UP-News Summer 2014

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Insect Population Outbreaks - Why Do They Occur?

 

cicada on leaf
Cicadas were a common sight in spring 2014. 

Spring and early summer of 2014 have been a season for high populations of some insects. Miller moths (army cutworms) flooded the Wasatch Front of northern Utah in record high numbers, a large emergence of cicadas wreaked havoc in fruit orchards, and large populations of Ips bark beetles continue to infest and kill landscape conifer trees. The Great Basin and desert landscapes of Utah are well known for their extreme environments and cycles of grasshopper and Mormon cricket outbreaks, but why do these insect outbreaks occur?

There isn’t a simple answer, but there are common factors, both environmental and biological, that contribute to higher than normal insect populations. Some of the environmental factors include drought, mild winter temperatures, lack of soil

army cutworm caught in trap
"Miller moths", or army cutworms, were commonly found in our orchard monitoring traps.

snow cover, and higher than normal heat unit accumulation in the late winter and early spring. Common biological factors thatinfluence insect population cycles include regulation of growth by food availability and quality, and suppression by natural enemies, including predators, parasitoids, and pathogens.

Late winter 2014 brought mild temperatures and rainfall, rather than snow, to the valleys of northern Utah. Southern Utah experienced a low-moisture winter and soils had little snow cover. A significant number of heat units (base 50°F) accumulated in the late winter and early spring. The mild conditions enhanced survival of overwintering insects such as armyworm and cutworm caterpillars on forage and field crops. Drought conditions can stress plants making them more

damage caused by spruce beetles
Spruce ips beetle tend to kill trees from the top down, and trees affected by drought are most susceptible.

susceptible to insects. This is the case for Ips bark beetles who can more easily overcome the natural defenses of conifers when tree sap is concentrated and turgor pressure within the cambium is low. Low sap levels are less effective in “pitching out” intruding bark beetles and concentrated nutrient content in sap provides a high quality food source for beetle offspring. Early spring warming and dry soils are optimal for hatch of grasshopper and Mormon cricket eggs in desert soils.

The upswing in abundance of some insect populations this year can be attributed to some or all of these factors. Can we predict when insect populations will peak?Yes, to some extent. If we analyze the key factors and track insect populations over time, we can often make reasonable predictions of insect abundance. However, nature always has the upper hand and can deliver surprises that we didn’t anticipate. We can be sure that whatever happens with insect populations, they will always provide an interesting show.

Most insects have natural population cycles within a growing season in a temperate climate such as Utah. Population density is low early in the season and typically builds in a logistic fashion over the summer as succeeding generations are completed. A high population increase within a year combined with conducive conditions can promote population

predator prey graph

growth in succeeding years leading to an upwards trajectory in abundance. If optimal environmental and biological conditions are combined with a low abundance of insect natural enemies, the result can be a spike in insect abundance. Abundance of natural enemies tends to lag behind that of their prey. Think of lemming and lynx population cycles. Typically, once natural enemy abundance builds to a high level, it will drive down numbers of its prey, and the insect population may crash.

- Diane Alston, USU Entomologist

  

  

Featured Picture of the Quarter

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The eggs on this raspberry resemble squash bug eggs, but they–and newly hatched nymphs–are leaf footed plant bugs. The name represents the leaf-like appearance of the femoral portion of the adult's legs. The nymphs and adults feed on a wide variety of plants, from foliage to flowers to fruit, by piercing the plant tissue with their proboscis and sucking the juices. Their saliva contains a toxic secretion, and feeding on fruit causes pitting, distortion and discoloration. Like stink bugs, leaf-footed bugs can exude foul-smelling fluids from pores on the sides of their bodies, helping to protect them from predators.

-Image by Marion Murray,
IPM Project Leader