Plant growth formulae for entomologists, what a great title for a paper or even a book J These two papers, separated by a decade had a great influence on my PhD and subsequent entomological career, or at least the lab based part of it. I started my PhD at the University of East Anglia on October 2nd 1977, where I was lucky enough to be supervised by that doyen of the aphid world, Professor Tony Dixon. I was, and still am, convinced that the sooner you get started on practical work the better and that is what I tell my students. Yes, reading is important but getting to know your organism early on, is just as, if not more, important. You can catch up on your in-depth reading later, but that early ‘hands on’ experience, even if what you first do is not publishable, is invaluable. I see from my lab notebooks that my first experiments* were examining the effects of host plant on the fecundity and longevity of my study aphid, Rhopalosiphum padi.
My first experiment as a PhD student!
It was doing these very simple experiments, collecting development, survival and reproductive data that introduced me to the idea of measuring life history parameters in the round,
One of my first data sheets – not used in my thesis but gave me invaluable experience for later on.
rather than as single factors, akin to how ecologists move from measuring species diversity as a simple species count to using diversity indices that combine other attributes and describe the community more holistically. So it was with me and growth and reproductive rates. I wanted to be able to infer what my laboratory results might mean in the field and to develop faster methods of screening for host plant effects. I came across, or was pointed in the direction of, two papers that had great influence on my research, Radford (1967)** about measuring growth rates, albeit of plants, and Wyatt & White (1977) on how to measure intrinsic rates of increase rm without having to go through the very laborious and time-consuming methods devised by Birch (1948); working on aphids makes you do things in a hurry 😊
Ian Wyatt and his colleague Peter White did a series of painstaking laboratory experiments to obtain reproductive figures for aphids and mites and came up with a simplified version of the Birch equation such that
rm = 0.738(lnMd)/d
where Md = the number of offspring produced over a period of time equal to the pre-reproductive period D and 0.738 is a constant (Wyatt & Wyatt, 1977)
The Radford paper, reinforced by reading a paper by another great aphidologist, Helmut van Emden, Professor of Horticulture at the University of Reading (van Emden, 1969) convinced me that Mean Relative Growth Rate (MRGR) was the way to go to obtain comparative measures of host plant suitability for my aphids. To save you looking it up, MRGR is calculated as follows:
The beauty of this, especially if you are working with very small animals such as aphids, is that you don’t need to weigh them at birth, you can if you want, just measure weights between two time periods.
Screening plants for resistance to aphids is an integral part of developing sustainable and environmentally friendly ways of protecting your crops. At the time I started my PhD several methods were in use, ranging from measuring direct fecundity and developmental time (Dean, 1974), to short cuts such as counting the number of mature embryos at adult moult (Dewar, 1977) and of course Mean Relative Growth Rate (van Emden, 1969). Although I tended to measure life-time fecundity and longevity for almost all my experiments, having the short cut of MRGR and in many cases the fecundity achieved in the first seven days of reproduction*** (e.g. Leather & Dixon, 1981) were useful tools to have. What was the world shaking discovery for me, and something that in retrospect, I find surprising that no one else cottoned on to, was that MRGR was highly correlated with fecundity and that this meant that MRGR was correlated with the intrinsic rate of increase (Leather & Dixon, 1984). This means that you can screen host plants and predict population trajectories with experimental observations that take less than half the time using the traditional measurements. That paper proved very popular and is Number 7 in my citation list, with, at the time of writing, 80 citations. A few years later, when I had moved on to working with other insect orders, I found that the relationship between MRGR and rm applied to Lepidoptera and that different insect orders followed the same rules (Leather, 1994).
Lepidoptera and aphids, singing from the same data sheets (Leather, 1994).
So truly, a paper that shook my world.
Birch, L.C. (1948) The intrinsic rate of natural increase of an insect population. Journal of Animal Ecology, 48, 15-26.
Dean, G.J.W. (1974) Effect of temperature on the cereal aphids, Metopolphium dirhodum (Wlk.), Rhoaplosiphum padi (L.) and Macrosiphum avenae (F.) (Hem., Aphididae). Bulletin of Entomological Research, 63, 401-409.
Dewar, A.M. (1977) Assessment of methods for testing varietal resistance to aphids in cereals. Annals of Applied Biology, 87, 183-190.
Fisher, R.A. (1921) Some remarks on the methods formulated in a recent article on ‘The quantitative analysis of plant growth’. Annals of Applied Biology, 7, 367-372.
Leather, S.R. & Dixon, A.F.G. (1981) The effect of cereal growth stage and feeding site on the reproductive activity of the bird cherry-oat aphid, Rhopalosihum padi. Annals of Applied Biology, 97, 135-141.
Leather, S.R. & Dixon, A.F.G. (1984) Aphid growth and reproductive rates. Entomologia experimentalis et applicata, 35, 137-140. 80 cites number 7 in my list
Leather, S.R. (1994) Insect growth and reproductive rates. In Individuals, Populations and Patterns in Ecology (ed. by S.R. Leather, A.D. Watt, N.J. Mills & K.F.A. Walters), pp. 35-43. Intercept, Andover.
Radford, P.J. (1967) Growth analysis formulae – their use and abuse. Crop Science, 7, 171-175.
van Emden, H.F. (1969) Plant resistance to Myzus persicae induced by a plant regulator and measured by aphid relative growth rate. Entomologia experimentalis et applicata, 12, 125-131.
Wyatt, I. J. & White, P. F. (1977) Simple estimation of intrinsic increase rates for aphids and tetranychid mites. Journal of Applied Ecology 14, 757-766.
and boy was I quick off the mark. I started my PhD on October 2nd and here I am 24 days later with cereal plants at GS 12 ready to receive aphids J
it is only fair to point out that Radford owed his inspiration to the work of that great statistician, Ronald Fisher (Fisher, 1921)
aphids like many insects produce over half their progeny in the first week or so