保幼激素类似物对德国小蠊卵黄发生及繁殖的影响

应用ELISA、单向免疫扩散法,研究了保幼激素类似物（灭幼宝）对德国小蠊Blattella germanica卵黄发生的影响;同时,观察了灭幼宝对德国小蠊繁殖的影响。灭幼宝促进了德国小蠊脂肪体Vg的合成,使血淋巴Vg的浓度明显增高,出现时间早于对照组;且变化规律不同于对照组,呈双峰模式。灭幼宝处理组昆虫卵巢中Vt的含量在第5天、第7天明显高于对照组,而在第9天明显下降,仅为对照组的50%左右,而此时对照组卵巢的Vt含量则上升到处理组第7天水平。灭幼宝处理新羽化的雌虫,导致不育。     

Management of a German cockroach population with a juvenoid bait formulation: an experiment in residential buildings

Baits including 0.1% of ethyl 2-(4-((1,4-dioxaspiro [4.5]dec-6-yl) methyl) phenoxy) ethylcarbamate were applied in two buildings containing 46 apartments with various degrees of infestation of the German cockroach, Blattella germanica (L.). During a yearly experiment, generally 8 baits were placed in each apartment and replenished monthly at which time cockroach population density, demographic structure, and percentage of morphologically deformed adults were determined from specimens captured by stickytrap sampling. Initial growth of the cockroach population observed during the first two months after deposition of the baits was followed by an asymptotical decrease. Only 2.5% of the initial population remained at the end of the experiment. Although adults with twisted wings (sterile) constituted only 70% of the total adult catches, the decrease of the nymph/adult ratio from 2.9 to 0.7 indicated a far more profound inhibitory effect on cockroach reproduction. This study shows that baits might be an efficient alternative to juvenoid formulations currently in use for control of the German cockroach.     

Influence of insecticide treatment on German cockroach (Dictyoptera: Blattellidae) movement and dispersal within apartments

Two similar studies were done to investigate effects of insecticide treatments on German cockroach, Blattella germanica (L.), movement and dispersal within individual units in multifamily housing. In the first study, Whitmire PT-565 Pyrethrum Insect Fogger (pyrethrins), Diazinon 4E (diazinon), Protector (permethrin), and Baygon 1.5 (propoxur) did not induce any significant changes in cockroach distribution, and rates of movement within apartments decreased. In the second study, populations treated with sublethal doses of pyrethrins and resmethrin settled to their original distribution 24 h after treatment. Results from both studies indicate that thorough insecticide applications do not significantly affect German cockroach population dispersal or movement patterns within apartments.     

Cost and efficacy comparison of integrated pest management strategies with monthly spray insecticide applications for German cockroach (Dictyoptera: Blattellidae) control in public housing

The long-term costs and efficacy of two treatment methodologies for German cockroach, Blattella germanica (L.), control were compared in the public housing environment. The "traditional" treatment for German cockroaches consisted of monthly baseboard and crack and crevice treatment (TBCC) by using spray and dust formulation insecticides. The integrated pest management treatment (IPM) involved initial vacuuming of apartments followed by monthly or quarterly applications of baits and insect growth regulator (IGR) devices. Cockroach populations in the IPM treatment were also monitored with sticky traps. Technician time and the amount of product applied were used to measure cost in both treatments. Twenty-four hour sticky trap catch was used as an indicator of treatment efficacy. The cost of the IPM treatment was found to be significantly greater than the traditional treatment, particularly at the initiation of the test. In the first month (clean-out), the average cost per apartment unit was dollar 14.60, whereas the average cost of a TBCC unit was dollar 2.75. In the second month of treatment, the average cost of IPM was still significantly greater than the TBCC cost. However, after month 4 the cost of the two treatments was no longer significantly different because many of the IPM apartments were moved to a quarterly treatment schedule. To evaluate the long-term costs of the two treatments over the entire year, technician time and product quantities were averaged over all units treated within the 12-mo test period (total 600 U per treatment). The average per unit cost of the IPM treatment was (dollar 4.06). The average IPM cost was significantly greater than that of the TBCC treatment at dollar 1.50 per unit. Although the TBCC was significantly less expensive than the IPM treatment, it was also less effective. Trap catch data indicated that the TBCC treatment had little, if any, effect on the cockroach populations over the course of the year. Cockroach populations in the TBCC treatment remained steady for the first 5 mo of the test and then had a threefold increase during the summer. Cockroach populations in the IPM treatment were significantly reduced from an average of 24.7 cockroaches per unit before treatment to an average 3.9 cockroaches per unit in month 4. The suppressed cockroach populations (< 5 per unit) in the IPM treatment remained constant for the remaining 8 mo of the test.     

Comparative study of integrated pest management and baiting for German cockroach management in public housing

This study assessed the cost and effectiveness of a building-wide cockroach integrated pest management (IPM) program compared with bait alone treatment in public housing. In total, 12 buildings (66 apartments) were treated and monitored for cockroach infestations over 7 mo. The buildings were divided into two groups: bait treatment and IPM. Apartments in the bait alone group were treated with Maxforce FC Select (0.01% fipronil) during the first 12 wk and Maxforce Roach Killer Bait Gel (2.15% hydramethylnon) from 16 wk when necessary. For the IPM group, cockroaches were flushed and vacuumed at the beginning of the study; sticky traps were placed in all apartments to monitor and reduce cockroach numbers; educational materials were delivered to the residents; and Maxforce FC Select and Maxforce Roach Killer Bait Gel were applied to kill cockroaches. Two seminars were presented to the manger, and Community Service Program staff of the Gary Housing Authority to help gain tenant cooperation in the program. Effects of the treatments were monitored using sticky traps (six per apartment) at 2, 4, 8, 12, 16, and 29 wk after treatment. More treatments were applied during each monitoring visit when necessary. Those apartments with high levels of infestations (> or =12 cockroaches in six traps) before treatment were used to compare the IPM and bait only treatments. IPM resulted in significantly greater trap catch reduction than the bait treatment. The IPM (n=12) and bait only treatment (n=11) resulted in 100.0 and 94.6%, respectively, reduction in trap catch after 16 wk. At 29 wk, only one apartment in the IPM group had a high level (>12 cockroaches) of cockroach infestation. In contrast, five apartments in the bait treatment group had high level infestations at 29 wk based on overnight trapping counts; thus, IPM is a more sustainable method of population reduction. Sanitation levels in the IPM group significantly improved at 29 wk (n=11) compared with that at the beginning of the study. The sanitation levels in the bait treatment group remained similar throughout the experiment (n=9). The cumulative cost of IPM was significantly higher than that of the bait treatment. The median costs per apartment during 29 wk were 64.8 dollars and 35.0 dollars for the IPM and bait treatment, respectively. The median amount of bait used per apartment in the IPM and bait treatment was 45.0 and 50.0 g, respectively. The cost of the IPM group for the 29 wk service was similar to that of the bait treatment group. We expect that IPM will provide better control at similar cost compared with bait treatment beyond 29 wk.     

Influence of Pyriproxyfen on the Expression of Haemolymph Protein Genes in the Colorado Potato Beetle,Leptinotarsa decemlineata

Pyriproxyfen, a potent juvenile hormone analogue for the Colorado potato beetle, Leptinotarsa decemlineata, was applied topically to last-instar larvae and short-day adults at different times after moulting. The effect of the hormone analogue on concentration and composition of protein in the haemolymph was studied at different intervals after pyriproxyfen application. The hormone analogue had little effect on total protein concentration of the haemolymph, but affected protein composition. Diapause protein 1 was prevented from being synthesized if pyriproxyfen was applied before the gene was activated and disappeared from the haemolymph if applied after the gene had been expressed. It therefore inactivated the gene for diapause protein in both larvae and adults. Pyriproxyfen also induced appearance of vitellogenin at both stages, indicating induction of expression of the vitellogenin gene. It also affected the stability of mRNA for diapause protein. The analogue caused mRNA for diapause protein 1 to disappear untimely compared to controls in last-instar larvae and short-day adults. The response of adults to the JHA was much more pronounced than that of larvae, although the analogue had a strong biological effect on last-instar larvae because it prevented metamorphosis at low doses. Copyright 1997 Elsevier Science Ltd. All rights reserved     

Susceptibility of immature stages of Chrysoperla rufilabris (Neurop., Chrysopidae) to pyriproxyfen, a juvenile hormone analog

Effects of pyriproxyfen (Knack), a juvenile analog, at three concentrations (10, 50 and100 mg [AI]/l), on survival and development of all immature stages of Chrysoperla rufilabris (Burmeister), were determined in the laboratory. Pyriproxyfen significantly reduced the survival rates when eggs, first and third instars were treated, but not when the second instars and pupae were treated. When eggs were treated, the two higher concentrations reduced the eggs viability by 33.3–50%, and only 0.0–6.7% developed to adults. The lowest concentration of pyriproxyfen (10 mg [AI]/l) caused high mortality on third instars, not on other stages, indicating the third instar was the most vulnerable stage. Pyriproxyfen had significant effects on development for all immature C. rufilabris that successfully developed to adults with variations among the developmental stages and concentrations. The overall developmental duration from eggs to adults when eggs, first, and third instars were treated were 2.6–4.2, 2.4–4.1 respectively, and 6.0–7.1 d longer than those in water control, respectively. However, the overall developmental durations from eggs to adults were 0.5–1.2 d shorter than in water control when the second instars were treated with pyriproxyfen. The compatibility of pyriproxyfen with natural enemies in integrated pest management programs is discussed.     

Laboratory and field performance of consumer bait products for German cockroach (Dictyoptera: Blattellidae) control.

Bait formulations containing boric acid, chlorpyrifos, or hydramethylnon were evaluated in field and laboratory experiments against the German cockroach, Blattella germanica (L.). In continuous exposure tests, chlorpyrifos formulations produced 100% mortality in less than 24 h, boric acid formulations had LT50's of approximately 5.5 d, and hydramethylnon had an LT50 of 0.86 d. LT50's estimated in Ebeling choice boxes were 1.6 to 10.8 times greater than those from continuous exposure tests for chlorpyrifos formulations and 2.5 times greater for the hydramethylnon formulation. The same LT50's were produced by choice box and continuous exposure tests for boric acid formulations. Relative repellency, measured as the mean percentage of live cockroaches in the light side of the choice box, was greatest for chlorpyrifos formulations, followed by boric acid formulations, then hydramethylnon. A performance index (PI) that combined choice box repellency and mortality data indicated that hydramethylnon (Combat) had the greatest potential for field effectiveness. Only hydramethylnon significantly reduced German cockroach populations in the field. Apartments treated with the IT WORKS formulation of boric acid bait had significantly greater cockroach trap catches. The PI is an accurate predictor of the field performance of toxic baits for German cockroach control.     

Residual activity and population effects of noviflumuron for German cockroach (Dictyoptera: Blattellidae) control

A benzoylphenyl urea insect growth regulator with the common name noviflumuron was evaluated for efficacy and residual activity on the German cockroach, Blattella germanica (L.). In laboratory studies evaluating residual activity, 0.05% noviflumuron suspension concentrate produced 100% nymphal mortality 120 d after application to steel and masonite substrates. Residual activity of noviflumuron was more variable on painted plywood substrates compared with stainless steel and masonite. In bioassay arenas, population reductions caused by noviflumuron were significantly greater than Archer and the untreated populations. After 16 wk, populations exposed to 0.05, 0.1, and 0.2% noviflumuron were reduced by 51.9 +/- 19.8, 62.2 +/- 6.5, and 62.6 +/- 18.4%, respectively. Control cockroach populations and populations exposed to 1.3% pyriproxyfen at labeled rate (Archer, 0.61 g/m2) increased by 1286.3 +/- 125.1 and 937.2 +/- 137.1%, respectively, at the end of 16 wk. A field study in multifamily housing complexes showed noviflumuron (0.2 and 0.5%) to provide 73.3 +/- 8.0 and 90.6 +/- 3.6% trap catch reduction at 4 wk posttreatment, respectively. There were no significant differences in the performance of noviflumuron, Maxforce FC Roach Bait Stations (0.05% [AI] fipronil), and Avert dust bait (0.05% [AI] abamectin B1). Noviflumuron shows excellent potential for use in cockroach management programs.     

Performance of gel and paste bait products for German cockroach (Dictyoptera: Blattellidae) control: laboratory and field studies.

Gel and paste bait formulations containing boric acid or hydramethylnon were evaluated in field and laboratory experiments with the German cockroach, Blattella germanica (L.). Bait formulations contained 14-62% water by weight and lost the most water after 1 d of desiccation. Bait age and water content did not alter the toxicity (LT50) of boric acid-based baits. Toxicity of hydramethylnon (Maxforce) bait, however, increased with bait age. In continuous exposure tests without harborage, boric acid formulations had LT50s of 8.5 and 10.1 d for Blue Diamond and IT WORKS, respectively. Maxforce bait had an LT50 of 4.2 d. LT50s determined with cardboard harborage or in Ebeling choice boxes were 1.5 to 2.2 times lower than those from continuous exposure tests without harborage. Relative repellency, measured as the mean percent of live cockroaches in the light side of the choice box, was greatest for IT WORKS (21.4%) and Maxforce (25.4%) formulations, followed by Blue Diamond (10.9%). A performance index that combined choice box repellency and mortality data indicated that Maxforce had the greatest potential for field effectiveness. Blue Diamond and Maxforce significantly reduced German cockroach populations in the field. Maxforce performed best in field trials when applied at 10 or 20 placements per apartment. Apartments treated with IT WORKS had significantly greater cockroach trap catches. Factors that may affect the predictive power of the performance index are discussed.     

Effects of a juvenile hormone analogue, pyriproxyfen, on the apterous form of soybean aphid (Aphis glycines)

The soybean aphid, Aphis glycines Matsamura, is an eastern Asian soybean [Glycine max (L.) Merr.] pest which can reduce soybean yield. We determined the effects of pyriproxyfen, a juvenile hormone analogue, on development, mortality, longevity and fecundity of A. glycines under laboratory conditions. Distance Insect Growth Regulator, containing ∼11.2% pyriproxyfen, was applied at two concentrations, 50 and 150 mg/l, to first and fourth instar nymphs. When first or fourth instar A. glycines were treated with pyriproxyfen, some nymphs became supernumerary‐molted nymphs with 1–3 extra molts or were sterilized. Mortality of treated first instar nymphs was >68% greater than the control group and longevity was reduced by >40%. The higher concentration of pyriproxyfen reduced fecundity of first instar nymphs when they reached adulthood by ∼79%. Pyriproxyfen similarly affected fourth instar nymphs. Mortality of treated fourth instar nymphs was ≥15% greater than the control group and longevity was reduced by >24%. Both concentrations of pyriproxyfen lowered the fecundity of fourth instar nymphs by >27%. Pyriproxyfen also had other sublethal effects on fourth instar nymphs which became apparent when they molted to adulthood. In a few instances they developed wing pads and many produced dead, deformed or abnormal neonates that lacked appendages.     

Sublethal effects of insecticides on the intrinsic rate of increase of cotton aphid

Insecticides are often implicated in causing outbreaks of the cotton aphid, Aphis gossypii (Glover) (Homoptera: Aphididae), through stimulation of reproduction. In this study we report the sublethal effects of dosages of bifenthrin, acephate, carbofuran or pyriproxifen on cotton aphid reproduction. We could not detect any increase or decrease in the intrinsic rate of increase of cotton aphids exposed to bifenthrin, acephate or carbofuran. However, we did detect some increases in the net reproductive rate of aphids treated with bifenthrin justifying further investigation of the effect on reproduction by this insecticide. Trends based on simple linear regression models suggest that sublethal dosages of bifenthrin or carbofuran have a negative impact on aphid population growth as dosages increase. These data suggest that stimulation of reproduction by these insecticides probably does not play a major role in cotton aphid outbreaks or resurgence. Pyriproxyfen is a juvenoid currently used for control of whiteflies in cotton. It demonstrated significant activity towards cotton aphid reared on treated cotton in our bioassays. Pyriproxyfen caused sterility in most aphids exposed to dosages exceeding 1 ppm, and reduced aphid longevity by approximately 50%. However, it did not appear to greatly influence the reproductive potential or longevity of reproductively mature aphids. A field study indicates that pyriproxyfen affects cotton aphid population structure and may have potential in managing cotton aphid outbreaks. Modifying aphid population structure and growth through the use of juvenoids such as pyriproxifen may prove to be an effective proactive approach to pest control without adversely impacting beneficial organisms or causing pest resurgence.     

Role of two insect growth regulators in integrated pest management of citrus scales

Portions of two commercial citrus orchards were treated for two consecutive years with buprofezin or three consecutive years with pyriproxyfen in a replicated plot design to determine the long-term impact of these insect growth regulators (IGRs) on the San Joaquin Valley California integrated pest management program. Pyriproxyfen reduced the target pest, California red scale, Aonidiella aurantii Maskell, to nondetectable levels on leaf samples approximately 4 mo after treatment. Pyriproxyfen treatments reduced the California red scale parasitoid Aphytis melinus DeBach to a greater extent than the parasitoid Comperiella bifasciata Howard collected on sticky cards. Treatments of lemons Citrus limon (L.) Burm. f. infested with scale parasitized by A. melinus showed only 33% direct mortality of the parasitoid, suggesting the population reduction observed on sticky cards was due to low host density. Three years of pyriproxyfen treatments did not maintain citricola scale, Coccus pseudomagnoliarum (Kuwana), below the treatment threshold and cottony cushion scale, Icerya purchasi Maskell, was slowly but incompletely controlled. Buprofezin reduced California red scale to very low but detectable levels approximately 5 mo after treatment. Buprofezin treatments resulted in similar levels of reduction of the two parasitoids A. melinus and C. bifasciata collected on sticky cards. Treatments of lemons infested with scale parasitized by A. melinus showed only 7% mortality of the parasitoids, suggesting the population reduction observed on sticky cards was due to low host density. Citricola scale was not present in this orchard, and cottony cushion scale was slowly and incompletely controlled by buprofezin. These field plots demonstrated that IGRs can act as organophosphate insecticide replacements for California red scale control; however, their narrower spectrum of activity and disruption of coccinellid beetles can allow other scale species to attain primary pest status.     

Effects on German cockroach nymphs of contact exposure to IGRs, singly and in combination

Late instar German cockroach male and female nymphs were exposed continuously for two weeks to surfaces treated with fenoxycarb, diflubenzuron, and pyriproxyfen, singly and in combination. Concentrations were determined that eliminated or nearly eliminated reproduction in matings with untreated mates, either through mortality, effects on reproduction, or a combination of mortality and sterility (no hatch). The major effect of fenoxycarb, pyriproxyfen, and pyriproxyfen plus fenoxycarb was on reproduction. The major effect of diflubenzuron was mortality. No hatch occurred in matings of females that were exposed to low concentrations of pyriproxyfen and fenoxycarb (2 ng/cm² and 6 ng/cm², respectively); sterility was incomplete when females were exposed to 600 ng/cm² of diflubenzuron. Mortality and sterility acted together to eliminate productive matings (matings that produced nymphs) when relatively high concentrations of diflubenzuron were combined with one or both of the other insect growth regulators (IGRs). In the triple combination, very small amounts of fenoxycarb and pyriproxyfen (total 1.1 ng/cm²) combined with 200 ng/cm² of diflubenzuron eliminated productive matings of treated females, but similar results with treated males were found only at higher concentrations of each IGR.     

Communication disruption of maleAgrotis segetum moths with one or several sex pheromone components

Attraction of maleAgrotis segetum Dennis & Schiffermüller (Lepidoptera: Noctuidae) to sex pheromone traps in fields, which were treated with one or three pheromone components was investigated. Small plots of 1/4ha size were treated with synthetic pheromone, released by 25 evenly dispersed latex rubber tube dispensers. The dispensers were loaded with either 500 μg Z5-10:OAc (50 mg/ha), or 1000 μg Z7-12:OAc (100 mg/ha), or a 3-component mixture consisting of 500 μg Z5-10:OAc+1000 μg Z7-12:OAc+1000 μg Z9-14:OAc. Pheromone traps were placed both within and outside of the treated area in a cross design, with an intertrap spacing of 15 m. Release rates from disruption dispensers were measured in the laboratory after being exposed in the field. The release rates of the components were estimated to be 0.44, 0.11, and 0.06 μg/h/dispenser for Z5-10:OAc, Z7-12:OAc and Z9-14:OAc, respectively. The highest effect of disruption was achieved by the three-component blend, resulting in a significant suppression of trap catches extending 5 m outside of the treated area. The Z5-10:OAc treatment resulted in reduced trap catches inside the treated area, but the effect did not extend outside. Z7-12:OAc alone did not result in any significant reduction in trap catch. The results indicate that different mechanisms may explain the disruptive effect of the treatments and that the single pheromone components are not as effective as the three-component blend.     

Pyriproxyfen enhances the immunoglobulin G immune response in mice

Pyriproxyfen is a juvenile hormone mimic of vital importance for insect development with little risk to humans. This study was performed to investigate whether large doses of pyriproxyfen affect the immune response in mammals. Mice were immunized thrice with ovalbumin in 5% ethanol, with or without pyriproxyfen or alum. Large doses of pyriproxyfen (9 or 15 mM) significantly enhanced specific total IgG immune response. This enhancement was no longer present 24 hr after treatment with pyriproxyfen. These results suggest that pyriproxyfen is a safe chemical. Moreover, pyriproxyfen induced higher titers of IgG2a and enhanced tumor necrosis factor‐alpha and gamma‐interferon responses whereas alum induced IgG1 with enhanced interleukin‐4 and ‐10. These observations indicate that the mechanism of immune enhancement by pyriproxyfen may differ from that of alum.     

Use of fecal extract trails to enhance trap catch in German cockroach (Dictyoptera: Blattellidae) monitoring stations

An aqueous extract of German cockroach, Blattella germanica (L.), fecal material was evaluated for inducing trail-following behavior in German cockroaches. In arena tests the fecal extract was found to stimulate trail following in 74% of adult male cockroaches. Significantly fewer cockroaches (22%) followed water-treated (control) trails. Residual activity of the fecal extract trails was evaluated by bioassay after the trails had been stored in the refrigerator or in the open air. Although trails stored in the refrigerator showed no decline in activity after 14 d, those stored in the open air declined significantly after 3 d, inducing only 40% of adult male cockroaches to follow the trail. After 7 d the activity of trails stored in the open air was further reduced to 23%. The ability of fecal-extract trails to influence trap catch in monitoring stations was determined by bioassay. Paper trails treated with fecal extract or water were positioned between cockroach harborages and monitoring stations inside 122-cm2 arenas. The presence of the fecal extract-treated trails significantly enhanced trap catch. Mean catch in the traps with fecal extract trails was 28 cockroaches compared with a mean of 11 cockroaches in the control traps. The trap catch ratios of adults to nymphs in the treated and control treatments were not significantly different.     

Impacts of two conventional insecticides on different stages of Encarsia inaron Walker parasitizing the whitefly,Trialeurodes vaporariorum Westwood under greenhouse condition

In this research, for the first time, compatibility of imidacloprid and pyriproxyfen was assayed with both pupa and adult stages of Encarsia inaron Walker parasitizing Trialeurodes vaporariorum Westwood. First, the insecticides were sprayed on bean plants containing the parasitoid pupae, thenceforth, survival of the adult parasitoid was evaluated 24 h post-exposure to fresh, 1-, 4-, 7-, 14-, and 21-day-old residues of them on bean leaflets in Petri dishes. Both insecticides significantly reduce E. inaron adult emergence. Mortality of the parasitoid pupa in treatment with imidacloprid (69.7%) was significantly higher than mortality with pyriproxyfen (28.6%). Pyriproxyfen was non-toxic to the adult parasitoid when residues were dried in fresh and aged through the experiment, while in the same condition, imidacloprid significantly killed the adult parasitoids up to 7 days after application. Results have been discussed on the potential compatibility of the insecticides with E. inaron in an integrated pest management (IPM) approach.     

Efficacy of CC traps and seasonal activity of adult Bemisia argentifolii (Homoptera: Aleyrodidae) in Imperial and Palo Verde Valleys, California

Adult whitefly Bemisia argentifolii Bellows & Perring trap (CC trap) catches were compared with suction type trap catches. CC trap catches were significantly correlated to the suction trap catches. Higher numbers of B. argentifolii adults were caught in CC traps oriented toward an untreated, B. argentifolii-infested, cotton field as compared with traps oriented toward Bermuda grass fields, farm roads, or fallow areas. CC trap catches at five heights above ground (from 0 to 120 cm) were significantly related to each other in choice and no-choice studies. CC trap catches were low in the Imperial and Palo Verde Valleys from late October to early June each of 1996, 1997, and 1998. Trap catches increased with increasing seasonal air temperatures and host availability. Trap catches were adversely affected by wind and rain. Abrupt trap catch increases of 40- to 50-fold for 1-2 d in late June to early July followed by abrupt decreases in adult catches suggest migrating activity of adults from other nearby crop sources.