Why wasps and bees hover over cabbage plants

Traditional PNG Highlands houseI grew up in a rural Papua New Guinea village, cared for by my grandpa as my parents were separated. Following the old man daily to his garden, I noticed he had a diversity of food plants from root to tuber crops, from more traditional to introduced vegetables (self-pollinating and vegetatively propagated). Maturing in a few months he sold some of the vegetables for cash and sometimes the takings was used to buy rice and tinned protein source. Grandpa would often place fern (Sphaeropteris cooperi) leaves over the cabbage heads. Wondering, I watched to see what happened next. I saw wasps, flies and bees flying in from all directions, hovering above the cabbage plants, some finding gaps and getting in through the fern leaves. Grandpa told me that the ferns reduced caterpillar damage to the cabbages.  With interest and enthusiasm I also started to grow vegetables around the backyard. During bright sunny days, caterpillars were in business, chewing my plants. Handpicking them, I noticed similar flies, wasps and bees, that I usually saw in my grandpa’s garden, flying just above the cabbage plants. Unsure of why this was, I went onto high school and, with the influence by my agriculture teacher, I ended up at the college studying in this area.  I then worked on an Integrated Pest Management (IPM) of Brassicas project. One day, my mind clicked about my childhood thoughts about wasps, bees and flies hovering above my cabbages. These species were beneficial insects.

Seeking to increase my knowledge on beneficial organisms, I applied to Lincoln University for postgraduate study looking into plant-aphid (plant lice) relationships. At Lincoln University, it was my supervisor, Michael Rostas gave me a book to read relating to my subject of interest. One of the chapters was by Michael and his colleagues Nicole Waschke and Torsten Meiners (from Freie Universitat Berlin, Germany). Recalling memories of my interest three decades back, I read through their paper on “Foraging strategies of parasitoids in complex chemical environments” in the book titled CHEMICAL ECOLOGY OF INSECT PARASITOIDS. They were interested in understanding the way in which parasitoids (parWasp ovipositingasitic insect that live on or in a host which they eventually kill) find suitable hosts for reproduction of offspring. Through this research, they found that parasitoids locate their hosts through various processes including; host habitat finding, host location, and host acceptance in a habitat containing various kinds of plant species diversity that are known to be producing complexity of chemical compounds. In a habitat with complex background volatiles (airborne odours) produced by the various plant species, the parasitoids use their behavioural, sensory and neurophysiological adaptations to successfully locate their hosts. When they have found the food plant of their host, they search for the host insects presence based on non-volatile compounds released by their host, such as from faeces, silk or honeydew, and host footprints on wax surface of the plant. Upon locating the host, the female parasitoid lays its egg using its ovipositor (a needle type tubeWasp emerged out) into its host. The egg, upon hatching within or on the host’s body, feeds from the host, and as a result the host is usually destroyed. Those wasps, bees and flies that I saw earlier  hovering over my grandpa’s cabbages were searching for their host insect species that were attacking the plants. I was seeing bioprotection in action.

The author Wesis Pus is a postgraduate student at Lincoln University. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.

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