Delia platura, also referred to as the seedcorn maggot and bean seed fly, is a member of the Anthomyiidae family within the order Diptera and the genus Delia.Capinera, J. (2001). Order Diptera—Flies and maggots. Handbook of vegetable pests, pp. 211–258. Academic Press. https://doi.org/10.1016/B978-0-12-814488-6.00007-8 During its adult stage, it is hard to distinguish from similar fly species because it has a thin body, long legs, and transparent wings. It inhabits numerous regions globally, and is known to be an agricultural pest, especially during its larval stage, causing substantial crop damage worldwide. During the larval phase, Delia platura damages crops at its beginning stages as a seed or seedling. This damage leads to an unsuccessful sprouting and, therefore, a decrease in crop production. Furthermore, the presence of Delia platura leads to decreased biodiversity, damage to ecosystems as organisms lose food sources and habitats, and negative economic impacts because farmers must regrow destroyed crops.
Delia platura prefers regions with mild climates, as it thrives best in those temperatures. Delia platura was first discovered in Germany and is currently found in many agricultural regions worldwide, such as North America, the United States, and Canada. Due to its widespread distribution and ability to infest and damage crops, Delia platura causes negative ecological and agricultural impacts worldwide.
Host plants
Found globally, the insect utilizes many host plants. Common host plants of Delia platura include both vegetables and fruits. Known vegetable host plants include members of the Brassicaceae family, comprising Brussels sprout, cabbage, cauliflower, kale, mustard, rutabaga, turnip, and sea kale; the Cucurbitaceae family, consisting of cantaloupe, cucumber, pumpkin, and squash; the Fabaceae family, containing alfalfa, lima bean, pea, and snap bean; the Solanaceae family, featuring potato, tobacco, and tomato; the Amaryllidaceae family, including onion and garlic; the Asteraceae family, comprising artichoke and lettuce; and the Amaranthaceae family containing beat and spinach. Additional host plants include carrot, corn, rhubarb, strawberry, cotton, and sweet potato, which belong to the Apiaceae, Poaceae, Polygonaceae, Rosaceae, Malvaceae, and Convolvulaceae families, respectively.
It has been debated whether or not Delia platura should be categorized as a primary pest, an organism that damages healthy plants, or a secondary pest, an organism that damages already harmed plants. However, previous research classifies it as a saprophagous pest, meaning it feeds on previously harmed plants and therefore best fits under the secondary pest category.
Host plant resistance
Different host plants have various responses to being attacked by Delia platura larvae. For instance, corn is the most resistant to harm, beans such as lima or snap beans are moderately impacted, while cantaloupe and watermelon are the most susceptible to damage. There are certain features of host plants that lead to lower risk in being harmed by the larvae. For example, beans containing a darker outer layer are less at risk of being harmed when compared to beans with a white outer layer.
Natural enemies
Global distribution often leads to a wide variety of natural enemies, which is the case for the Delia platura. It is associated with a large range of natural enemies such as parasitoids, arthropod predators, and fungi. Parasitoids, organisms that develop inside their host and eventually kill it, found in Delia platura include Aleochara bilineata (Coleoptera: Staphylinidae), Aphaereta pallipes (Hymenoptera: Braconidae), Trybliographa rapae (Hymenoptera: Figitidae), and Aleochara bimaculata (Coleoptera: Staphylinidae). Arthropod predators of this species include ants and spiders. Lastly, the fungus Entomophthora muscae (Entomophthorales: Entomophthoraceae) infects Delia platura and causes disease.
Due to their known ability to negatively impact agriculture, ecosystems, and economic systems, chemicals are used to minimize Delia platura populations. Chemicals that are used include, but not limited to, chlorpyrifos, clothianidin, imidacloprid, and spinosadWilson, R. (2015). Evaluation of pesticides and application methods to protect onions from onion maggots, Delia antigua, and seed corn maggot, Delia platura, damage. Crop Protection pp. 102-108. Elsevier. https://doi.org/10.1016/j.cropro.2014.10.002. These chemicals have been used on seeds or part of planting methods in order to protect onion seedlings from Delia platura larvae damage. However, this can lead to detrimental effects on human health as well as the environment because it can cause air and soil pollution, which in turn has the ability to kill valuable organisms. Therefore, a more natural solution is recommended when reducing their population from farms.
Researchers identified biological pest control as a natural solution for decreasing Delia platura populations. The three most common parasitoids found in agricultural fields within Mexico were Aphaereta pallipes, Trybliographa rapae, and Aleochara bimaculata, with Aphaereta pallipes being observed the most. This has encouraged farmers to introduce parasitoids into Delia platura environments, as a crop conserving mechanism, instead of chemicals to prevent damage to crops.Nava-Ruiz, P., Meraz-Álvarez, R., Valdez-Carrasco, J., Chávez-López, O., & Bautista-Martínez, N. (2021). Parasitoids of Delia planipalpis and Delia platura (Diptera: Anthomyiidae) in Mexico. ZooKeys, 1046, 177–187. https://doi.org/10.3897/zookeys.1046.64405
In addition to using biological pest control, entomopathogenic nematodes, like Steinernema, can also limit Delia platura communities in farms, as these nematodes use carbon dioxide to detect moving hosts and then make their way through the body of the insect, where they release toxins that kill it.Jaramillo, C., Celeita, J., & Sáenz, A. (2013). Susceptibility of Delia platura to seven entomopathogenic nematode isolates from the Central Andes region of Colombia. Universitas Scientiarum, 18(2), 165–172. https://doi.org/10.11144/Javeriana.SC18-2.sdps
Using these natural enemies offers a sustainable solution for reducing agricultural pests, allowing for an increase in plant biodiversity and a reduction in excess economic costs. Furthermore, this method prevents farmers from needing to use chemicals that can damage human health and the environment.
Oviposition
Previously, oviposition was thought to be a straightforward process in which Delia platura simply laid its eggs on common host plants. However, it is now known that oviposition is a much more complex mechanism, and there are many factors that influence their egg-laying behavior. It is essential for female Delia platura to oviposit in the right places as their larval offspring will not be able to move far afterwards.
Female Delia platura use their volatile cues to determine where to lay their eggs. They detect chemicals, such as 1-octen-3-ol and 3-octatone which trigger oviposition.Gouinguené, S. P., & Städler, E. (2006). Oviposition in Delia platura (Diptera: Anthomyiidae): The role of volatile and contact cues of beans. Journal of Chemical Ecology, 32, 1399–1413. https://doi.org/10.1007/s10886-006-9058-3 These chemicals are oftentimes associated with bacteria found on or near host plants, suggesting that bacteria plays a major role in oviposition, and that females prefer surfaces with bacteria over clean ones. Specifically, the three bacterial species that strongly promote oviposition were identified as Flavobacterium species, Erwinia herbicola, and Xanthomonas campestris. One reason why egg laying near bacteria is preferred is because it is commonly linked with food, meaning the mothers prefer to lay eggs where food can later be used by their offspring.Hough-Goldstein, J. A., & Bassler, M. A. (1988). Effects of bacteria on oviposition by seedcorn maggots (Diptera: Anthomyiidae). Environmental Entomology, 17(1), 7–12. https://doi.org/10.1093/ee/17.1.7
Furthermore, Delia platura does not need to directly interact with bacteria, so long as it can smell them, it can still be influenced to lay eggs nearby. Even when growth-inhibiting bacteria are present and decrease the volume of bacteria, Delia platura can still detect them and continue laying eggs there, demonstrating that bacterial odors play an important role in oviposition behavior.
Life cycle and development
Delia platura can typically complete an entire life cycle, progressing through the egg, larval, pupal, and adult stage, in around 15–77 days. However, this timeline is heavily affected by temperature. Warm temperatures of around 25–30°C lead to faster developmental times, while colder temperature ranges of 10–15°C slow development down. During warm temperatures, the insect could complete its life cycle within 17 days, while it took 240 days in colder temperatures.Throne, James, and C. J. Eckenrode. (1986). Developmental Rates for the Seedcorn Maggots Delia platura and D. jlorilega (Diptera: Anthomyiidae). Environmental Entomology, 15, 1022-1027. https://doi.org/10.1093/ee/15.5.1022
Each year, there are 2 to 4 generations of Delia platura produced, but this number is influenced by temperature. During intense weather conditions, such as high heat in the summer or extremely cold temperatures in the winter, Delia platura enters a dormant state in order to save energy and survive unfavorable conditions. Entering these states leads to lower reproduction rates and thus fewer generations.
Due to different regions having different temperatures and climates, different numbers of generations may arise per location. In the United States, specifically New York and Wisconsin, as well as in England and Ontario, there are around 4 new generations annually. The generations are easily distinguishable due to their flight times. New generations are identified in May, June, July, and August, However, other regions, like South Carolina, make it harder for generations to be differentiated from one another. Delia platura in South Carolina are active year-round due to favorable weather conditions that allow them to stay continuously active. Because of this, generations can blend together, making it hard to differentiate the separate generations. This leads to South Carolina experiencing the emergence of 3 new generations annually; however, this number may not be accurately represented. Meanwhile, in Iowa, around 2 generations are produced each year due to extreme seasonal weather that stops development.
Egg
Delia platura females typically lay their eggs in temperatures between 10–27°C near decaying seedlings and fresh soil. The eggs are white, long, and oval-shaped. The eggs are extremely small, typically with a length of 0.95 mm and a width of 0.3 mm. How long the insect stays in its egg stage is dependent on temperature. Colder temperatures cause them to remain in their egg stage longer than in hot temperatures. At temperatures of 7°C, 15°C, and 28°C, they hatch within 9, 3, and 1 day(s), respectively.
Larva
The larval stage comes immediately after egg hatching, and this stage is when Delia platura does the most damage by eating seeds and seedlings. The larval stage consists of 3 phases, otherwise known as instars. From the first to third instar, they grow from 0.7 mm to 7 mm. During this stage, the larvae eat in groups. The larva's optimal temperature for development is 21–23°C, and they can mature into pupae in 7–12 days, while development takes 20–30 days when the temperature is 10°C. They can also develop at lower temperatures of 4–7°C, but at an even slower rate. Due to the fact that Delia platura larvae are small and extremely similar to onion maggots, researchers use anterior spiracles, structures that support larval breathing, to differentiate between them, and aiding in their identification are the 5 to 8 lobes that they possess.