humane solution, scientists have found a way to use the hormone secreted by the males to attract the beetles into a trap, where they can be removed from the site.^[24] Once the beetles are in the trap, the next step is very difficult, as releasing the individuals into another field would cause them harm. Killing the individuals immediately is likely quicker than causing them to starve.

For the adult individuals, which feed on leaves, a couple of options exist to control population. The first is Hyalomyodes triangulifer, a tachinid fly that parasitizes adult O. melanopus individuals.^[10]

Another type of biological control consists of animals that parasitize O. melanopus larvae. Diaparsis carnifer, Lemophagus curtis, and Tetrastichus julis are three examples of wasps that do this ^[10] D. carnifer and L. curtis both consume the O. melanopus larvae, and T. julis lays eggs inside of the body of the O. melanopus larvae. Specifically, T. julis does not seem to pose too much risk to the crops themselves, but are extremely effective at dispatching large populations. Effectiveness is around 90%.^{[4][9][10][13][18][25]} With all parasitic wasps in general, some experiments are proving that spraying a sugar solution on the fields encourages predators of O. melanopus to grow fast and hearty where they can better kill off the population in the field.^[26]

Anaphes flavipes is an egg parasitoid that lays its eggs inside of the O. melanopus egg, killing it in the process. Effectiveness is around 90%. It is a good control agent because it is hearty and its life cycle is in synch with that of O. melanopus, the disadvantage being that it is not host-specific, and can cause other unforeseen deaths in other bugs, disrupting the balance of the ecosystem.^[4][9][18]>.

Coccinellidae, otherwise known as ladybug beetles, are known to eat the eggs and larvae of O. melanopus, and are known to be effective in some locations.^[10]

Other solutions

Even without human intervention, O. melanopus has experienced a decline in the United States due to general weather conditions like drought or excessive rainfall.^[4] In the early days of outbreak in the United States, California experimented with quarantine as the first option. They did not allow entry into California any material suspected to contain O. melanopus.^[27] O. melanopus populations traveling in hay can be suppressed by compressing the hay into bales.^[28]

Transgenic plants are still another option for control. Meissle et al. found that mortality of O. melanopus on Cry3Bb1 transgenic maize was double that of wild type. Many transgenic plants seek to change the physical properties of the plant, making things like adhering eggs a difficult task.^[29]

In Europe, where the species is native, scientists have experimented with several pesticides. In one experiment, four pesticides belonging to two groups (organophosphates and pyrethroids) were tested to see their effects on the beetles' numbers. These were tested at several concentrations and mixtures. The results found that a chemical named Vantex was superior; however the other pesticides were averaging 80% efficiency and were not to be counted out.^[19] With many pesticides, farmers must be cautious not to spray too early, because the eggs will not be affected much, and pesticide use will actually increase the numbers of O. melanopus because predators are killed.^[9] The optimum time to begin spraying is when approximately 50% of the larvae have hatched. An alternative is spraying when the adults are laying their eggs, which is also shown to be effective.^{[9][11][30][31][32]}

References

External links

Media related to Oulema melanopus at Wikimedia Commons
Data related to Oulema melanopus at Wikispecies