Effects of Crowding on Aphid Alate Frequency ...

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Aphids are one of the affected organisms and are the subject of a number of ..... Dynamics of Soybean Aphid (Hemiptera: Aphididae).” Biological Control, 2017,.
Toluwalope Toluhi  Principles of Ecology  Biology 261  Professor Emily Mohl   

Effects of Crowding on Aphid Alate Frequency  (Hemiptera: Aphididae)  Introduction  The phenomena of global warming is one that has been extensively discussed in  recent times as the world seems to come unglued with melting glaciers, erratic weather  patterns and overall higher average temperatures(Karl, 2003). While there are two sides to  the debate, a major undeniable fact is the rise of CO​2​ and greenhouse gases in recent years  and the corresponding rise in factors such as temperature and drought conditions around the  world. These observable, largely quantifiable phenomena have far reaching effects that we  are yet to fully comprehend, one being the effect on the life cycle of a vast number of  organisms on the planet. Aphids are one of the affected organisms and are the subject of a  number of studies(Hulle, 2010). These inquiries examine factors such as temperature,  precipitation, seasonal changes and duration of daytime light. Many effects have been  uncovered such as the increased migration of aphids from their local populations to  previously unoccupied locations by these species, which in some cases presents with the  symptoms of a serious pest invasion(Watson et al., 1999).  There are many facets to this development that have far reaching implications to  public health and ecology. One of these is the spread of diseases and viruses by aphid  vectors(Escriu et al., 2003). While in their natural habitats we can assume the parasitized  organisms had sufficient defences so as not to be decimated by aphid parasitoids. We can  also assume predators that acted as a population control and reduced carrying capacity of the  aphid populations(Bannerman, 2017). However in the event of migrations, aphid  populations might arrive at locations where they effectively had no predatorial control(Ehler  Toluhi 1

& Kinsley, 1995) and in high densities virus transmission even to local aphids is an event that  might occur(Betancourt et al., 2016). A counter argument is that events that cause such  migration will sometimes result in the same effect in the predator species, but this has only  been noticed in specific species(Conti et al., 2018; Baaren, 2010).  Another possibility is the lack of defences or adaptation by the organisms that would  become parasitized by the aphids. Such a situation could decimate a certain niche of the  biome and effectively remove a key player in that ecosystem from the trophic level(Bertness,  1984). In the event that the decimated organism were not vital to the survival of other  organisms, the effect would be minimal and the aphid migration would have done little to no  damage. If it is the case that the decimated organisms are important to the new system, we  would then have a breakdown of the system and a possible reorganization of the trophic  levels, possibly causing some extinctions.  The aphids status as disease or virus vectors could be one that compounds their  effect in a new habitat. The various species of aphids are known to be able to transmit  viruses to their hosts. While the virus might have had a certain negative effect on a former  population, a new habitat might be unprepared for the virus. A study on the swine flu virus  showed how the reintroduction of even formerly familiar viruses can have catastrophic  effects due to mutation and reassortment(Neumann et al., 2009). Therefore, the similarity of  the habitats being migrated from and to is more relevant in terms of how easily the new  habitat would be occupied and less relevant in regards to the possible similarity of viral load  of the symbiont vectors present in the biomes. Hence we are aware that depending on the  new habitat and the virus that the specific aphid is a vector to, the effects of migration might  present a health risk to the habitat and any populations nearby.  Aphid migration is made possible by the alates who develop in response to crowding  on a host plant, temperature changes, precipitation, lower food quality and longer nights.  Alates are winged aphids who enable migration after the aforementioned factors precipitate  their presence(Mehrparvar et al., 2013). There have been studies on the specific rates at 

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which alates have been produced for the Cereal Aphid, ​Sitobion avenae,​ with formulaic  methods:Percentage alates = 2.603 × Aphid density + 0.847 × GS − 27.189(Carter et al.,  1982). There is the assumption of a max flight time of 2 hours serving as a limiting factor to  how far they may emigrate(Duffy et al., 2017). However the progression towards earlier  migratory dates might enable more intense niche carving by aphids(Hughes, 2000; Hulle,  2010).  Continuing advancement in this vein, this paper focuses on how the aphid  populations of the particular species ​Aphis Nerii g​ row and aims to quantify the effect of  crowdedness on the development of alates who are perfect migrants and serve as virus  vectors. The viruses infect the plants mostly and the expurgation of unsuspecting plant hosts  might have unforeseen negative impacts on the inhabitants of the biome. We hypothesize  that the effects of density are sufficient enough to cause a significant increase in alate  frequency. Since high alate frequency may result in increased migration attempts, this would  lead to successfully established populations of aphids in non native areas that may be less  beneficial to the particular biome. Among other effects, new plant viruses from the migrant  aphids would present a challenge the new host plant and population might be unprepared  for.   

Materials and Methods  Data for this project was gathered from a experiment run by students of an  Ecological Principles Class taught by Professor Emily Mohl at St. Olaf College in the Fall  Semester of 2017. The experiment consisted of 148 milkweed plants that were housed in  netted cages(n = 18) in the greenhouse at St. Olaf with an average of 8 plants to a cage.  There were 67 control plants and 71 damaged plants in the final pool of milkweed. The  damage criteria was included because the original study was interested in the effects of  damaged leaves on the parasite-host interaction between the plant and a parasite we would  introduce. Of the 8 plants, 4 of them had the top half of their leaves damaged by cutting off  Toluhi 3

half the leaf along the midrib which reduces overall photosynthesis and gas  exchange(Delaney & Leon, 2006). So if a plant had 12 leaves, the top 6 leaves would be  damaged in this manner and the bottom 6 would be untouched. Plants were distributed  randomly into the cages by genotype(samples were collected from various locations with  varying genotypes).  Aphids(n = 5) of the ​Aphis nerii​ species, the milkweed aphid, were then put on the  plants and allowed to grow for 14 days. This number was used to simulate migration of  aphids when overcrowding happens on a host plant. The ​A.​ ​nerii​ were used because they are  the native parasites of the milkweed plant. In order to obtain useful data we tried to replicate  the organisms and conditions as they would occur in the world, hence the use of the natural  parasite and its natural host. Counts were taken daily of winged and unwinged aphids on 1  damaged and one control milkweed plant. On day = 1 and day = 14 the following  measurements were taken from the host plant: Height(cm), nodes(n), leaves(n), Largest leaf  width(cm), largest leaf length(cm), and stem width(mm).  This analysis takes the counts from day 14 of the control plants(n = 581) to select for  ideal conditions and performs regressions to search for any correlation between population  number and alates. The regression plots the number of unwinged aphids against the winged  aphids(alates) and gives an R-squared value to indicate correlation. ​In trying to quantify the  extent to which alate production is dependent on or correlated to aphid population, a  regression test is a helpful tool.​ A higher R-squared number signifies greater correlation. R, a  statistical modeling freeware was used for all analyses. P values were taken into account in  deciding how significant any values were. P values of ≤0.05 were considered significant. All  the conditions for regressions and were satisfied as the graphs included show.      

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A number of plants had unusable date and had to be excluded from the analyses in order to obtain trustworthy results

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Results  The experiments went smoothly as students came in to do counts of aphid numbers  on assigned days. No plant casualties were recorded and plants with abnormalities in counts  were excluded from analyses. The counts that were used in this analysis were collected on  the 14th day. A range in unwinged aphids was observed from 74 to 1400. Winged ranges  were more conservative and went from 0 to 80. A regression done resulted in ​multiple  R-squared of 0.02766, adjusted R-squared of 0.0103 and p-value of 0.2121 

  Figure 1. Each dot represents a milkweed plant and its winged and unwinged population. A line of best fit has been drawn to  show a trend and supply us with R-squared values. Adjusted R-squared = 0.0103 and p-value = 0.2121 

There would seem to be a slight correlation from the graph however t​hey actually only  indicate that only 1% of the data points can be predicted based on the data supplied and  alongside that, the p value is >0.05(Hence our F-statistic is also insignificant). These give us  insignificant results.    

Discussion  In regards to populations grow and and the quantification of the effect of  crowdedness on alate frequency who are perfect migrants and serve as virus vectors, the data  Toluhi 5

available is inadequate to support or dispute the idea that an increase in aphid density per  plant results in higher alate frequency. The p-value does not meet the maximum 0.05  rendering the results from the analyses insignificant. This might be due to a number of  factors: (i) difference in counts among various students. It could be that there were some  conservative counters and some more liberal counters of a proportion and to an extent that  did not even out. That could be an aspect that is more standardized in a sequel of this study.  (ii) difference in densities. Because not all milkweed plants grew at the same rate, some were  larger than others, giving more room to reproduce to the aphids. In this case, there might  have been some populations that reached carrying capacity(a function of available resources),  which would stimulate alate production, while others were still below the line at which  reproduction would begin to slow down. (iii) difference in genotypes. The project utilized a  number of different phenotypes collected from different regions of the US. Because they  were now all in a common garden study due to the similar conditions, they grew at different  rates as some of them were better suited for the environment provided.   Based on these, it would be necessary to conduct more experiments in order to  obtain significant data. More consistency with the data collection would be needed. More  uniform numbers would help to solidify trends wherein confidence can be couched. It would  also be beneficial to use a single genotype for the plants to give more uniform growth indices  and food quality. While the effect of different plants on aphid population size has been  studied(Wu et al., 2013; Zhang et al., 2016), intraspecies, genotype specific comparisons have  not been carried out and these various genotypes could be used in a study that explored the  response of aphids to the quality or robustness of the host. However, for the purpose of  quantifying how population quantity relates to proportion of alates the study would need to  be as uniform as possible. Another aspect that might be related to genotype in some studies  is plant height. Height dictates amount of space available for aphid growth before carrying  capacity is reached. If this were to be studied, plants would need to be selected for specific  heights for the desired study as aphid habitation of plants has been shown to affect 

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phenotypic traits which could impact resource availability and quality for the aphid  populations(Park ​&​ Blossey, 2008). In this particular study plant height could definitely have  affected the rate at which aphid populations grew and an improved analysis could normalize  for plant height to observe whether predictability increased once this variable was removed.  This paper will not go into how to identify potential migration destinations, but  studies have shown how to predict aphid migration patterns using factors such as time of  year, precipitation, uplift at the source, wind velocity, etc.(Sands, 1965; Parry, 2013). Niche  construction theory helps in understanding the various options aphids may take advantage of  inhabiting new areas(Laland et al., 2016; Laland et al., 2017), though conditions would need  to be extremely favorable as aphids seem unlikely to be able to exert these changes except  due to sheer number and intensity of grazing. Since these are all factors that have been  mentioned previously in respect of the likelihood of their increase in response to symptoms  of global warming, it follows that there should be concern about how they might further  impact systems affected by global warming. There seems to have been a diversity of thought  in academia about the effects of global warming on parasitoids and their hosts(Bezemer et  al., 1998; Romo ​&​ Tylianakis, 2013), but newer studies tend to state that their results are  limited based on region, even though functionally aphids would respond similarly(Newman,  2005; Tekle, 2016). They already are finding significant correlations between winged aphids  migrating and the effects of global warming(Newman, 2005; Tekle, 2016).  Further research should then examine more micro influences of the effects of global  warming on aphid-milkweed interaction as there is an abundance of big picture reasoning.  These micro effects might be a clue into localized solutions for their mitigation. Additionally,  in future studies, temperature could be manipulated in order to examine how the  proportions of alates differ at different levels. While it is known that determinants such as  drought and rises in temperature enhance aphid fitness on trees(Dale ​&​ Frank, 2017;  Doherty, 2017) and other plants with varying results(Bezemer et al., 1998; Romo ​&  Tylianakis, 2013), it is unknown how the same or similar conditions would impact aphid 

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fitness on milkweed. This quantification would also further the field in understanding the  rates at which alates as migratory vectors are produced and how specific stimuli affect their  noxiousness.   

 

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