Wheat Yield in a Tolerant Winter Wheat Line Infested by Hessian Fly ( Mayetiola destructor )

The Hessian fly, Mayetiola destructor, is a harmful pest of wheat (Triticum aestivum). Pioneer variety 25R78 is putatively tolerant, meaning that the plant can survive successful Hessian fly infestation with reduced growth effects. To understand if Hessian fly-tolerance in wheat results in reduced yield effects and to analyze the economic feasibility of tolerant wheat as a Hessian fly control method, this study focused on analyzing the effect of infestation on tolerant wheat yield. This study analyzed tolerant Pioneer variety 25R78, resistant Pioneer brand variety 25R32, and susceptible Pioneer brand variety 25R47 through harvest. Treated plants were infested using a plastic cover and allowing 1 - 2 female flies to lay eggs for two hours. We measured head, fertile head number and tiller number. Seeds were analyzed by measuring total seed number and weight, as well as average seed number and weight. Tolerant and resistant plants showed no significant effects on yield in comparison to susceptible wheat. The infested tolerant plants were comparable in yield to infested resistant plants. Therefore, we propose that tolerance incorporated into wheat varieties will lower selection pressure on Hessian fly populations and increase the durability of these wheat lines.

greatest source of food calories and being the most widely cultivated crop [1].
Unfortunately, the Hessian fly, or Mayetiola destructor (Say) (Diptera: Cecidomyiidae), is one of the leading contributors to wheat yield loss. Hessian fly infestations can lead to losses of up to $100 million per year in the United States as reported in 1953, a converted value of approximately $886 million per year [2].
The Hessian fly range includes the wheat-growing regions of North America, Europe, and North Africa [3] [4] [5]. While no North American studies have been done to evaluate percentage of crop loss, in Morocco, heavy infestations have been shown to cause can cause 32% -100% yield loss [4] [6] [7].
Unlike most fellow members of the gall midge family (Cecidomyiidae), Hessian flies induce nutritive tissue at a feeding site rather than a normal gall, feeding at the site for 10 -14 days [5] [8] [9]. In compatible interactions, epidermal and mesophyll cells at the feeding site accumulate organelles and free amino acids, cell walls thin, and cells rupture, releasing nutrients for the larva to ingest [10]. Larval feeding permanently stunts plants, reduces stem elongation, prevents nutrient allocation to the developing grain head, and can kill the plant [11] [12].
In incompatible interactions caused by plant resistance genes, feeding sites fail to form and larvae die after 3 -5 days [10] [13] [14] [15]. Thirty-five identified resistance genes are currently the primary method of Hessian fly control and research [16] [17] [18]. R genes confer plant resistance to Hessian fly by recognizing secreted effectors from avirulent larvae. Then, the plant defense response, called effector-triggered immunity, occurs, triggering an up-regulation of the gene encoding Hfr-1. This protein is a plant lectin that might affect the avirulent larva's ability to feed by preventing the establishment of a successful feeding site and causing the larvae to experience writhing and increased searching time [19].
However, the lethal larval antibiosis places selection pressures on surviving fly populations, leading to an increased frequency of virulent biotypes that are capable of overcoming resistance genes. These virulent flies act similarly to flies on susceptible wheat, leading to stunted plants, yield loss, or plant death. This leads to the question of whether there is another method of managing Hessian flies which would either reduce selection pressures or increase the longevity of resistance genes. Can wheat plants survive infestation and not kill the feeding larvae?
One possible solution would be to combine the ability to survive initial infestation and the inability to kill the larvae, thus potentially reducing or eliminating selection pressures on fly populations caused by resistance genes. This might be possible with tolerance in wheat to Hessian fly damage. In the context of insect herbivory, tolerance has been defined several ways. These definitions include the ability for a plant to recover, grow, and potentially reproduce despite pest attack, without placing selection pressures on the pest populations and the ability to tolerate damage which minimizes yield effects caused by pests without killing the pests [20] [21] [22].
A benefit of tolerance is that it can support and maintain natural enemy populations by not decreasing prey numbers through antixenosis (non-preference)  [20]. Tolerance does not impose selection pressures that lead to the formation of novel biotypes, unlike insecticides or resistance genes [20] [21].
The absence of selection pressure is due to tolerance being a plant reaction to insect attack without direct effect on insect physiology, growth, reproduction, biology, or fitness making tolerance more evolutionarily stable than defense such as antibiosis [20].
The only research on wheat tolerance response to Hessian flies has been through indirect research on partial tolerance. One of the first proposed partially tolerant lines was the Marquillo hybrid. Marquillo hybrids, a cross of R gene H18-containing Marquillo and winter wheat, were able to survive and provide yield even under heavy infestation of Hessian fly infestation (Agricultural Experiment Station Kansas State 1940).
Another partially tolerant line is the "Superb" line. The wheat line "Superb" can reduce yield loss from fly infestations by up to 65% compared to susceptible lines such as "AC Barrie" due to the partial tolerance and antibiosis present in the line [23] [24]. These researchers considered tolerance as the ability of wheat stems to survive larval feeding without snapping [23]. "Superb" showed partial larval death which could place selection pressure on fly populations. Pioneer variety 25R78 was screened by Sue Cambron in the USDA-ARS greenhouse in West Lafayette, IN. The screening showed potential tolerance in that flies continued to emerge from the plants after infestation, but plants did not die or stunt as dramatically. To investigate this potential tolerance, we tested Pioneer variety 25R78 was chosen as the putative tolerant line to be studied in this experiment.
The susceptible Pioneer variety 25R75 was chosen to act as a control for this tolerant line.
Theoretically, tolerance in wheat might be used alongside resistance to help offset allocation expenses for R genes such as reduced yield, seed protein, and seed weight [19] [25]. Initially, there might be an allocation cost for a resistant and tolerant plant, but increased growth would eventually compensate for this cost [25]. This compensation could be completed via carbon deployment to larvae-inaccessible regions or subsequent redistribution of the carbon for growth [19].
In addition to previous studies, there are multiple reasons why tolerance to Hessian flies may to be present in wheat. Susceptible "Newton" can trigger growth through tiller production from an axillary coleoptile meristem to survive infestation, despite costs in yield production [10]. Second, some infested resis- could grow to successfully produce yield without directly killing the larvae via antibiosis. We also asked whether tolerant plants stunt and recover or stunt to the point of reducing yield. If yield is unaffected, this would potentially prevent yield and economic loss from Hessian fly infestations while not killing the flies and creating virulent biotypes.

Plant and Insect Preparation
To

Plant Transplant
For the plants to have sufficient space to produce roots and seeds, the plants

Measurements Taken
The following measurements allowed for an initial understanding of tolerance in wheat. The measurements used were modeled after the research on the fitness

Statistical Analysis
To determine effects of infestation on the three lines, statistical analyses were

Heads and Fertile Heads Number
The results indicated that there was no significant effect from infestation on head number for either resistant with a p-value of 0.790 (Figure 1)

Tiller Number
Infestation did not significantly decrease tiller numbers for resistant (P = 0.935;

Total Seed Number and Weight
Hessian fly infestation did not significantly decrease total seed number for resistant or tolerant plants (P = 0.678 and P = 0.993, respectively; Figure 4)     Hessian fly infestation did not significantly decrease total seed weight for resistant or tolerant plants (P = 0.450 and P = 0.960, respectively; Figure 5). However, infested susceptible Pioneer variety 25R47 was significantly smaller than the uninfested plants (P = 0.003). There was no significant difference in total seed number between infested tolerant plants and infested Pioneer variety 25R32 plants or infested Pioneer variety 25R47 plants (P = 0.999 and P = 0.681, respectively) ( Table 4).

Average Seed Number and Weight
Hessian fly infestation did not significantly decrease average seed number for resistant, tolerant, or susceptible plants (P = 0.997, P = 0.991, and P = 0.999, respectively; Figure 6).

Plant Survival
There is no evidence that plant survival depends on wheat line and infestation

Discussion
Managing Hessian fly infestations has been a challenge, focusing efforts on  [25]. That study analyzed fitness costs for H-gene mediated resistance by looking at yield effects of infestation [25]. Although this experiment was focused on yield effects in tolerant plants and not fitness costs, the measurements are a useful tool for looking at yield loss. With the results, it was clear that infestation reduced the yield of the susceptible plants, but not the tolerant or resistant plants.
There were no larval effects on tiller number for tolerant and resistant plants and negative effects on tiller number for susceptible plants ( Table 4). The number of tillers per plant is one of the primary contributors to wheat grain yield [26] [27]. Due to this connection between tiller number and grain yield, tiller loss from Hessian fly infestation could reduce yield. In the case of tolerant Pioneer variety 25R78, the prevention of tiller loss might prevent yield loss, making the tolerant line more economically appealing to farmers. Also, infestation does not reduce seed production through seed number or seed weight per head. Each of the infested lines showed no significant differences in total seed number or total seed weight compared to the corresponding uninfested plants. This indicates that this measurement of yield showed no adverse effects from infestation, regardless of wheat line.
No negative effects of infestation were observed for the average seed number or weight per head for any line. These results indicate that larval attack does not reduce these measurements, only the number of heads and total seed number.
This varies from the results of Anderson and Harris [25], in which the suscepti- The absence of significant yield effects from Hessian flies on tolerant wheat demonstrates the potential for tolerant lines to be used in solely tolerant or mixed fields with resistant wheat. Evidence shows that tolerant wheat has the potential to be used as a stand-alone crop due to the ability of the line to survive infestation and produce yield. Tolerant lines could also be used in fields mixed with a resistant line to act as a refuge crop.

Conclusions
The yield measurements, including the primary contributors, tiller number, seed