Christmas Tree Insects

Blow Fly

Blow Fly

Christmas trees are popular holiday decorations. Occasionally, the carefully selected tree will harbor a number of insects and small arthropods. These may be a nuisance but are typically not harmful. Penn State has a guide to commonly encountered insects and how to address them.

First, it is useful to not introduce insects. A tree should be inspected before bringing it indoors. Some vendors have shakers to physically remove loose needles and twigs. Shakers will also dislodge small insects and birds nests that can harbor insects and mites. Egg cases of praying mantids are occasionally attached to trees and should be removed and left outdoors. Mantids overwinter as eggs and time their emergence to coincide with prey. Brining them indoors can trigger the development and hatch of mantids that will starve for lack of prey.

Most insects cannot survive in the dry barren indoor climate. A simple effective method of removing them is by vacuum cleaner. Spraying a tree with insecticide is not advised as some aerosol propellents are flammable or can increase the flammability of a cut tree.

My most memorable experience with insects and Christmas trees involved a potted live tree. In Spring, we place our live potted tree outdoors and bring it inside when temperatures free in Fall. Shortly after moving the pot from the cold outdoors to the warm indoors, our house was buzzing with dozens of blow flies. Why? The tree had been next to a window. Late in the year, a bird had flown into the window and bounced into the pot where it died. Blow flies colonized the bird and pupated in the soil.  Seasonal factors halted their emergence and the pupa were dormant awaiting the arrival of warm Spring weather.   The warm indoor temperatures caused development to resume and the blowfly adults to emerge. Fortunately blow flies are slow and eliminated in less than a day.

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Finding a Safe Space

Fruit Flies

Drosophila Flies

Nests are special locations for social insects to store food and rear brood. A nest is a unique location unlike all other locations. The survival of social insects depends on the ability of foragers to navigate back to a nest after a trip to collect resources.

For insects that do not make nests, no one location is of unique importance to survival. Numerous locations may contain equally suitable resources and the need to navigate back to the same resource is not as critical. Can insects that do not have nests orient to unique locations using landmarks?

A group of scientists* asked Drosophila flies this question by creating a unique location of importance to the flies. They built an arena with a floor made of tiles. The temperature of each tile could be individually controlled. One tile in the arena was held at a temperature suited to long term survival. All the other tiles were kept at an unsuitably warm temperature. The tile floor was surrounded by walls containing patterns useful as landmarks.

Naive flies released into the arena eventually located the one suitable tile within 5 minutes through random movements. In subsequent trials, Drosophila flies located the suitable tile in less time and with more direct path. If the landmarks were moved by rotating the arena walls, no learning occurred. They concluded that Drosophila flies can and do use landmarks to navigate to unique locations.  Navigation using landmarks may be a widespread ability in insects and not solely confined to those that have nests.

Ousted TA, Zuker CS, Reiser MB. 2011. Visual place learning in Drosophila melanogaster. Nature 474:204–7.
doi:10.1038/nature10131

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Finding the Way Home

Cockroach

Female German Cockroach with Ootheca (egg case) protruding from her abdomen

Insects that make nests have a well developed navigation system that allows them to forage outside the nest and return. Hymenoptera make extensive use of visual cues and use a series of landmarks to navigate back to their nests. German cockroaches do not have nests but they aggregate in a harborage with a defined location. German cockroaches forage outside of the harborage, but return to the same harborage. How do they find the way home?

Rivault and Durier* studied the behavior of cockroaches in an arena where visual and olfactory cues could be manipulated. Moving either the olfactory or the visual cues increased the length of the path a cockroach traveled to return to the harborage. The return path was shortest when both olfactory and visual cues were presented in agreement. When cockroaches encountered a visual cue without the olfactory cues, they investigated the area around the visual cue before moving on. If cockroaches encountered olfactory cues without the visual cues, they also investigated the area before moving on.

This behavior may be adaptive to those environments inhabited by cockroaches. Under high light intensity, visual cues are readily apparent and allow a cockroach to reach a harborage by a direct path. Where low light intensity eliminates visual information, a cockroach must rely on other modalities such as olfaction.

*Colette Rivault & Virginie Durier. (2004)  Homing in German Cockroaches, Blattella germanica (L.) (Insecta:Dictyoptera): Multi-Channelled Orientation Cues.  Ethology 110, 761—777.

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Discriminating Sounds

Katydid Knee

Sound Detector (Tympanum) On the Front Leg of a Katydid

Insect hearing organs vary in their ability to discriminate among sounds. Insects that need hearing to do one thing well, such as warning of an approaching bat, can have an auditory system that only responds to sounds made by bats. Some sphingid moths have a single auditory neuron in each tympanum that triggers evasive flight activity.

Using sound to identify, locate and find mates requires more sophisticated auditory processing systems. The auditory organs of cicadas can contain a couple thousand sensory neurons. The Johnston’s organs of male mosquitoes that are capable of sophisticated tone matching can contain 15,000 neurons. In comparison, the human ear contains around 16,000 sensory hair cells in the cochlea.

Martin C. Gopfert and R. Matthias Hennig. Hearing in Insects. Annu. Rev. Entomol. 2016. 61:257–76.

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Zika Transmission in Texas

Mosquito Bite

Mosquito Bite

The Texas Department of State Health Services reported on November 28, 2016 the first documented case of Zika transmission in Texas from Cameron County along the Mexican border. A woman tested positive for Zika virus in urine but not blood indicating that she was exposed but no longer infective. It is unknown if Zika was contracted through mosquito bite or through one of the other transmission pathways. Zika transmission has been reported for communities across the border in Mexico. The question has always been “When will Zika be transmitted in Texas” not “Will it or will it not”.

Texas officials are intensively trapping mosquitoes in an effort to identify local mosquito populations containing Zika. They are also collecting voluntary urine samples to identify the extent of the infection in the local population. The CDC has been notified, but as of yesterday, have not issued travel warnings for Texas.

Texas is recommending Zika testing for all pregnant women. However, Texas has challenges including one of the highest uninsured populations in the US due to the refusal of Texas politicians to expand Medicaid coverage under the ACA.  Comprehensive coverage of women’s health care is not in place because of political and religious objections. Symptoms of Zika are often mild and people who lack medical coverage are not likely to pay out of pocket for a doctor visit or testing.  This creates a climate where new infections and their spread may be ignored.  A culture that views health issues as an individual responsibility clashes with volumes of medical and scientific data on the benefits of public health programs.

Lack of attention to public health can have bad consequences. The West Nile outbreak of 2012 was especially bad with over 200 West Nile associated deaths. Many of the deaths were in Texas. Weather conditions are correlated with the spike in disease and deaths. However, the response and lack of attention to mosquito control in Texas have been questioned. How effective will the Texas response to Zika be? At a national level, we hope that a vaccine will soon be available to protect the population from the worst effects. In the meantime, it is important for all to do what we can to slow the spread into the US.

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Vive La Différence

Mosquito

Mosquito

If two sounds of different frequencies are played together, they can produce a “difference tone” or a sound that has a frequency that is determined by subtracting the frequency of the lower tone from the higher tone. For example, two adjacent strings on a string instrument are separated by a perfect fifth are played together.  In addition to the tone of each individual string, an additional tone that is an octave below the lower tone (or difference tone), is also produced..

Difference tones are used for mate identification and finding by the Culex mosquitoes. A female is larger and beats her wings more slowly than the smaller male. When a male and female of the same species approach the antennae vibrate in response to both the female wingbeat frequency and the male wingbeat frequency. Together, the vibrations create another lower frequency vibration of the antennae that is the difference tone. Although the male and female wingbeats create a high frequency harmonic, the harmonic is above the response range of the antenna. However, the difference tone is in the range of detectable frequencies and it is the difference tone that is important to mate recognition.

Ben Warren, Gabriella Gibson and Ian J. Russell. Sex Recognition through Midflight Mating Duets in Culex Mosquitoes Is Mediated by Acoustic Distortion. Current Biology 19, 485–491, March 24, 2009.
DOI 10.1016/j.cub.2009.01.059

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The Mechanics of Hearing

Cartoon of Mechanically Activated Nerve Channel

Cartoon of Mechanically Activated Nerve Channel*

Flies such as Drosophila hear with their antennae. The base of the antennae, the Johnston’s organ vibrates in response to vibrations in the air (sound). The vibrations mechanically move hairs that are innervated by sound detectors. How is the vibration in hairs and neurons converted into nerve signals?

Nerve signals are generated by ions such as sodium and potassium moving through channels in the nerve membrane. Channels are made from proteins that span the membrane from inside to outside. Ions can cross the membrane through a pore in the center of a protein channel. The pore in the channel has a protein that acts as a gate to opens and close the pore. (See Cartoon) In mechanically activated channels, the protein gate is attached to the actin cytoskeleton of the nerve cell. Deforming the nerve cell causes the actin cytoskeleton to move, which moves the gates and opens pores.

Mechanical coupling acts rapidly. A nerve impulse may be generated in less than a nanosecond after the nerve is deformed. This is much faster than any known chemical activation. The inability of chemical coupling to act rapidly enough led scientists to search for other mechanisms that couple vibration to nerve impulses. The mechanism identified in Drosophila is similar to the mechanisms found in human ear hairs. Thus, Drosophila may be a good model to explore hearing with application to humans.

*Susanne Bechstedt, Jonathon Howard. Hearing Mechanics: A Fly in Your Ear, Current Biology, Volume 18, Issue 18, 23 September 2008, Pages R869-R870.
http://dx.doi.org/10.1016/j.cub.2008.07.069

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