Diabrotica virgifera

Diabrotica virgifera

The Western corn rootworm (Diabrotica virgifera LeConte, 1868) is an insect belonging to the Chrysomelidae family.

Systematics –
From a systematic point of view it belongs to the Eukaryota Domain, Animalia Kingdom, Eumetazoa Subarign, Ramo Bilateria, Phylum Arthropoda, Subphylum Tracheata, Superclass Hexapoda, Classe Insecta, Subclass Pterygota, Cohort Endopterygota, Superorder Oligoneoptera, Section Coleopteroida, Order Coleopteroida Cucujiformia, Chrysomeloidea superfamily, Chrysomelidae family, Galerucinae subfamily and therefore to the Diabrotica genus and to the D. virgifera species.
Two subspecies are known of this insect:
– Diabrotica virgifera virgifera;
– Diabrotica virgifera zeae, widespread mainly in Mexico.

Geographic Distribution and Habitat –
The Western corn rootworm is an insect of American origin and precisely of the United States, where it is considered the main phytophagous of corn. Until 1955 the chrysomelid remained confined to its range of origin, that is in the western part of the USA, then it rapidly spread eastwards and now it has settled in Mexico, Central America, where there are also other species of Diabrotica and in South America.
This insect arrived for the first time in Europe in 1992 in Serbia while in Italy it was caught for the first time in 1998 at the Venice airport. In the following years it was reported in other regions of Northern Italy.
It is currently widespread in particular in Northern Italy in the main maize areas. It is present in numerous European countries (in particular Serbia, Croatia, Hungary, Romania, Bulgaria, Bosnia and Switzerland); in America it is widespread in Canada and the United States.

Morphology –
Diabrotica virgifera, at the adult stage, has a yellow – brownish color and with dimensions of 5 – 6 mm in length, with evident sexual dimorphism.
Males have dark elytra with yellowish caudal part; the females have three symmetrical dark stripes on a yellowish background. Furthermore, the antennae of the males are longer than those of the females.
The eggs are recognized by their size of about 0.65 x 0.45 mm and with a color that is generally pale yellow.
Mature larvae measure 12 – 18 mm in length and appear wrinkled, whitish in color with a dark head and a brown plaque on the upper part of the last abdominal segment. Before reaching maturity these go through three stages of development.
The pupae are white in color.

Attitude and Life Cycle –
The Diabrotica virgifera overwinters in the egg stage which has been laid in the ground at a depth of about 15 cm in the period of July – August.
After about a week from the flickering, the first mating occurs, which continue for the entire life period of the adults; both males and females can mate multiple times in their life. Thanks to the action of the wind or by exploiting different vectors including humans, adults can move up to 25 – 40 km a year. Only one generation per year is carried out.
The females lay the eggs in small groups and complete the laying in about 20 days preferring moist and loose soils. A female lays a few hundred eggs in the ground at a depth of about 15 cm. Female fecundity is influenced by temperature, photoperiod and food availability; on average 400 eggs are laid even if their number can vary from about 100 to 1000. Furthermore, the depth of deposition is influenced by the texture of the soil, becoming more superficial in heavy soils. The egg begins to hatch from the middle of the following May with a peak of births that comes around the middle of June.
The larvae that are born from it move in the ground feeding on the roots of corn and various other grasses and their development lasts about a month. The larvae are found in the areas of Northern Italy starting from the month of May with a peak around mid-June, although the hatching can last until the beginning of July.
After three larval stages, the pupa is hardly visible in the soil as the metamorphosis takes place in 1-2 days.
This insect makes one generation a year.
In the areas of the Po Valley, adults fly from June to October with a peak between the end of June and the first ten days of July, in correspondence with the flowering of the corn, after which the adult populations remain in constant decline. Adults are more active at a temperature between 23 and 27 ° C; while in the hottest hours, when the temperature exceeds 30 ° C, they aggregate on the portions of the plant less exposed to the sun, protected by the bracts of the ear or by the leaf sheaths.
Adults live between 50 days and three months depending on climatic variables and food availability.
The damage is due to the larvae which, feeding on the rootlets and digging tunnels in the largest roots, cause the greatest damage to the corn. The plant, damaged as a result of their trophic action, has a reduced root development that makes it more subject to enticements, reduces its capacity to absorb water and nutrients and, during harvesting operations, causes greater production losses. A fairly typical symptom that is found in cases of high larval infestation is the presence in the field of “goosenecked plants”, that is, of enticed plants that tend to rise from the ground by bending and forming elbows.

Ecological Role –
As seen, the Diabrotica virgifera is an insect that carries out its life cycle on corn and in the case of considerable infestations it can cause very serious damage and huge economic losses; however, the larvae are able to develop on other Poaceae. Adults are occasionally found on species belonging to the Asteraceae, Fabaceae, Cucurbitaceae, Convolvulaceae and Rosaceae families.
The damage done to corn is greater if the crop is repeated for two or more consecutive years on the same plot.
In the United States, in fact, the defense against the two main American species (Diabrotica virgifera and Diabrotica barberi) is carried out by means of granular geodisinfestants for the control of the larvae, to be distributed in a band or in the seeding furrow, and by subsequent treatments against adults.
It is a struggle linked and greatly subordinated to the specialization of corn crops which often return to the same plot causing explosions of infestations of this insect.
The chemical strategy certainly needs to be reviewed and, if absolutely necessary, it must in any case be integrated with the adoption of appropriate agronomic techniques.
The key factor in preventing or delaying the emergence of large populations is the abandonment of single succession, which can be combined with other agronomic practices that make the proliferation of insect populations more difficult.
Rotation is therefore important. In fact, the alternation with other species causes the death of the larvae in the ground that cannot find plants on which to feed. Generally, the choice of the crop that replaces the corn favors business needs and profitability. In this regard, soybean, grain sorghum, or canola or protein pea with autumn sowing can be grown.
Diabrotica virgifera is one of those insects whose infestations are the daughters of an industrial-type agriculture that has escaped the rules of eco-systemic equilibrium and regular biocoenosis.
Another useful precaution can be that of early sowing which are to be preferred since they allow the plant to reach the most developed roots in the period of maximum insect appearance. A vigorous root system allows better resistance to lodging even after damage suffered by the larvae. Furthermore, plants that have completed flowering are less attractive towards Diabrotica and cause less egg-laying activity. The choice of the hybrid must fall on varieties with a particularly vigorous root system and good resistance to lodging, for the same reasons expressed above. On the contrary, late sowing (from May onwards) coincides with the hatching of the eggs and with the appearance of the larval populations when the roots are still weak and not very developed.
Furthermore, the climatic trend strongly influences the populations of the insect. In particular, rainy springs favor the maintenance of high populations of eggs and larvae; on the contrary, mild and dry winters and springs cause high egg mortality.
It should also be remembered that the pre-sowing processes must aim at favoring the drainage of excess water in the soil as humid soils are preferred by females for laying eggs. The tamping favors the formation of adventitious roots and therefore improves the stability of the plant.
The collection period is also important. It is advisable to harvest the corn after October 1st to avoid transporting female Diabrotica to new areas that are still able to ovipose; for the same reason, it is necessary to avoid bringing plants or parts of them in a fresh state outside the infested area (for example chopped corn) as well as the ground that hosts the corn.
Also interesting is the aid of biological control. It is in fact possible to use insecticides based on Bacillus thuringiensis, which are allowed in organic farming. Alternatively, preparations based on the nematode Steirnenema carpocapsae can be used which have shown, in some studies, a good ability to contain the Diabrotica populations.
The chemical fight, which we recall is the worst solution to a corporate setting error and which then causes worse repercussions on the entire ecosystem, uses the use of tanning products, geodisinfestants, and adulticide treatments. Geodisinfestants are applied to the soil when corn is sown, to target the larvae that will hatch from the eggs. However, this treatment does not always prove effective due to the long time between the application of the product and the escape of the larvae from the egg.
The tanning agents are applied to the seed and allow partial containment of the larval populations. In the event of heavy infestations, however, the economic damage is not avoided.
Adulticidal treatments are carried out when the threshold is exceeded, with products based on: Fenitrotion, Clorpyrifos, Deltamethrin, Alphacipermethrin; these allow a good containment of adults and oviposition. Furthermore, with the same treatment it is also possible to contain the populations of Ostrinia nubilalis. To be effective, chemical treatments should be carried out by mid-July and this then interferes with the pollinating pollinating insect population of the area.
A good method to understand if measures must be taken against this insect is to monitor adults.
The monitoring of adult populations is essential to prevent the occurrence of damage and to decide the best strategies for fighting to be implemented. It is performed with special chromotropic traps of the Pherocon AM type placed in the field after the start of the flights (approximately starting from mid-June, but variable every year). We will use 3 traps per plot. The monitoring duration will be 6 weeks with weekly check and replacement of traps after the third week. The replacement can take place even earlier if damaged or if they have captured a large number of adults. If after the end of the monitoring period 6 adult catches per trap per day have been exceeded (corresponding to 756 total adults caught at the end of monitoring, ie after 42 days) and which represents the “intervention threshold”, it will be appropriate consider the rotation for the following year, as the risk of a strong infestation will be very high, or protect the crop for sowing. If the catches do not exceed 300 total adults caught after the first two weeks of monitoring (corresponding to 50 catches per trap per week 300 = 50 (catches) x3 (traps) x2 (weeks) a chemical intervention will not be advisable. were instead higher than 450 (ie 75 catches per trap per week, after the first two weeks, the treatment will be advisable because the probability of exceeding the intervention threshold indicated above at the end of the monitoring period will be high. treatment will be carried out in the third week after the start of monitoring. If after the second week there is an intermediate situation (between 300 and 450 catches), their trend will be evaluated in the third week, deciding whether or not to treat the following week ( the fourth). The evaluation of the presence in the field of at least 10% of pregnant females is the most reliable parameter to decide the best time to implement re treatment, but it is also difficult to estimate by the farmer.
Finally, it should be remembered that in Italy the fight against corn Diabrotica is mandatory pursuant to the D.M. of 30 June 2004, “Emergency phytosanitary measures intended to prevent the spread of Diabrotica virgifera virgifera Le Conte in the Community.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– Russo G., 1976. Agricultural Entomology. Special Part. Liguori Editore, Naples.
– Tremblay E., 1997. Applied entomology. Liguori Editore, Naples.