Diabrotica is a large, widespread genus of beetles in the family Chrysomelidae. Members of this genus include several destructive agricultural pest species, sometimes referred to as corn rootworms.
There are an estimated 400 species grouped into the genus Diabrotica. Within Diabrotica there are 3 series: fucata, virgifera, and signifera groups. The fucata series contains the majority of Diabrotica diversity with 354 species. Fucata are characterized as multivoltine, producing two broods of offspring per year. The virgifera series comprises 24 species, and the signifera series contains 11. Additionally, virgifera and signifera are characterized as univoltine, and will only brood one set of offspring annually.
There is very limited information on most species of Diabrotica. Majority of the research conducted on the Diabrotica genus focuses on investigating species of consequential economic importance, such as D. balteata, D. barberi, D. undecimpunctata howardi, D. virgifera, D. speciosa. Multiple Diabrotica species are considered major agricultural pests, therefore the control and management of populations is of significant importance to farm management strategies.
Taxonomy and evolutionary history The Diabrotica genus arose in the Cretaceous period and began to diversify and speciate ~60 million years ago, which culminated ~30-40 million years ago. Contrary to previous suggestions, Diabrotica speciation was not linked to the onset of corn and other crop cultivation. Gene sequencing has led to a phylogenetic reconstruction of the Diabrotica genus that indicates monophagy is the ancestral trait. The genus began to diversify when certain lineages expanded their diets to include a multitude of different types of plants, leading to a polyphagous trait. However, subsequent reversals back to monophagy or oligophagy have occurred in some evolutionary branches.
The diversification and evolution of Diabrotica species is closely linked to their relationship with wild plant species in the family Cucurbitaceae (cucurbits), which characteristically produce cucurbitacin secondary compounds. Cucurbitacins are bitter and toxic. While cucurbitacins deter most herbivores, Diabrotica beetles are attracted to cucurbitacins and compulsively feed on cucurbit species, especially the tissues that contain high concentrations of cucurbitacins, such as roots, seeds, and cotyledons. Diabrotica beetles favour cucurbitacin-containing plants to the extent that they will leave another nutritious plant host for a cucurbit plant. The mouthparts of Diabrotica beetles display receptors that bind cucurbitacins to stimulate this compulsive feeding behaviour. By feeding on cucurbit plants and sequestering cucurbitacin in their haemolymph, Diabrotica beetles are afforded some protective advantages, such as chemical defenses against natural predators. This is an example of pharmacophagy, in which insects consume plant metabolites for reasons besides nutrition. The aforementioned demonstrates a situation of chemically mediated coevolution between Diabrotica and cucurbit plants, and even Diabrotica species that have evolved to no longer rely on cucurbitacin-containing plants still demonstrate this compulsive feeding behavior in the presence of these plants.
Diabrotica species are separated into 3 groups: virgifera, fucata, and signifera. These are ‘groups of convenience’ as they are based on host range, diet, life history, and other ecological traits, rather than being supported by molecular and genetic data.
Distribution and historical changes Diabrotica is a neotropical genus that evolutionarily originated in Central America and is native to North and South America. Central America is the most rich in Diabrotica species (i.e. has the highest number of Diabrotica species), but Mexico and Brazil are also high in Diabrotica diversity. Although the tropical areas are significantly more diverse in Diabrotica species, the US Diabrotica fauna has a greater proportion of pest species. 4 out of the 7 Diabrotica species native to the US are pests. However, only 6 out of the 338 Diabrotica species found in the tropical regions are pests.
Climate is the main factor that puts constraints on the distribution of different groups of Diabrotica species. For example, the entire fucata group is incapable of overwintering. Consequently, members of the fucata group inhabit host plants primarily located in the tropics and subtropics. However, variations in weather conditions from year to year lead to annual fluctuations in the geographic ranges of fucata group species. D. speciosa sensu lato, regarded as the best known pest species within the fucata group in South America, has a broad distribution covering the majority of the continent. Signifera group species are exclusively found within South America. A multitude of species in the virgifera group are capable of overwintering. Specifically the US virgifera group species overwinter as cold-resistant eggs.
The western corn rootworm, D. virgifera, is the most damaging pest of corn crops in the US and was accidentally brought to Serbia. As a result, its range expanded to threaten corn crops in the Eastern and Central areas of Europe, especially in Germany and Hungary. Belgium, Netherlands, and the UK were successful in extirpating in western corn rootworm. Through various strategies, such as crop rotation, Diabrotica beetles have been kept at non-damaging levels in Germany and France.
Habitat and diet Since adult Diabrotica are highly motile and have the ability to migrate, the genus occupies diverse habitats. Diabrotica movement patterns are driven by the appeal and availability of food sources. Typically, the preferred food source of adult Diabrotica includes pollen and reproductive structures of plants. Species of the fucata group are polyphagous, which means that they feed on various plant species. Species of the virgifera and signifera groups are oligophagous, which means that they consume only a few plant species.
Diabrotica species in the virgifera group that feed on corn primarily inhabit huge maize monocultures. Both D. virgifera and D. longicornis barberi are corn-feeding species within the virgifera group; however, their feeding and egg-laying behaviours exhibit some key differences. Both species feed on silks, pollen, and young kernels of corn. Adult D. virgifera also feed on corn leaves, but D. l. barberi do not. In fact, D. virgifera will remain in a corn field past the seasonal availability of pollen and silk to feed on the corn foliage, especially when their egg-laying period is prolonged. However, once silk and pollen are no longer available in a corn crop, D. l. barberi will move on and inhabit other fields of plants. As a result, high densities of D. l. barberi larvae are commonly present in crops where corn was not cultivated during the prior year. This is seldom observed for D. virgifera larvae, rather they usually remain in the corn field. Research has revealed that D. virgifera populations tend to rise when corn is continuously planted in the same field each year, but D. l barberi populations tend to benefit more from crop rotation. All together, it suggests that D. virgifera are much more dependent on corn as a habitat and food source than D. l. barberi. D. undecimpunctata howardi is a member of the fucata group that uses a broad range of plant species and does not rely on corn as long as other nutritive food sources are present.
Lifecycle
Adult female Diabrotica that are feeding on the leaves and pollen of the host plant release their eggs (~300-400 eggs) in the soil adjacent to the plant's roots. Eggs can be deposited as deep as 15 cm into the soil. After the eggs hatch, larval development occurs. In Diabrotica larval development, there are 3 instar larva, which are stages in Arthropod life cycles that occur between moults. Following the 3rd instar, is a mature pupa, which is the stage exhibiting complete metamorphosis to the adult form. As larvae develop, they feed on and thus damage the roots of the host plant.
For multivoltine species (2 or more broods of offspring each year), the whole lifecycle (egg to adult) lasts around 30 days. In addition, these species usually spend the winter as dormant adults suspended in their development (diapausing adults). In contrast, eggs overwinter in univoltine species. The virgifera group species are univoltine and fucata group species are multivoltine.
The corn rootworms, which include D. virgifera virgifera, D. virgifera zaea, and D. longicornis barberi, are the 3 most economically relevant taxa due to their damage on agricultural crops, especially corn. These taxa are univoltine, thus they spend the winter as eggs deposited in soil and hatch in the late spring. Adults can be seen feeding on corn silk, leaves, and pollen from the midsummer through to the frost.
Behaviour Foraging techniques
Diabrotica larvae, specifically Diabrotica viridula, are prominent pests of agricultural cornfields, as the larvae's preferred food source primarily consists of corn roots. Adult females intentionally deposit the eggs of their brood in the soil of cultivated farm fields. This oviposition upon successfully hatching, allows larvae the ability to easily locate and tunnel towards the roots of the host plant. There exists a proportionality between the density of laid eggs around the targeted host plant, and the damage inflicted upon the roots of the plant. The closer eggs are laid to the host results in greater propensity for damage to incur on the host as a result of larval feeding. Other Diabrotica species larvae within the fucata series are predominantly polyphagous in nature, feeding off a variety of diverse vegetation.
Adult Diabrotica species are herbivores, with a diet predominantly consisting of foliage and pollen. Due to the vast richness of diversity in the genus a broad variety of plant species are used as host plants and food sources. Host sources of economic importance in prairie habitats include, but are not limited to, corn, squash, beans, and soybean varieties. Many Diabrotica species exhibit a preference for plants containing cucurbitacin's as they can be metabolized by the beetle into an effective chemical defense mechanism.
Threat responses Chemical defense is a key mechanism implemented by many Diabrotica species to act as an important mode of protection against predation. Cucurbitacin is a bitter tasting compound found in certain plants that is often toxic to insects. Diabrotica species will relentlessly ingest cucurbitacin compounds synthesized by their preferential food sources. After acquiring the cucurbitacin compounds, they metabolize the consumed cucurbitacin into the haemolymph. The Diabrotica beetle will have then successfully incorporated the bitter tasting properties of cucurbitacin compounds into itself. Therefore, the adoption of the deterring bitter, toxic characteristics of the cucurbitacin's results in the Diabrotica species significantly reducing the likelihood of predation.
Breeding and courtship Female members of Diabrotica species will release sex pheromones and begin expressing a characteristic calling posture signaling their receptivity to commence mating. The calling posture consists of exposing the membranes of the females abdominal segment's to facilitate the secretion of the aforementioned sex pheromones. Reproducing males will then approach the female and engage in tactile stimulation of the female's abdomen. This likely facilitates relaxation and increases the females receptiveness prior to and throughout the duration of copulation occurring. The mating procedure may last for 1–6 hours, with copulation lasting approximately 10–60 minutes. Mating behavior appears to be influenced by circadian rhythms, with the majority of mating events occurring around the dusk hours.