Many landscapers are familiar with the larger beneficial insects such as lady beetles, praying mantids, lacewings, and flower flies. Although common, parasitic wasps/flies (parasitoids) are examples of landscape beneficials that are typically less recognized because of their small size, and that magnification is needed for best viewing. Also many parasitoids feed unseen on the interior organs within their hosts. Although the majority of parasitoids are found in the two orders mentioned above (Hymenoptera & Diptera), there are more than 50 families that have been identified. Many of these insects do not have distinctive differences in general appearance & therefore attempting to ID the specific species or even family is not practicable for the landscaper or nursery grower. Learning about & being aware of the activity of these less observed but exceptionally important biological control organisms are photographed & reviewed in this blog.

An apparent parasitoid wasp inadvertently captured on the edge of a yellow sticky trap. Most of these adult wasps are exceptionally small & will often have a constricted waist & beaded antennae. The vast majority of observed landscape parasitoids will be wasp or fly species. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Too often, landscape plant managers ignore or confuse beneficial organisms with insect pests and inappropriately apply control materials. This is especially the case with the larvae or immature stages of beneficial insects. An observant and knowledgeable IPM scout needs to learn how to recognize and conserve these “good guys,” so they are not needlessly destroyed. Remember, “we must look before we shoot,” when spraying pesticides and take advantage of natural pest control that works for free!

Home entrance walkway with diverse plantings of numerous species. An ideal setting to attract many beneficial insects. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

The classical definition of biological control is the use of natural enemies to control insect pests. These natural enemies include predators, parasitoids, and pathogens. Pathogens are microorganisms (bacteria, viruses, fungi, protozoan, and nematodes) that kill pests. Parasitoids are parasites that kill their hosts by their feeding activities. Most parasitoids of landscape pests are wasps and flies. This blog will discuss some of the more valuable ornamental landscape predators.

Over one hundred praying mantids can emerge from a typical sized egg case (ootheca). Although excellent predators, praying mantids are NOT considered to be valuable in the home landscape. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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The Dilemma Regarding Birch Trees

Bronze birch borer adult female laying an egg on the bark surface of a birch tree. (Photo Credit: Ohio State Coop. Ext.)

Bronze Birch Borer (Agrilus anxius) adults (440-800 GDD) are now actively searching landscapes for various species of Betula (Birch) to lay eggs upon. The adult beetles will be attracted to susceptible trees that are genetically defenseless or are compromised from stress (e.g., especially drought stress).

It is well known that the native River Birch (Betula nigra) has a strong resistance against the BBB. As a result, this species has been extensively planted in NJ landscapes during the past few decades to the extent of arguably being over-planted. And it is also widely understood that the non-native Asian & European birch species have virtually no or little resistance to the BBB. Therefore, the planting of these non-natives has rightfully been reduced over the decades. However, what about some of our other native species such as the paper, gray, & yellow birches. Is it best to avoid planting these species as well?       

River birch (Betula nigra) is a native birch that is strongly resistant to the bronze birch borer. (Photo Credit: Steven K. Rettke, Rutgers Coop.Ext.)

Is it best to avoid planting other native birch trees in our landscapes, such as the gray birch shown above? (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Definition

The Plant Stress Hypothesis generally states that plants subjected to stressful conditions become more susceptible to attack by insects and diseases. This is often thought to be due to the plant’s increased suitability as a food or due to the reduced ability of the stressed plant to defend itself. It is commonly believed that when a plant is stressed it becomes better food for “bugs.” Is it correct to state that plant stress leads to pest outbreaks, or is the statement too simplistic?

The Experiments

A publication written in 1998 (Koricheva et. al.) summarized approximately 70 controlled experiments that were performed to study how distinct types of plant stresses affected insect/mite attacks. Diverse types of environmental stresses were included in these studies (e.g., drought stress, waterlogged roots, ozone exposure, excess shade, as well as many others). Plant nutrient stress & insect impacts has been researched but was not performed within these studies.

Plant roots stressed from waterlogged soils were included in these studies. Research also included other types of environmental stresses such as excess shade & ozone exposure. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Dogwood leaves scorched from environmental drought stress. How does this plant stress impact insect herbivores? (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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