Field Crops News

Greenbug Predation and Escape Response

Greenbugs are important aphid pests of grain sorghum throughout much of the United States. A variety of lady beetles have been demonstrated to be important greenbug predators.

Aphids use a variety of defenses to avoid insect natural enemies (predators and parasitoids). One of these defense measures is to drop off the host plant when disturbed. As lady beetles begin feeding on greenbugs, the first contacted aphids secrete a chemical alarm pheromone that causes nearby aphids to drop off the plant.

In laboratory and field studies on grain sorghum, McConnell & Kring (1990) documented the number of greenbugs fed upon and dislodged by adult sevenspotted lady beetles (Coccinella septempunctata). Using a range of aphid densities and sorghum growth stages in the lab and field, each lady beetle adult was found to be able to consume 0.5-1.1 greenbugs per minute. However, from 1.7-3.5 greenbugs per minute were dislodged from plants. Aphids dislodged to the ground may be exposed to additional biological and climatological mortality factors (ground predators, high soil temperatures, rainfall). Mortality of aphids dislodged from plants was not quantified in this study.

Using only the number of aphids consumed as a measure of predator efficacy may underestimate lady beetle importance due to the large number of aphids dislodged. This study showed that 3-5 times as many greenbugs were dislodged as consumed. If only 25% of the dislodged aphids fail to reestablish on a plant, lady beetle efficacy would be 75-125% greater than that estimated based on the numbers consumed.

Source

J. A. McConnell & T. J. Kring. 1990. Predation and dislodgment of Schizaphis graminum by adult Coccinella septempunctata. Environ. Entomol. 19: 1798-1802.

What Do Sevenspotted Lady Beetle Adults Really Eat?

The sevenspotted lady beetle (Coccinella septempunctata) is a European introduction to North America. Although their primary food source is aphids, they do feed on other foods as well.

During a two year study, adult sevenspotted lady beetles were collected throughout the year in wheat, oat and rye fields at three locations in Germany. The different food types ingested was determined using gut dissection.

During the course of the year the composition of the gut contents changed as the lady beetles went through their seasonal cycle.

The alimentary canal was generally empty when the beetles came out of hibernation. In the spring, as the lady beetles began dispersing, soil particles, fungal spores, and aphids were found most frequently in the gut. Feeding on pollen was also observed in the spring, but only 6% of the beetles dissected had pollen in their gut.

During the summer, when the beetles were reproducing, aphid remains were present in over 85% of the individuals. Surprisingly fungal spores occurred more frequently than remains of other arthropods during this time, with half of the beetles dissected containing fungal spores.

In late summer and autumn, fungal spores occurred most frequently. Aphids had been eaten by only 37% of the lady beetles. Pollen, primarily from goldenrod, was also an important food in late summer as the beetles prepared to enter hibernation again in the fall.

Sevenspotted lady beetle adults showed very uniform food preferences. Other than aphids, nearly all other arthropods found were thrips; only a few fly or beetle larvae remains were found. Cannibalism was rare (it is more common for larvae). Nearly all the spores found were of two types: Alternaria sp. (80%) and uredospores of Puccinia sp. These spores were a dominant food component during the whole active life period. Spores were consumed from April until September, in the absence and presence of aphids. Fungal spores may be an obligatory food source for these lady beetles.

Source:

Triltsch, H. 1997. Gut contents in field sampled adults of Coccinella septempunctata (Col.: Coccinellidae). Entomophaga 42(1/2): 125-131.


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